Triggered Activity in Cardiac Muscle Fibers of the Simian Mitral Valve

Transcription

1 TRIGGERED MITRAL VALVE ACTIVITY/Wi, and Cranefwld 85 tin f the release f the adrenergic transmitter. Naunyn Schmiedebergs Arch Pharmacl 267: , Muschll E: Muscarinic inhibitin f the nrepinephrine release frm peripheral sympathetic fibres. In Pharmaclgy and the Future f Man: Prceedings f the 5th Internatinal Cngress f Pharmaclgy, vl 4, edited by FE Blm, GH Achesn. White Plains,.New Yrk, Albert J. Phiebig, 1973, pp Hllenberg M, Carriere S, Barger AC: Biphasic actin f acetylchline n ventricular mycardium. Circ Res 16: , Dempsey PJ, Cper T: Ventricular chlinergic receptr systems: Interactin with adrenergic systems. J Pharmacl Exp Ther 167: , La Raia PJ, Snnenblick EH: Autnmic cntrl f cardiac C-AMP. Circ Res 28: , Lee TP, Ku JF, Greengard P: Rle f muscarinic chlinergic receptrs in regulatin f guansine 3':5'-cyclic mnphsphate cntent in mammalian brain, heart muscle, and intestinal smth muscle. Prc Natl Acad Sci (USA) 69: , Gerge WJ, Wilkersn RD, Kadwitz PJ: Influence f acetylchline n cntractile frce and cyclic nucletide levels in the islated perfused rat heart. J Pharmacl Exp Ther 184: , De Geest H, Levy MN, Zieske H, Lipman RI: Depressin f ventricular cntractility by stimulatin f the vagus nerves. Circ Res 17: , Levy MN, Ng M, Martin P, Zieske H: Sympathetic and parasympathetic interactins upn the left ventricle f the dg. Circ Res 19: 5-10, Pace JB, Randall WC, Wechsler JS, Prila DV: Alteratins in ventricular dynamics induced by stimulatin f the cervical vagsympathetic trunk. Am J Physil 214: , Harmn MA, Reeves TJ: Effect f efferent vagal stimulatin n atrial and ventricular functin. Am J Physil 215: , Levy MN, Zieske H: Effect f enhanced cntractility n the left ventricular respnse t vagus nerve stimulatin in dgs. Circ Res 24: , Prila DV, Fultn RL: Psitive and negative respnses f the atria and ventricles t vagsympathetic stimulatin in the isvlumic canine heart. Circ Res 25: , Martin PJ, Levy MN, Zieske H: Analysis and simulatin f the left ventricular respnse t autnmic nervus activity. Cardivasc Res 3: , Levy MN, Ng ML, Zieske H: Functinal distributin f the peripheral cardiac sympathetic pathways. Circ Res 19: , Antn AH, Sayre DF: A study f the factrs affecting the aluminum xide-trihydrxyindle prcedure fr the analysis f catechlamines. J Pharmacl Exp Ther 138: , Laverty R, Taylr KM: Flurmetric assay f catechlamines and related cmpunds. Anal Bichem 22: , Siegel JH, Gilmre JP, Sarnff SJ: Mycardial extractin and prductin f catechl amines. Circ Res 9: , Hukvic S, Muschll E: Die Nradrenalin-Abgabe aus dem islierten Kaninchenherzen bei sympathischer Nervenreizung und ihre pharmaklgische Beeinflussung. Nauyn Schmiedebergs Arch Pharmacl 244:81-96, Yamaguchi N, de Champlain J, Nadeau R: Nradrenaline liberatin frm the dg heart in viv. Can J Physil Pharmacl 51: , Kch-Weser J, Blinks JR: The influence f the interval between beats n mycardial cntractility. Pharmacl Rev 15: , Sarnff SJ, Brckman SK, Gilmre JP, et al.: Regulatin f ventricular cntractin; influence f cardiac sympathetic and vagal nerve stimulatin n atrial and ventricular dynamics. Circ Res 8: , I Williams JF, Snnenblick EH, Braunwald E: Determinants f atrial cntractile frce in the intact heart. Am J Physil 209: , Lindmar R, Lffelhlz K, Muschll E: A muscarinic mechanism inhibiting the release f nradrenaline frm peripheral adrenergic nerve fibres by nictinic agents. BrJ Pharmacl Chemther 32: , Hirsch EF, Willman VL, Jellinek M, Cper T: The terminal innervatin f the heart. Arch Pathl 76: , Kluda MA: Distributin f catechlamine in the dg heart. Prc Sc Exp Bil Med 112: , Angelaks ET: Reginal distributin f catechlamines in the dg heart. Circ Res 16: 39-44, Jellinek M, Kaye MP, Kaiser GC, Cper T: Effect f cervical vagsympathectmy n mycardial catechlamine cncentratin. Am J Physil 209: , Hiksaka H: The effects f stellate ganglinectmy n the distributin, uptake and strage f nradrenaline in the dg heart. Jap J Pharmacl 16: , MacLean LD, Hedenstrm PH, Kim YS: Distributin f bld flw t the canine heart. Prc Sc Exp Bil Med 107: , Lve WD, O'Meallie LP: Clearance f Rb" frm crnary bld by the atria f dgs. Prc Sc Exp Bil Med 112: , Mir TW, Driscl TE, Eckstein RW: Thebesian drainage in the left heart f the dg. Circ Res 14: , 1964 Triggered Activity in Cardiac Muscle Fibers f the Simian Mitral Valve ANDREW L. WIT, PH.D., AND PAUL F. CRANEFIELD, M.D., SUMMARY The actin ptential f cardiac fibers in the anterir mitral valve leaflet f the mnkey heart is fllwed by an after-hyperplarizatin. The additin f catechlamines causes a delayed after-deplarizatin t fllw the after-hyperplarizatin. The amplitude f the after-deplarizatin increases as the stimulus cycle length is decreased, r after premature stimulatin, and as a result can reach threshld t yield nndriven, sustained rhythmic activity which we term triggered activity. This sustained rhythmic activity can be terminated by a single, apprpriately timed, premature stimulus. The amplitude f the actin ptentials f mitral valve fibers is increased PH.D. by catechlamines; the amplitude and rate f deplarizatin are depressed by verapamil. The amplitude f the actin ptentials is little affected by tetrdtxin (TTX) but the maximum rate f deplarizatin is reduced by TTX. The delayed after-deplarizatin induced by catechlamines is ablished by verapamil, as is triggered activity. These bservatins suggest that mitral valve fibers generate slw respnse actin ptentials, that triggerable sustained rhythmic activity may be a prperty f the slw respnse and that such activity may cause the types f cardiac arrhythmias that usually are attributed t reentry. WE HAVE described, in Purkinje fibers expsed t sdiumfree slutins, a frm f sustained rhythmic activity that can be initiated and smetimes terminated by a single stimulus and that is prbably nt reentrant The driven actin Frm the Department f Pharmaclgy, Clumbia University Cllege f Physicians and Surgens, and The Rckefeller University, New Yrk, New Yrk. ptential that triggers such activity is fllwed by an early after-hyperplarizatin and a delayed after-deplarizatin. Supprted by U.S. Public Health Service Grants HL and HL frm the Natinal Heart and Lung Institute. Dr. Wit was a Senir Investigatr f the New Yrk Heart Assciatin during this study and is a Career Scientist f the Irma T. Hirschl Trust. Received April 3, 1975; accepted fr publicatin Octber 13, 1975.

2 86 CIRCULATION RESEARCH VOL. 38, N. 2, FEBRUARY 1976 The delayed after-deplarizatin reaches threshld and evkes the next actin ptential. Withut an initial driven actin ptential a quiescent fiber remains quiescent. Triggered activity als has been described fr sinatrial ndal fibers expsed t lw [Na + ] 0, 3 embrynic fibers, 4 and Purkinje fibers expsed t digitalis. 5 ' 6 Sustained rhythmic activity that depends n deplarizing afterptentials is f bvius interest as a pssible cause f arrhythmias. We nw reprt that fibers f the mitral valve f the mnkey heart shw this behavir when expsed t catechlamines and a nrmal inic envirnment. Sustained rhythmic activity can be triggered in such fibers by a gradual increase in drive rate, by a single premature stimulus interpsed in a train f regular driven impulses, r by a single driven impulse evked in a quiescent fiber; this triggered rhythmic activity can be arrested by a premature impulse. In additin, the upstrkes f the actin ptentials f fibers f the mitral valve f the mnkey heart are sensitive t verapamil; they are mre resistant t tetrdtxin (TTX) than are the upstrkes f actin ptentials in rdinary atrial fibers; and they persist at high levels f [K + ] when the resting membrane ptential is markedly reduced; these actin ptentials thus depend n the slw inward current. These findings, tgether with previus reprts n triggered activity, suggest that the phenmenn f triggered activity may be a prperty f the slw respnse. Methds Islated anterir mitral valve leaflets r leaflets attached t the antermedial atrial wall were btained frm the hearts f 30 pigtail mnkeys. The mnkeys (weighing 5-8 kg) were anesthetized with sdium pentbarbital (30 mg/kg, iv); the hearts were rapidly remved thrugh a thractmy and dissected at rm temperature in a mdified Tyrde's slutin, as previusly described fr the canine heart. 7 The preparatin was pinned t the wax base f a 50-ml tissue superfusin chamber with the atrial surface f the mitral valve and endcardial surface f the atrial wall facing upward. The tissue was superfused at a rate f abut 15 ml/min with the mdified Tyrde's slutin 7 that was saturated with a 95% O 2-5% CO 2 mixture. Temperature was maintained at 36 ± 0.5 C. The preparatin was stimulated thrugh biplar Teflncated silver electrdes n the atrial wall r n the valve leaflet by rectangular pulses 1.5x-2x threshld and 3 msec lng. Premature impulses culd be applied t the preparatin at any given interval after the preceding basic drive stimulus; the basic drive fllwing the premature stimulus culd be mitted t determine whether nndriven actin ptentials ccurred. When the effects f premature stimuli were studied the stimuli were induced after every 10th basic drive stimulus. Actin ptentials were recrded frm atrial wall fibers and mitral valve fibers thrugh intracellular electrdes filled with 3 M KG. 7 Atrial muscle fibers usually extend nly 1-4 mm int the simian mitral valve near its attachment t the atrial wall and nt int the midvalve leaflet r mre distal regins as in the dg. 7 Epinephrine r nrepinephrine was added either directly t the bath withut stpping the inflw, t achieve an estimated transient cncentratin f Ojig/ml, r t the reservir f Tyrde's slutin t give the desired cncentratin. When catechlamines were added t the reservir ethylenediaminetetraacetate (EDTA) was als added t give a cncentratin f 10"* M. In three experiments nrepinephrine was applied t the surface f the mitral valve thrugh a micrpipette with a tip diameter f abut 30 /i and cnnected t a syringe system. 8 The cncentratin f epinephrine r nrepinephrine in the injectin fluid was 2 jig/ml; the cncentratin that reached the tissue was prbably much lwer. When the effects f verapamil r TTX were studied, the drugs usually were added t the reservir t give the desired cncentratin, but t attain a very high cncentratin f TTX it smetimes was added t the tissue bath with the nw stpped. In three experiments the sequence f excitatin was determined during sustained rhythmic activity; an extracellular electrgram recrded frm the atrial wall served as a time reference. The time f activatin f sites n the valve leaflet and atria relative t the reference site was determined at a recrding speed f 5-20 msec/cm. We determined the lcatin f each site by using the stage mvement n the micrmanipulatr and pltted the times f activatin n a diagram f the preparatin, as we have previusly described. 7 TERMINOLOGY In this reprt we have used several recently intrduced terms, 2 defined as fllws: After-hyperplarizatin. An afterptential that is cntinuus with phase 3 replarizatin but that carries the membrane ptential t a level mre negative than that seen later in diastle. Delayed After-deplarizatin. A deplarizing afterptential that ccurs after replarizatin has brught membrane ptential t the level characteristic f diastle. The delayed after-deplarizatins described in the present reprt invariably fllwed an early after-hyperplarizatin. Spntaneus r Autmatic Activity. The activity that arises in the absence f an external cause, i.e., activity that des nt have t be triggered by a stimulated actin ptential. An example f such activity is that f the sinatrial nde. In fibers that remain quiescent until driven at least nce, and thereafter initiate nndriven actin ptentials, the resultant activity des nt arise spntaneusly even thugh it persists nce it is evked. We call this persistent activity sustained rhythmic activity (see belw). Triggered Activity. Activity in which nndriven actin ptentials are initiated by ne r mre driven actin ptentials. The term triggered underlines the fact that a quiescent fiber will remain quiescent until driven either by a lcally evked actin ptential r by an actin ptential that prpagates int the fiber frm a distant site. Sustained Rhythmic Activity. This is a purely descriptive term that embraces bth triggered and truly spntaneus activity as well as repetitive activity arising frm sustained circus mvement f excitatin. It implies sustained generatin f actin ptentials in the absence f artificial stimulatin.

3 TRIGGERED MITRAL VALVE ACTIVITY/Wit and Cranefield 87 Results CHARACTERISTICS OF THE ACTION POTENTIAL OF FIBERS OF THE SIMIAN MITRAL VALVE In atrial muscle fibers f the anterir atrial wall the maximum diastlic ptential was 83 ± 3 mv, actin ptential amplitude was 98 ± 5 mv, versht was 15 ± 2 mv and V mbx (maximum value fr the first time derivative f transmembrane vltage) f phase 0 was 125 ± 15 V/sec (mean ± SD, values frm 50 cells). In cntrast, the maximum diastlic ptential f mitral valve fibers was 65 ± 5 mv, the amplitude f the actin ptential, measured frm maximum diastlic ptential t the peak f the versht, was 80 ± 4 mv, the versht was 15 ± 2 mv, and K mal f phase 0 was 5 ± 2 V/sec (mean ± SD, values frm 150 cells). The actin ptentials f mitral valve fibers f the simian heart thus resemble thse f fibers f the canine mitral valve 7 but differ markedly frm thse f atrial cells. As in the canine heart, the actin ptentials shwed a marked early after-hyperplarizatin after which the ptential declined t a steady resting level (Fig. 1, tp). Neither triggered nr autmatic impulse initiatin culd be demnstrated in the simian mitral valve during superfusin with Tyrde's slutin which did nt cntain a catechlamine. I A --35 FIGURE 1 Epinephrine-induced delayed after-deplarizatins in mitral valve fibers. Panels IA and IB shw recrdings frm the same fiber. Only the lwer pan f the actin ptential (mre negative than -35 mv) is shwn. In IA, in the absence f catechlamines, there is a prminent after-hyperplarizatin but n after-deplarizatin. IB shws the after-deplarizatin (arrw) during superfusin with epinephrine (1.0 ng/ml). IIA shws the effect f lcal applicatin f epinephrine n a mitral valve fiber. The epinephrine was ejected thrugh a micrpipette (see Methds) and the actin ptential in the bttm trace was recrded at the site f ejectin. The actin ptential in the middle trace was recrded at a site 5 mm away. Nte the develpment f the delayed afterdeplarizatin (slid arrw) recrded frm the fiber at the site f epinephrine ejectin (n after-deplarizatin was present prir t epinephrine). N after-deplarizatin develped in the fiber remte frm this site (pen arrw). Calibratins fr 11A: vertical = 50 mv, hrizntal = 200 msec. TRIGGERED ACTIVITY Delayed After-deplarizatin. In the presence f epinephrine r nrepinephrine ( /ig/ml) the after-hyperplarizatin f all fibers increased and a delayed after-deplarizatin appeared msec after the upstrke f the actin ptential (Table 1, Fig. 1). The magnitude f the increase in the after-hyperplarizatin was nt related t the amplitude f the delayed after-deplarizatin. The amplitude f the delayed after-deplarizatin, defined as the difference between the afterptential at its peak and the steady diastlic ptential, varied greatly frm fiber t fiber in the same valve and frm valve t valve and ranged frm 2 t 30 mv at a drive rate f 30/min (Table 1). The additin f catechlamine ( /ig/ml) t unstimulated, quiescent preparatins never caused them t becme spntaneusly active. The additin f catechlamine in this cncentratin t fibers driven at rates f 30/min r less rarely caused them t shw triggered r sustained rhythmic activity. The delayed after-deplarizatin that ccurred in the presence f catechlamine was nt caused by electrtnic spread frm a secnd impulse blcked in the vicinity f the recrding electrde; detailed explratin f each f 10 preparatins never revealed such a secnd actin ptential. In three f these experiments, a slutin cntaining a catechlamine als was applied lcally t mitral valve fibers thrugh a micrpipette; the after-deplarizatin invariably appeared nly in fibers within a distance f 1 mm frm the site f applicatin f catechlamine (Table 1, Fig. 1). Fibers utside this zne did nt shw a delayed after-deplarizatin. In all f these experiments the amplitude f the after-deplarizatin was maximal immediately beneath the tip f the micrpipette used t apply the catechlamine and gradually decreased with increasing distance frm this site (Fig. I). Triggering by Increasing Stimulatin Rate. In 18 experiments we supervised the islated valve leaflet with epinephrine r nrepinephrine (0.5 t 2.0 fig/ml) and determined the effects f the interval between stimuli (cycle length) n the delayed after-deplarizatin. The respnses t epinephrine and nrepinephrine were identical. An initial cycle length f greater than 3 secnds was decreased first in steps f 200 msec and later in steps f 50 msec. Fifteen impulses were initiated at each cycle length and then stimulatin was interrupted. After the drive had been stpped, we measured the amplitude f the delayed after-deplarizatin that fllwed the last driven impulse. Nndriven activity smetimes supervened either during the test perid r at its end (Fig. 2). A lw initial rate was chsen s that each driven actin ptential ccurred lng after cmpletin f the preceding after-deplarizatin (Fig. 3). As the cycle length was reduced the delayed after-deplarizatin ccurred at the same time relative t the preceding actin ptential, althugh there were slight changes frm ne cycle t the next, and its amplitude increased. In five preparatins, reductin f the stimulus cycle length did nt increase the amplitude f the after-deplarizatin unless each driven actin ptential ccurred during the replarizatin phase f the preceding

4 CIRCULATION RESEARCH VOL. 38, N. 2, FEBRUARY 1976 TABLE 1 Effects f Catechlamines n Afterptentials f Mitral Valve Fibers After-hyperplariza- After-deplarizatin Stimulus cycle tin(mv) (mv) length (msec) n (Range) Mean ± SD (Range) Mean ± SD Superfusin experiments Cntrl 3, ( 1-15) 10 ± Catechlamine ( , (10-30) 16 ± 2.0* (2-30) 20 ± 4* Lcal catechlamine applicatin Cntrl 3,000 Catechlamine ( ,000 6 ( 2-12) 8 ± ( 6-23) 14 ± 2.0* (5-27) 18 ± 2* n = number f fibers frm which data were btained in 10 superfusin experiments and in three experiments during lcal catechlamine applicatin. * Statistically significant frm cntrl (P <0.001). after-deplarizatin. This ccurred at cycle lengths f msec (Figs. 2 and 3). Further reductin f the cycle length resulted in a further increase in the amplitude f the delayed after-deplarizatin bth during stimulatin and after the final driven actin ptential. In five ther preparatins a reductin in cycle length caused an immediate increase in amplitude f the after-deplarizatin, even when each driven actin ptential ccurred after cmpletin f the preceding after-deplarizatin (Fig. 2, Fig. 4-1). Five t 10 trials were perfrmed in each study and the respnse always was reprducible. In fibers shwing either type f respnse, in 10 f the 18 experiments shrtening the cycle length t a critical value caused single nndriven actin ptentials t ccur either befre the next stimulus r after stimulatin stpped (Fig. 3). This critical cycle length varied widely amng the different preparatins (2, msec) (Fig. 2). The amplitude f the after-deplarizatin ften increased gradually ver the first 5-10 impulses during stimulatin at this critical cycle length until, when the amplitude f the delayed afterdeplarizatin reached a critical value (18-22 mv), the nndriven actin ptentials ccurred (Fig. 4-1B). The nndriven actin ptential arse frm the peak f the delayed afterdeplarizatin; therefre the cupling interval between the nndriven actin ptential and the preceding driven actin ptential always was the same as the interval between the upstrke f the driven actin ptentials and the peak f their after-deplarizatins. Further reductin in the cycle length beynd that necessary t elicit single nndriven actin ptentials triggered sustained rhythmic activity at a rate mre rapid than the rate at which the fiber was being driven; this activity cntinued fr several secnds t 15 minutes after stimulatin was terminated (Fig. 4). The cycle length f sustained rhythmic activity ranged frm 350 t 600 msec fr the 10 preparatins but was cnstant during repeated trial in each preparatin. Althugh the first nndriven actin ptential ccurred at a cupling interval crrespnding t that f the after-deplarizatin, the rate f nndriven activity increased during the first few beats f sustained rhythmic activity. The same acceleratin is seen after triggering by a premature stimulus (Fig ) but we d nt knw why it ccurs. Fur valves in which triggered activity culd be prduced were cut int small pieces, sme n larger than 2-4 mm 2. Stimulatin induced triggered activity in 75-80% f the small pieces; the upstrkes f actin ptentials recrded simultaneusly frm different cells in a given fragment ccurred virtually simultaneusly; this suggests that reentry was nt taking place (see Discussin). In eight f the 18 preparatins, nndriven actin ptentials culd nt be elicited in mitral.valve fibers by rapid drive r premature stimulatin. In three f these preparatins muscle fibers did nt extend int the mitral valve leaflet. In the remaining five preparatins the amplitude f the after-deplarizatins after catechlamine superfusin was nly 2-8 mv, and in fur f these the after-deplarizatins did nt increase in amplitude when cycle length was shrtened, r did s nly slightly, even thugh we used cncentratins f catechlamine f up t 5.0^g/ml (Fig. 5). Triggering by a Premature Stimulus. In fur experiments, when mitral valve fibers were driven at a fixed rate that did nt lead t triggering the delayed after-deplarizatins that fllwed premature impulses were larger than thse fllwing the driven actin ptentials and triggered nndriven activity. As the cupling interval between the basic and premature impulse was decreased, the after-deplarizatin f the premature impulse prgressively increased in amplitude until triggering ccurred (Fig. 2; Fig ). In tw fibers sustained rhythmic activity was triggered by premature impulses that ccurred after the peak f the preceding after-deplarizatin; in tw thers, the impulse that triggered rhythmic activity ccurred befre the expected peak f the preceding after-deplarizatin. In all cases the first triggered actin ptential arse frm the peak f an after-deplarizatin (Fig. 4). Initiatin f Rhythmic Activity by a Single Applied Stimulus. Superfusin f quiescent mitral valve fibers with catechlamine ( /ig/ml) never caused spntaneus activity. In each f 10 separate trials n tw preparatins during such superfusin, a single electrical stimulus resulted in a single driven actin ptential that was fllwed by sustained rhythmic activity which cnsisted f impulses (Fig. 4). Spntaneus and Induced Terminatin f Sustained Rhythmic Activity. Once sustained rhythmic activity had been triggered, it either stpped spntaneusly r culd be

5 TRIGGERED MITRAL VALVE ACTIVITY/ Wit and Cranefield 89 stpped by a single premature impulse. When sustained rhythmic activity stpped spntaneusly, the last actin ptential was fllwed by ne r mre subthreshld deplarizatins at intervals crrespnding t that f the preceding rhythmic activity (Fig. 4). A premature impulse terminated sustained rhythmic activity if it was induced either very early in the cycle (tw f five experiments) r very late in the cycle (five f five experiments). Early premature impulses that terminated sustained rhythmic activity ccurred while the fibers were partially refractry and evked nly an abrtive respnse, after which the after-deplarizatin did nt reach threshld (Fig. 6A). The cupling intervals f these premature impulses ranged frm 250 t 350 msec. Actin ptentials evked slightly later shwed a nrmal amplitude and did nt terminate the sustained rhythmic activity, but ften demnstrated an increased after-hyperplarizatin f 2-5 mv. This increased the time needed fr deplarizatin t reach threshld and initiate the next impulse. As a result the E Q. <t 14 - O 12- Ul Q 10 - a ui I- 8 - u. * CYCLE LENGTH msec FIGURE 2 Effects f stimulus cycle length n the amplitude f the delayed after-deplarizatin f mitral valve actin ptentials. All preparatins are superfused with a calechlamine. In bth panels each symbl designates a different preparatin. Each pint represents the mean ± SD fr 5-7 cells in the preparatin. The tp panel shws the effects f decreasing the basic cycle length (abscissa) n after-deplarizatin amplitude (rdinate). Nte that there are tw general types f respnses: in five preparatins after-deplarizatin amplitude increased prgressively as cycle length decreased and in five preparatins cycle length was decreased t 750-1,750 msec befre after-deplarizatin amplitude began t increase. The bttm panel shws the effects f premature stimuli induced al a basic stimulus cycle length f'2,800 msec: the cupling intervals f the premature impulses are n the abscissa and the after-deplarizatin amplitude f the premature impulses is n the rdinate. in bth panels the shrtest cycle length shwn fr each experiment is the cycle length which induced triggered impulses.

6 90 CIRCULATION RESEARCH VOL. 38, N. 2, FEBRUARY 1976 A -30mV -70mV SOO msec FIGURE 3 The effect f decreasing stimulus cycle length n the amplitude f the delayed after-deplarizatin. Recrdings frm the same fiber are shwn in each panel. Only the lwer part f the actin ptential is shwn. The preparatin was superfused with epinephrine (1.0 ng/ ml). The last several driven ptentials f a series f 15 are shwn in each panel. Cycle length in A is 2,600 msec. In B (cycle length, 1,300 msec) the amplitude f the delayed after-deplarizatin is increased. In C (cycle length, 850 msec) each actin ptential is elicited prir t cmplete replarizatin f the previus after-deplarizatin; the amplitude f the after-deplarizatin has increased further. In D (cycle length, 800 msec) a nndriven actin ptential ccurs (arrw) after the stimulus has been turned ff. The nndriven actin ptential is fllwed by a delayed after-deplarizatin. next upstrke was delayed (Fig. 6B). Premature impulses induced still later in the cycle were fllwed by a slightly greater after-hyperplarizatin; the subsequent after-deplarizatin failed t give rise t an actin ptential, and quiescence ensued (Fig. 6C). Late premature impulses that terminated sustained rhythmic activity ccurred at cupling intervals f msec. Absence f Triggered Activity in Atrial Wall Fibers. Nne f the phenmena described abve culd be prduced in the islated antermedial atrial wall when it was nt attached t the mitral valve leaflet. In five experiments unstimulated preparatins f the atrial wall remained quiescent during superfusin with catechlamine, and stimulatin did nt prduce either afterptentials r sustained rhythmic activity. Spntaneus activity ccurred in the atrial wall in five ther preparatins at cycle lengths f 800 ± 85 msec, i.e., at rates much slwer than thse seen during triggered activity f fibers in the mitral valve. We were unable t find cells shwing phase 4 deplarizatin in the atrium during this spntaneus activity. Activatin f the Atrium frm the Mitral Valve during Sustained Rhythmic Activity. We studied the sequence f activatin in five preparatins in which the atrial wall was stimulated and sustained rhythmic activity was induced in the mitral valve fibers by impulses that prpagated frm the atrium int the valve (Fig. 7). Once rhythmic activity had been triggered in the mitral valve leaflet, the spntaneusly ccurring actin ptentials prpagated back t activate the atrium (Fig. 7) and the sequence f activatin was identical t that described previusly. 7 That the impulse riginated in the mitral valve was cnfirmed in three f the preparatins when, during the curse f sustained rhythmic activity, cnductin blck develped between the valve and atrium. At such times rhythmic activity ccurred nly in the valve leaflet and nt in the atrium (Fig. 7). IS THE ACTION POTENTIAL OF MITRAL VALVE FIBERS A SLOW RESPONSE? Triggered activity previusly has been demnstrated in fibers in which the actin ptential prbably is a slw respnse, i.e., in fibers in which actin ptential upstrke depends n inward current flwing thrugh the slw channel. 2 The slw respnse can be characterized by the effects f catechlamines, verapamil, and TTX and by effects f resting ptential n the upstrke f the actin ptential.

7 TRIGGERED MITRAL VALVE ACTIVITY/Wit and Cranefteld 91 Effects f Catechlamines. In additin t enhancing the delayed after-deplarizatin, epinephrine r nrepinephrine ( ng/mx) increased the amplitude f the mitral valve actin ptential, ften withut affecting the maximum diastlic ptential (Table 2). This effect was particularly prnunced when the actin ptential amplitude was lw (<65 mv) and the versht absent even thugh resting ptential was nrmal (-65 t -70 mv). In such depressed fibers an increase in actin ptential amplitude after expsure t catechlamine resulted frm an increase in versht. In fibers whse actin ptentials initially had a nrmal amplitude f mv and vershts f mv, the additin f catechlamine increased the amplitude nly slightly by increasing the versht (Table 2). Effects f Verapamil. In fur studies we were able t btain actin ptential recrdings frm the same single fiber during a cntrl perid and during superfusin with verapamil ( mg/liter). In the absence f catechlamine the upstrke f the actin ptential f the mitral valve was slwed by verapamil and the versht and amplitude were diminished. Maximum diastlic ptential was nt changed (Table 3, Fig. 8). Similar cncentratins had n effect n the maximum diastlic ptential, actin ptential upstrke velcity, r versht f fibers f the atrial wall, but they depressed the level at which the plateau ccurred (results frm three fibers in which impalement was maintained during cntrl and drug superfusin) (Table 3). The depressant effects f verapamil n the upstrke f the mitral valve actin ptential culd be partially reversed by the additin f nrepinephrine r epinephrine ( ng/m\) t the superfusate. Verapamil depressed the delayed after-deplarizatin seen during catechlamine superfusin (five experiments) (Table 3, Fig. 9). During superfusin with catechlamine ( ^g/ml) verapamil (0.5 mg/liter) reduced the rate f deplarizatin and amplitude f the actin ptential less than in the absence f catechlamine but almst ablished the delayed after-deplarizatin (Table 3). Verapamil thereby ablished the ability f a decrease in cycle length r premature stimulatin t trigger activity. This was true whether the stimulus was applied directly t the valve r t the atrium (Fig. 9). If the catechlamine cncentratin in the superfusate was increased after verapamil had suppressed the delayed after-deplarizatin, the amplitude f the after-deplarizatin increased and triggered activity nce again culd be induced. -30mV -30mV FIGURE 4 Triggering f sustained rhythmic activity. Only the lwer part f the actin ptential is shwn in each panel. Hrizntal lines in I and 11 are at minus 30 m V, tp trace in III is at -20 m V. Vertical calibratin is 30 m Vfr I and II and 60 m Vfr III. Hrizntal calibratin = 1,400 msec is fr I and II nly. Time pips in tp trace f III are at 100-msec intervals. Panels IA and B shw triggered activity induced by decreasing the basic cycle length. In IA cycle length is 3,400 msec and a delayed after-deplarizatin is present. In IB cycle length is 1,750 msec and there is a prgressive increase in the after-deplariiatin which fllws each f the first fur driven actin ptentials. The last driven actin ptential (arrw) is fllwed by a nndriven actin ptential and by sustained rhythmic activity at a rate greater than the driven rate. Panels IIA and B shw triggering f sustained rhythmic activity in a mitral valve fiber by a single driven impulse in a preparatin superfused with epinephrine (1.0 fig/ml). In the absence f electrical stimulatin there are n spntaneus actin ptentials. In 11A, after a erid f quiescence, a single stimulated actin ptential (arrw) is fllwed by an after-deplarizatin but n nndriven actin ptentials. In I Id, after a subsequent 2-minute perid f quiescence anther single actin ptential is stimulated (arrw) and is fllwed by sustained rhythmic activity which lasted fr 5 minutes. Panels III A and B shw triggered activity induced by premature stimulatin in the presence f epinephrine (1.0 Hg/ml). The basic cycle length is 2,800 msec. In III A, when the premature impulse (arrw) was induced n the replarizatin phase fthe previus after-deplarizatin, the amplitude f the after-deplarizatin f the premature impulse was increased. In I1IB the premature impulse (small arrw) was induced prir t the time at which the after-deplarizatin f the preceding basic impulse wuld have ccurred, a nndriven actin ptential arises frm the after-deplarizatin (large arrw), and nndriven activity is sustained. The bttm right panel shws this sustained rhythmic activity terminating in a subthreshld after-deplarizatin (arrw).

8 92 CIRCULATION RESEARCH VOL. 38, N. 2, FEBRUARY 1976 IB l > 14' E O '0- Q_ UJ Q a UJ CYCLE LENGTH (msec) FIGURE 5 Effect f basic stimulus cycle length (abscissa) n the amplitude f the delayed after-deplarizatin (rdinate) f mitral valve actin ptentials in preparatins in which triggered activity culd nt be induced. Each symbl designates a different preparatin. Each pint represents the means ± SD f 5-7 cells in the preparatin. All preparatins are superfused with a catechlamine. _JL 50cnV FIGURE 6 Terminatin f sustained rhythmic activity by a premature stimulus. Each panel shws the last tw nndriven actin ptentials which preceded the premature stimulus at the arrws- Recrdings frm the same fiber are shwn in each panel. In panel A, the premature stimulus is induced prir t cmplete replarizalin f the preceding actin ptential, and induces nly an abrtive respnse which is fllwed by a lw amplitude scillatin and terminatin f the rhythmic activity. In B, a premature impulse induced later in the cycle is fllwed by a slight lengthening f the cycle, but sustained rhythmic activity cntinues. In C, a premature impulse ccurring still later in the cycle is fllwed by an after-deplarizatin which fails t reach threshld, and sustained rhythmic activity is terminated. Time pips in lp trace f each panel are at 100-msec intervals. Effect f Tetrdtxin (TTX). We studied the effect f TTX n mitral valve actin ptentials in five preparatins. We were nt able t recrd mitral valve actin ptentials frm the same fiber bth during the cntrl perid and after expsure t TTX; therefre recrdings were btained frm five r six fibers cnsecutively at each perid during the experiment. We were able t btain recrdings frm three atrial fibers during cntrl and drug superfusin. Mitral valve fibers were mre resistant t the effects f TTX than were muscle fibers f the anterir atrial wall (Table 4). The steepness f the upstrke and the amplitude f the actin ptential f atrial muscle fibers were prgressively depressed at cncentratins f TTX greater than 2.0 mg/liter. At 10.0 mg/liter atrial fibers became inexcitable, althugh the level f resting membrane ptential was unaltered (Table 4). The slpe f the actin ptential upstrke f mitral valve fibers als was prgressively depressed by TTX in cncentratins greater than 2.0 mg/liter; this ccurred bth in the presence and absence f catechlamine (Table 4, Fig. 8). Hwever, the amplitude f deplarizatin was nt greatly affected by TTX, and actin ptentials with nearly nrmal amplitude but greatly reduced upstrke velcity culd be elicited after expsure t TTX (5.0 mg/liter) and, in tw experiments, after 10.0 mg/liter (Fig. 8). The effect f TTX n atrial muscle fibers was nt altered by the presence f catechlamine. Hwever, TTX caused less decline in steepness and amplitude f mitral valve actin ptentials in the presence f either epinephrine r nrepinephrine (Table 4). TTX (5.0 mg/liter) did nt have any

9 TRIGGERED MITRAL VALVE ACTIVITY/ Wit and Cranefteld mv 250 msec FIGURE 7 Prpagatin f triggered activity frm the anterir mitral valve lea/let t the adjacent atrial wall in the presence f epinephrine, 1.0 fig I ml. Tp trace, biplar atrial electrgram; bttm trace, actin ptential f mitral valve muscle fiber. Stimulus electrdes are n the atrial wall. At cycle lengths greater than 1,200 msec triggering did nt ccur (nt shwn). The first tw actin ptentials shwn in the tp panel were initiated after the cycle length was reduced t 1,150 msec. Atrial activity precedes activatin f the valve fiber, and an after-hyperplarizatin and a delayed after-deplarizatin fllw the first driven actin ptential. The after-deplarizatin fllwing the secnd driven actin ptential prgresses int the upstrke f a nndriven actin ptential which nw precedes the atrijl electrgram, suggesting that the impulse is nw prpagating in the ppsite directin (frm valve t atrium). Sustained rhythmic activity cntinues in the valve, and the atrium is activated after each valve actin ptential. After several minutes cmplete cnductin blck develped between the valve leaflet and the atrial wall, as shwn in the bttm panel; sustained rhythmic activity cntinues in the valve fiber but there is n evidence f electrical activity in the atrial electrgram. effect n the delayed after-deplarizatin that ccurred in the presence f catechlamine; the effects f changes in rate and rhythm n the amplitude f this afterptential were nt altered, and sustained rhythmic activity culd be triggered by stimulatin patterns similar t thse that induced it befre expsure t TTX (Table 4). Als, the rate at which sustained rhythmic activity appeared was nt affected. Effects f Resting Ptential n the Rate f Deplarizatin and Oversht. The resting ptential f atrial fibers in the atrial wall and mitral valve was reduced by increasing [K + ] in the absence f catechlamine. In five experiments, increasing [K + ] frm 4 mm t 12, 20, 30, 50, and 80 mm resulted in a prgressive decline in maximum diastlic ptential f atrial wall fibers. The upstrke velcity, amplitude, and versht f the actin ptential fell as the fibers deplarized. When [K + ] was 12 mm the resting ptential declined t less than -70 mv and these cells became inexcitable (Fig. 10). Further elevatin f [K + ] t 20, 50, and 80 mm caused additinal decreases in membrane ptential and cntinued inexcitability. In fur experiments increasing [K + ] abve 4 mm als TABLE 2 Effects f Catechlamines n Mitral Valve Fibers MDP (mv) Depressed fibers Cntrl 6 68 ± 4 Caiechlamines ( ± 2 NS caused a lss f resting membrane ptential f mitral valve fibers and a decrease in actin ptential upstrke velcity, but the versht did nt decrease as much as fr atrial wall fibers (Fig. 10). When [K + ] was mm, resting ptential fell t abut -55 mv and actin ptentials culd nt be evked by extracellular stimulatin. Hwever, when [K + ] was 20 mm and resting ptential was abut -50 mv, actin ptentials with a nearly nrmal versht f mv nce mre culd be elicited (Figs. 10 and 11). These actin ptentials were nt fllwed by an after-hyperplarizatin. Actin ptentials with versht still culd be elicited when [K + ] was 50 mm and resting ptential -30 mv. In mitral valve fibers elevatin f [K + ] t 30 mm in the absence f catechlamine resulted in the appearance f nndriven activity at a cycle length f 1,100 ± 75 msec (mean ± SD fr the fur preparatins) (Fig. 11). This activity seemed t be spntaneus; that is, it appeared withut being triggered by a driven r prpagated impulse. It was nt affected by cutting the valve int several small pieces, and therefre prbably was nt reentrant. As [K + ] APA (mv) 64 ± 3 85 ± 4 <0.001 Overshtt (mv) -4 ± ±2 <0.001 Nrmal fibers Cntrl 6 70 ± 4 Catechlamines ( ± 3 NS 81 ±4 89 ± 5 < ± ± 2 <0.01 All values are mean ± SD. n = number f fibers studied (single impalements maintained during cntrl and after drug); MDP = maximum diastlic ptential; APA = actin ptential amplitude; NS = nt significant. * P values are derived frm cmparisn f electrphysilgical values after drug perfusin with cntrl values. t Values under versht are the difference between the level f membrane ptential attained at the peak f the actin ptential and zer transmembrane ptential. Psitive values therefre indicate a true versht r reversal and negative values indicate the degree t which the actin ptential fell shrt f zer.

10 94 CIRCULATION RESEARCH VOL. 38, N. 2, FEBRUARY 1976 CONTROL CONTROL i-» IADA E 2 I -H I H -H O «TT O OO fn I I 1 1 VERAPAMIL 0.5mg/L TTX 5.0mg/L 2 O O O r-l ' " IN CT\ Tt TTX 10.0 mg/l I- -H I I E.. I = iali c Pt tin 1 Valve ditral c m UJ CO Ovei sht' E, a. < E MDPi < 2 i- _,< 2 f- < 2 l- 2 I -H r O H S3 00 a. -H -«r^ I I Us _. "5 _ 'I "S M) _ 'g aj^. 0^3 O-. "5 li t r- "5. = si 8 s = «; Is IS E - B.-Sr, C u U u S- >. «w «2! g ^ * 2 I j ^ j J j i FIGURE 8 The effects fverapamil and tetrdlxin (TTX) n mitral valvefibers.thefibersare nt being superfused with catechlamine. The left panels shw a cntrl mitral valve actin ptential (tp) and the actin ptential recrded frm the same fiber after superfusin with verapamil (0.5 mg I liter) fr 30 minutes (bttm). The right panels shw the cntrl actin ptential in anther preparatin and the effects f superfusin with TTX, 5.0 and 10.0 mg/ liter. was increased frm 30 t 50 mm and membrane ptential declined, the rate f this spntaneus activity increased prgressively; at 50 mm [K + ] the spntaneus cycle length was 820 ± 55 msec. The additin f catechlamine t the perfusate at this time substantially increased the rate but did nt induce after-deplarizatins. At a membrane ptential f -25 mv ([K + ] = 80 mm) actin ptentials n lnger were generated either spntaneusly r in respnse t electrical stimulatin but small scillatins in the membrane ptential did ccur. Discussin MIGHT THE SUSTAINED RHYTHMIC ACTIVITY BE REENTRANT? The triggered activity described in this study des nt appear t be caused by reentry. Triggered actin ptentials ccurred in the mitral valve leaflet even when it was detached frm the atrium; this indicates that a reentrant pathway invlving bth valve leaflet and atrium was nt respnsible. Furthermre, when the valve leaflet was divided int small pieces, many f the pieces exhibited triggered activity. It is pssible that reentry invlving nly a few fibers ccurred within each f the small pieces; hwever, the upstrkes f actin ptentials recrded frm different fibers within each piece were virtually simultaneus and thus did nt shw the pattern f cnductin expected frm circus mvement f excitatin.' The ccurrence f spntaneus actin ptentials in the

11 i ' TRIGGERED MITRAL VALVE ACTIVITY/Wit and Cranefteld 95 FIGURE 9 The effects f verapamil n the delayed after-deplarizatin f fibers f the islated mitral valve superfused with epinephrine, 0.5 Hg/ml. Reference 0 line is shwn fr first actin ptential in each panel. A, cycle length = 4,000 msec; delayed after-deplarizatins fllw each actin ptential but there is n triggered activity. B, cycle length = /,500 msec; the third driven actin ptential is fllwed by a nndriven respnse (arrw) and by sustained rhythmic activity. C, a different fiber f the same mitral valve, superfused with verapamil (0.2 mg/liter)fr 30 minutes, is driven at a cycle length f 1,500 msec. N delayed after-deplarizatin was evident in this fiber r in 15 ther fibers frm which recrdings were btained. Sustained rhythmic activity culd never be triggered in the presence fverapamil, which als reduced the upstrke velcity and amplitude f the driven actin ptential. 300 msec 50 mv mitral valve leaflet f canine hearts has previusly been reprted. 7 The phenmenn f triggering was nt nticed in this earlier study. THE ACTION POTENTIAL OF MITRAL VALVE FIBERS AS A SLOW RESPONSE The slw respnse is a prpagated actin ptential the upstrke f which results frm inward current thrugh the slw channel. 2 The slw respnse can ccur in atrial, ventricular, r Purkinje fibers in which a lw resting ptential has inactivated the fast inward current 10 " 12 and it may be the nrmal respnse f the sinus and atriventricular ndes.* 13 ' M The upstrke f the slw respnse is insensitive t TTX, 13 ' 15> '* but is depressed by verapamil, an agent that appears t blck the slw channel " 18 Catechlamines can increase the magnitude f the inward current in the slw channel and thereby increase the amplitude f the slw respnse withut changing the resting ptential. 12 ' " The actin ptential f mitral valve fibers appears at lw resting ptentials, is enhanced by catechlamines, and is depressed by verapamil; it therefre resembles the slw respnse. On the ther hand, the upstrke is depressed by TTX and may arise at a membrane ptential smewhat negative t the generally accepted threshld fr the slw inward current. Nr is the respnse f mitral valve fibers t elevatin f [K*] and lss f resting ptential that expected f either the pure fast respnse r the pure slw respnse. As resting ptential falls frm -70 mv t -60 mv the steepness f the upstrke is diminished but the versht des nt change. At resting ptentials between -60 mv and -55 mv the fibers cannt be excited by extracellular stimuli, but when resting ptential falls t -50 mv extracellular stimuli elicit actin ptentials with vershts. It is pssible therefre that the upstrke depends bth upn the slw channel and upn a partially inactivated, TTXsensitive fast channel. Deplarizatin t -55 mv wuld whlly inactivate the fast channel. The fact that fibers are mre easily excited frm a resting ptential f -50 mv merely indicates that a smaller stimulus-induced deplarizatin is adequate t bring such fibers t their threshld ptential. It is knwn that slw respnse activity cannt be elicited in fibers expsed t Na-free slutins if the resting ptential is t high, but can be elicited if resting ptential is reduced. 2 ' l6 20 ' Mitral valve fibers als becme spntaneusly active if markedly deplarized by increased [K + ]. We knw f n ther cardiac fiber in which spntaneus activity is caused by elevatin f [K + ]. Purkinje fibers that are quiescent when expsed t Na-free slutins shw repetitive activity when deplarized by an applied current, 20 as may frg atrial fibers 21 and guinea pig ventricular fibers," " but such fibers d nt becme spntaneusly active when expsed t high [K + ] ; elevatin f [K + ] causes nrmal Purkinje fibers t lse spntaneus activity even in the presence f catechlamines. 24 It is cnceivable that elevatin f [K + ] releases catechlamines frm adrenergic nerves in the mitral valve and thereby enhances the slw inward current, althugh it is nt clear why this wuld ccur nly at such high levels f [K + ]. THE MECHANISM OF TRIGGERED ACTIVITY Triggered activity differs frm spntaneus activity in that triggered activity is dependent n the prir ccurrence f an actin ptential. The delayed after-deplarizatin that fllws a driven actin ptential may r may nt reach the threshld ptential. If it des reach threshld (because f a decrease in stimulus cycle length r a premature stimulus) a nndriven actin ptential ccurs. If this nndriven actin ptential is fllwed by a delayed after-deplarizatin that reaches threshld, and if each subsequent actin ptential als is fllwed by a delayed after-deplarizatin that reaches threshld, sustained rhythmic activity ccurs. Spntaneus activity des nt ccur after catechlamines are added t a nnstimulated, quiescent mitral valve. Truly autmatic fibers presumably develp phase 4 deplarizatin and spntaneus activity withut prir activatin, althugh Vassalle and Carpentier" fund that electrical stimulatin can induce autmaticity in Purkinje fibers expsed t lw cncentratins f nrepinephrine.

12 1X0, 96 CIRCULATION RESEARCH VOL. 38, N. 2, FEBRUARY 1976 ADA (mv) > I I SO v-> *N H I -H -H m * fn O _» r> r-i <N I -H ' <N in v -H I I CN (N H -H \& v *< ^j" r-- \G The inic mechanism fr the delayed after-deplarizatin that causes triggered and sustained rhythmic activity is unknwn. The effects f verapamil, TTX, and catechlamines n the delayed after-deplarizatin suggest that it may be caused, at least in part, by inward current in the slw channels. The delayed after-deplarizatin may result frm a phasic increase in slw-channel inward current during diastle r it may arise because inactivatin f that inward current flwing during the deplarizatin phases f the actin ptential lags behind cmplete replarizatin. If the terminal phase f replarizatin is caused by a transient increase in K cnductance and the slw-channel inward current persists after K cnductance has returned t its nrmal value (r pssibly even belw the value characteristic f diastle) an afterptential shuld result. The absence f the after-deplarizatin in the presence f elevated [K + ] and catechlamines suggests that a lw K cnductance is imprtant fr its generatin. The ccurrence f the afterptential at levels f membrane ptential mre negative than the generally accepted threshld fr activatin f slw inward current may be explained by this mechanism r alternatively by a catechlamine-induced shift f threshld t mre negative values. In view f the prpsed mechanism it is als pssible that ablitin f the after-deplarizatin by verapamil might be due t effects f this agent n plateau currents ther than the slw-channel inward current. 26 The cause f the increase in amplitude f the after-deplarizatin f mitral valve fibers in respnse t an increase in the frequency f activatin is unknwn..a I APA 0. -H i\ -a CM OJ r^ N I I r- r- jn * I I LJ W ^J "^ u M O, OX) OO C E O 6 3 s = s -; > S S - V a. HI TRIGGERED ACTIVITY AS A CAUSE OF CARDIAC ARRHYTHMIAS Fibers that are anatmically similar t thse described in this article exist in the human heart, and the ccurrence f triggered activity in such fibers is at least pssible. 27 The additin f catechlamines in vitr may merely restre the fiber t the state in which it exists in situ; it is als pssible that such activity ccurs in situ nly in the presence f increased levels f catechlamines r f increased activity f adrenergic nerves that richly innervate the valve leaflet. 28 Scherf and Schtt" have lng maintained that nndriven actin ptentials arising frm after-deplarizatins might cause extrasystles r tachycardia. Fibers in the mitral valve that exhibited triggerable activity might accunt fr several types f atrial arrhythmias. Fr example, parxysmal atrial tachycardia is ften initiated by a premature atrial impulse, can be terminated by a premature impulse, and is generally thught t result frm circus mvement f excitatin in a pathway part f which passes thrugh the atriventricular nde. 80 Hwever, sme f these arrhythmias may be triggered, because triggered arrhythmias als can be evked by a single critically timed premature impulse and interrupted in the same way. In fact, it is widely assumed that spntaneus r evked premature impulses r rapid stimulatin may initiate r terminate nly reentrant rhythms. In view f ur data and the data f thers suggesting that sustained rhythmic activity which is nt reentrant can be induced in much the same manner, perhaps this criterin shuld be reevaluated.

13 X TRIGGERED MITRAL VALVE ACTTVITY/W» and Cranefield 97 ATRIAL WALL FIBERS MITRAL VALVE FIBERS 4 ^ g 125- E > D 0 O 0 RESTING MEMBRANE POTENTIAL (mv) FIGURE 10 Relatinship f versht and phase 0 V max (rdinate) f atrial wall fibers (left panels) and mitral valve fibers (right panels) t membrane ptential at which the upstrke is initiated (abscissa). Each symbl represents a different single fiber frm a different preparatin. Resting membrane ptential was reduced by increasing [K] in the superfusate. Fr the atrial wall fibers, versht and phase 0 V max decreased in each experiment as membrane ptential declined when [K] was increased. Fr the mitral valve fibers, the versht was nt affected when membrane ptential decreased, althugh phase 0 V max was reduced. The shaded clumn in the right panels dentes the membrane ptentials at which mst mitral valve fibers were transiently inexcitable. K-IOmM K-30rtiM K*IOmM K=30mM K=50mM K=l5mM 200 msec _J A. K-50mM 1000 msec 75 mv References 1. Cranefield PF, Arnsn RS: Initiatin f sustained rhythmic activity by single prpagated actin ptentials in canine cardiac Purkinje fibers expsed t sdium-free slutin r t uabain. Circ Res 34: , Cranefield PF: The Cnductin f the Cardiac Impulse: The Slw Respnse and Cardiac Arrhythmias. Mt. Kisc, Futura Publishing C., West TC: Effects f chrntrpic influences n subthreshld scillatins FIGURE 11 The effects f increasing [K + ] n mitral valve actin ptentials. At the tp, actin ptentials were recrded frm the same cell at K = 4, 10, and 30 mm cncentratins, and frm a different cell at K = 50 mm. As [K*] was increased, there was a decrease in resting membrane ptential frm 78 mv at [K + ] c = 4 mm t 50 mv at [K + ] = 30 mm, but actin ptential versht remained unchanged. Ala [K + ] = 50 mm resting membrane ptential f a different cell was 36 mv and an actin ptential with a slight versht culd still be elicited. The bttm three panels shw the ccurrence f nnstimulated actin ptentials after graded deplarizatin caused by increasing [K + ] t. At K = 10 mm stimulated actin ptentials ccurred but when [K + ] was increased t 15 mm n actin ptentials culd be elicited and there was n spntaneus activity (membrane ptential is still declining and has nt yet equilibrated in the trace which is shwn). At [K + ] = 30 mm nndriven actin ptentials ccur, and the spntaneus rate increases with further deplarizatin at [K + ] = 50 mm.

14 98 CIRCULATION RESEARCH VOL. 38, N. 2, FEBRUARY 1976 in the sin-atrial nde. In The Specialized Tissues f the Heart, edited by A Paes de Carvalh, WC de Mell, BF Hffman. New Yrk, Elsevier, Sperelakis N: Electrical prperties f embrynic heart cells. In Electrical Phenmena in the Heart, edited by WC dc Mell. New Yrk, Academic Press, 1972, pp Rsen MR Gelband H, Merker C, Hffman BF: Mechanisms f digitalis txicity; effects f uabain n phase fur f canine Purkinje fiber transmembrane ptentials. Circulatin 47: , Ferrier GR, Saunders JH, Mendez C: A cellular mechanism fr the generatin f ventricular arrhythmias by acetylstrphanthidin. Circ Res 32: , Wit AL, Fengli JJ Jr, Wagner BM, Bassett AL: Electrphysilgical prperties f cardiac muscle in the anterir mitral valve leaflet and the adjacent atrium in the dg; pssible implicatins fr the genesis f at rial dysrhythmias. Circ Res 32: , Paes de Carvalh A, Hffman BF, Paula de Carvalh M: Tw cmpnents f the cardiac actin ptential. I. Vltage time curse and the effect f acetylchline n atrial and ndal cells f the rabbit heart. J Gen Physil 54: , Wit AL, Cranefield PF, Hffman BF: Slw cnductin and reentry in the ventricular cnducting system. II. Single and sustained circus mvement in netwrks f canine and bvine Purkinje fibers. Circ Res 30: 11-22, Pappan, AJ: Calcium-dependent actin ptentials prduced by calechlamincs in guinea pig atrial muscle fibers deplarized by ptassium. Circ Res 27: , Mascher D: Electrical and mechanical respnses frm ventricular muscle fibers after inactivatin f the sdium carrying system. Pfluegers Arch 317: , Carmeliet E, Vereecke J: Adrenaline and the plateau phase f the cardiac actin ptential. Pfluegers Arch 313: , Zipes DP, Mendez C: Actin f manganese ins and tetrdtxin n atriventricular ndal transmembrane ptentials in islated rabbit hearts. Circ Res 32: , Wit AL, Cranefield PF: Effect f verapamil n the sinatrial and atriventricular ndes f the rabbit and the mechanism by which it arrests reentrant atriventricular ndal tachycardia. Circ Res 35: , Arnsn RS, Cranefield PF: The electrical activity f canine cardiac Purkinje fibers in sdium-free, calcium-rich slutins. J Gen Physil 61: , Rugier O, Vassrt G, Gamier D, el al.: Existence and rle f a slw inward current during the frg atrial actin ptential. Pfluegers Arch 308:91-110, Cranefield PF, Arnsn RS, Wit AL: Effect f verapamil n the nrmal actin ptential and n a calcium-dependent slw respnse f canine cardiac Purkinje fiber. Circ Res 34: , Zipes DP, Fischer JC: Effects f agents which inhibit the slw channel n sinus nde autmaticity and atriventricular cnductin in the dg. Circ Res 34: , Reuter H: The dependence f slw inward current in Purkinje fibers n the extracellular calcium cncentratin. J Physil (Lnd) 192: , Arnsn RS, Cranefield PF: The effect f resting ptential n the electrical activity f canine cardiac Purkinje fibers expsed t Na-free slutin r t uabain. Pfluegers Arch 347: , Brwn HF, Nble SJ: Membrane currents underlying delayed rectificatin and pace-maker activity in frg atrial muscle. J Physil (Lnd) 204: , Katzung BG: Effects f extracellular calcium and sdium n deplarizatin-induced autmaticity in guinea pig papillary muscle. Circ Res 37: , Surawicz B, Imanishi S: Mechanism f autmaticily in deplarized guinea pig ventricular mycardium (abstr). Circulatin 50 (suppl 2): 84, Hffman BF, Cranefield PF: Electrphysilgy f the Heart. New Yrk, McGraw-Hill, Vassalle M, Carpentier R: Overdrive excitatin: nset f activity fllwing fast drive in cardiac Purkinje fibers expsed t nrepinephrine. Pfluegers Arch 332: , Kass RS, Tsien RW: Multiple effects f calcium antagnists n plateau currents in cardiac Purkinje fibers. J Gen Physil 66: , Mntiel MM: Muscular apparatus f the mitral valve in man and its invlvement in left sided cardiac hypertrphy. Am J Cardil 26: , Ehinger B, Falck B, Stenevi U: Adrenergic and nnadrenergic valvular nerves f the heart. Experientia 25: , Scherf D. Schtt A: Extrasystles and Allied Arrhythmias. Lndn, Heinemann, Bigger JT Jr, Gldreyer BN: Mechanism f superaventricular tachycardia. Circulatin 42: , 1970