Effect of Serotonin and Thromboxane A 2 on Blood Flow Through Moderately Well Developed Coronary Collateral Vessels

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1 lacc Vl. 19, N EXPERIMENTAL STUDIES Effect f Sertnin and Thrmbxane A 2 n Bld Flw Thrugh Mderately Well Develped Crnary Cllateral Vessels LAUREL WRIGHT, MD, DAVID C. HOMANS, MD, FACC, DAVID D. LAXSON, MD, XUE-ZHENG DAI, MD, ROBERT J. BACHE, MD, FACC This study was perfrmed t determine whether thrmbxane A 2 (as the analgue U46619) and sertnin can cause vascnstrictin f mderately well develped crnary cllateral vessels. Studies were carried ut in seven adult mngrel dgs 2 t 4 mnths after emblic cclusin f the left anterir descending crnary artery had been perfrmed t stimulate cllateral vessel grwth. At the time f study this artery was cannulated t determine interarterial cllateral flw frm measurements f retrgrade bld flw. Radiactive micrspheres were administered during retrgrade flw cllectin t determine cntinuing tissue flw fr evaluatin f micrvascular cllateral cmmunicatins. Sertnin (50 Jtg!min) resulted in a 48 ± 11 % decrease in retrgrade flw (p < 0.01), with a 36 ± 10% decrease in ttal cllateral bld flw (p < 0.02). Infusin f U46619 (0.01 pg/kg per min) caused a 38 ± 13% decrease in retrgrade bld flw (p < 0.01), with a 34 ± 13% decrease in ttal cllateral flw (p < 0.05). Sertnin caused a significant increase in tissue flw t the subepicardium f the cllateral-dependent regin, whereas U46619 caused n change in tissue bld flw. These data demnstrate that bth sertnin and thrmbxane A 2 can cause vascnstrictin f interarterial crnary cllateral vessels. The findings suggest that platelet activatin in crnary arteries frm which cllateral vessels riginate has ptential fr causing cllateral vascnstrictin, thereby cmprmising bld flw t the dependent mycardium. (J Am Cll CardiI1992;19:687-93) The transfrmatin f native intercrnary micrvascular anastmtic channels int mature cllateral vessels in respnse t crnary artery cclusin has been well dcumented (1). These mature cllateral vessels have structural characteristics that are similar t small arteries and are capable f vasmtr activity (1-3). Hwever, the respnses f cllateral vessels t vasactive stimuli may differ frm the respnses f ther segments f the crnary artery vasculature. Fr example, althugh bth epicardial arteries and cllateral vessels underg vasdilatin in respnse t nitrglycerin and atrial natriuretic peptide (3-5), epicardial arteries but nt cllateral vessels cnstrict in respnse t alphaadrenergic agnists (6-8), and epicardial arteries dilate, whereas cllateral vessels cnstrict in respnse t vaspressin (7-9). Thus, knwledge f the actin f vasactive substances n large crnary arteries r n crnary resistance vessels cannt be generalized t the crnary cllateral circulatin. The platelet prducts sertnin and thrmbxane A 2 cause cnstrictin f epicardial crnary arteries, but the effects f these agents n crnary cllateral bld flw have nt been reprted (10-12). Frm the Divisin f Cardilgy, Department f Medicine, University f Minnesta Medical Schl, Minneaplis, Minnesta. This wrk was supprted in part by U.S. Public Health Service Grants HL20598 and HL34701 frm the Natinal Heart, Lung, and Bld Institute, Bethesda, Maryland. U46619 was supplied by the Upjhn Cmpany. Kalamaz, Michigan. Manuscript received May 21, 1991; revised manuscript received August 28, 1991, accepted September 12, Address fr reprints: Rbert J. Bache, MD, University f Minnesta, Bx 338 UMHC, 420 Delaware Street S.E., Minneaplis. Minnesta by the American Cllege f CardIOlgy Cnsequently, the present study was perfrmed t determine whether thrmbxane A 2 (as the analgue U46619) and sertnin cause vascnstrictin f mderately well develped crnary cllateral vessels. Studies were carried ut in dgs in which cllateral vessel grwth had been stimulated by placement f a gradually ccluding intravascular plug int a crnary artery 2 t 4 mnths befre study. Retrgrade flw frm the cannulated cllateralized artery was used t assess interarterial cllateral vasculature; cntinuing tissue flw was measured with micrspheres t assess micrvascular cllateral cmmunicatins (13). Methds Inductin f cllateral vessels. These studies were carried ut in accrdance with the "Psitin f the American Heart Assciatin n Research Animal Use" adpted Nvember 11, 1984 and under the supervisin f the Animal Care Cmmittee f the University f Minnesta. Crnary cllateral vessel grwth was stimulated by placement f an intravascular plug int the left anterir descending crnary artery. Seven adult mngrel dgs weighing 20 t 30 kg were premedicated with atenll (50 mg rally), aspirin (325 mg) and mrphine sulfate (1 mg/kg bdy weight subcutaneusly) 1h befre inductin f anesthesia with sdium thiamylal (20 t 30 mg/kg intravenusly). The dgs were intubated and ventilated with a respiratr using rm air supplemented with xygen. A right cartid arteritmy was perfrmed under sterile surgical cnditins. After administratin f /92/$5.00

2 688 WRIGHT ET AL. COLLATERAL EFFECTS OF SEROTONIN AND THROMBOXANE Az lacc Vl. 19, N.3 March I, I99Z: heparin sdium (200 U/kg intravenusly) and lidcaine (2 mg/kg intravenusly), a 7F Judkins JR4 crnary catheter was intrduced int the right cartid artery and advanced t the stium f the left main crnary artery under flurscpic guidance. A in. (0.036 cm) angiplasty guide wire was inserted int the crnary catheter, guided int the anterir descending crnary artery and advanced t the cardiac apex. The catheter was withdrawn and a hllw stainless steel plug (3 mm length x 2.3 t 3 mm uter diameter; 1.1 mm inner diameter) was advanced ver the guide wire until it was firmly wedged in the mid-prtin f the anterir descending artery. The guide wire was then remved and radipaque cntrast material was administered t cnfirm the psitin f the plug. The arteritmy was repaired and the animal allwed t recver. Surgical preparatin. Studies were perfrmed 2t 4mnths after placement f the intravascular plug. The dgs were premedicated with mrphine sulfate (1.2 mg/kg subcutaneusly), anesthetized with alpha-chlralse (100 mg/kg intravenusly), intubated and ventilated with rm air supplemented with xygen. A 7F Natinal Institutes f Health catheter was intrduced int the right femral artery and advanced int the left ventricle fr pressure measurement. A similar catheter was intrduced int the left femral artery and advanced int the left ventricle fr pressure measurement. A similar catheter was intrduced int the left femral artery and advanced int the ascending arta. A thractmy was perfrmed in the left furth intercstal space. A pneumatic cuff ccluder was placed arund the descending thracic arta fr cntrl f artic pressure. The heart was suspended in a pericardial cradle and a PVC catheter (3 mm uter diameter) was inserted int the left atrium thrugh the atrial appendage. The intravascular plug was lcated in the crnary artery by palpatin, and the artery was mbilized in this regin. A PESO catheter was intrduced prximal t the plug in a retrgrade directin and psitined with the catheter tip in the left main crnary artery fr drug infusin. Heparin sdium (200 U/kg intravenusly fllwed by 1,000 U/h) was administered, and the crnary artery plug was remved thrugh an arteritmy, allwing free retrgrade flw f bld t displace any residual thrmbus. The artery was then cannulated with a thin wall stainless steel cannula (4 mm uter diameter) in which 23-gauge tubing had been incrprated t mnitr pressure at the cannula tip. Mycardial bld flw measurement. Reginal mycardial bld flw was measured using IS-pm diameter micrspheres labeled with idine-125, nibium-95, strntium-85, chrmium-51, scandium-46 r cerium-141 (3M Cmpany and New England Nuclear). Micrspheres were btained as 1 mci in 10 ml f 10% lw mlecular weight dextran and agitated in an ultrasnic bath fr at least 15 min befre injectin. Fr each interventin, 3 x 10 6 micrspheres were injected int the left atrium; a reference sample f arterial bld was withdrawn frm the artic catheter at a cnstant rate f 15 mumin with a peristaltic pump (Harvard mdel 551). Reference bld sampling was begun at the time f micrsphere injectin and cntinued fr 90 s. Experimental prtcl. Left ventricular, artic and crnary cannula pressures were mnitred with use f Spectramed TNF-R pressure transducers. Left ventricular pressure was displayed at nrmal and high gain fr measurement f end-diastlic pressure. Data were recrded n a Hewlett Packard 8800 eight-channel direct-writing scillgraph. Interarterial cllateral bld flw measurements were perfrmed by cllecting retrgrade flw frm the crnary cannula int a graduated cylinder ver 30 s while the cannula tip was maintained at the level f the heart. Cntrl retrgrade flw cllectins were repeated until cnsistent measurements were btained. Retrgrade pressure measurements were mnitred cntinuusly. An initial injectin f micrsphere injectin was then perfrmed with the cannula pen t air t assess the degree f cntinuing tissue flw while the retrgrade flw cllectin was perfrmed. Sertnin. After cntrl measurements were cmpleted, the respnse t sertnin was examined. A lw dse f sertnin (10 JLg/min) was infused at a cnstant rate (0.49 ml/min) int the left main crnary artery catheter fr 10 min with a syringe pump (Harvard Instruments mdel 701). Retrgrade bld flw measurements were btained at 2, 4 and 8 min f drug infusin. The sertnin dse was then increased t 50 JLg/min by increasing the cncentratin f sertnin with n change in infusin rate. Retrgrade flw measurements were again btained at 2, 4 and 8 min f drug infusin. A third injectin f micrspheres was perfrmed simultaneusly with the 8-min retrgrade flw cllectin. The sertnin infusin was then discntinued and the dg allwed t recver fr 30 min. Thrmbxane A 2 Hemdynamic measurements and retrgrade flw cllectins were repeated t cnfirm return t baseline levels. The effect f thrmbxane Az was then assessed by infusin f U46619 int the left main crnary artery catheter at a dse f }lg/kg per min and an infusin rate f 0.49 mumin. Retrgrade flws were cllected at 2, 4 and 8 min. The dse was then increased t 0.01 J,tg/kg per min; retrgrade flw measurements were repeated at 2, 4 and 8 min, and a micrsphere injectin was perfrmed simultaneusly with the 8-min cllectin. Identificatin f cllateral-dependent mycardium. After cmpletin f study, the shadw technique f Pattersn and Kirk (14) was used t delineate the perfusin bed f the cannulated left anterir descending crnary artery. The crnary cannula was perfused with nnradiactive arterial bld previusly cllected frm the dg in a pressurized reservir. Cannula tip pressure was maintained 5 t 10 mm Hg abve mean artic pressure during the micrsphere injectin. In this way, the area f left ventricle perfused by the anterir descending artery distal t the site f cclusin was marked with nnradiactive bld t distinguish it frm the remainder f the heart, which was perfused frm the arta with micrsphere-cntaining bld. Tissue preparatin. At the cnclusin f the study, the animal was killed and the heart excised and fixed in 10% buffered frmalin. The left ventricle was sectined int five

3 JACC Vl. 19, N.3 WRIGHT ET AL. 689 COLLATERAL EFFECTS OF SEROTONIN AND THROMBOXANE A 2 transverse rings frm base t apex, In fur dgs a thin rim f subendcardial scar was fund in the cllateral-dependent regin. This healed infarct, which accunted fr <5% f the weight f the cllateral regin, was remved befre the tissue was prepared fr cunting. Each ring was sectined radially int 16 segments, which were then subdivided int epicardial and endcardial valves, weighed n an analytical balance and placed int vials fr cunting. Mycardial and bld reference specimens were cunted in a Packard mdel 5912 gamma cunter at windw settings crrespnding t the peak energies f each radinuclide. The activity in each energy windw was crrected fr backgrund and fr verlapping cunts between istpes with a digital cmputer. Bld flw t each mycardial specimen (Qm) was cmputed with the frmula: Qm == Qr x Cm/Cr, where Qr == reference bld flw rate (ml/min), Cm == cunts/min f the mycardial specimen and Cr == cunts/min f the reference bld specimen. Bld flws were expressed as ml/min per g f mycardium. T btain ttal mycardial tissue flw t the cllateral-dependent mycardium, abslute bld flws were summed fr all specimens identified by the shadw technique t represent mycardium perfused by the left anterir descending crnary artery. Duplicate specimens frm the psterir wall were used t assess bld flw frm nrmally perfused mycardium. Data analysis. Hemdynamic data were measured directly frm the strip chart recrdings. Hemdynamic variables and retrgrade bld flw during cntrl cnditins and with each interventin were cmpared by using analysis f variance fr repeated measures. A value f p < 0.05 was required fr statistical significance. When a significant difference was fund, individual cmparisns were made with the Scheffe methd. Tissue flw measurements made with micrspheres were cmpared with use f the Wilcxn signed-rank test. All data are expressed as mean values ± SEM. Results Hemdynamic data. Hemdynamic measurements btained during cntrl cnditins, and during infusin f tw dses f sertnin and tw dses f U46619 are shwn in Table 1. Heart rate and left ventricular end-diastlic pressures were nt significantly different between the cntrl perid and either drug interventin. Mean artic pressure was slightly but significantly increased by bth dses f U46619 and left ventricular systlic pressure was increased during high dse U A trend tward decreased mean artic pressure with sertnin (50 JLg/min) did nt achieve statistical significance. Retrgrade bld flw. Retrgrade flw cllectins frm the left anterir descending crnary artery cannula are shwn in Table 2. During cntrl cnditins befre infusin f sertnin, mean retrgrade bld flw was 36.4 ± 12 ml/min (range 10 t 101 ml/min). Sertnin in a dse f 10 JLg/min caused a 27 ± 7% decrease in retrgrade bld Table 1. Hemdynamic Data Frm Seven Dgs During Cntrl Cnditins and During Infusins f Sertnin and U46619 Pressures (mm Hg) Heart Rate Mean LV LV End- (beats/min) Artic Systlic Diastlic Cntrl sertnin 130 ± 5 98 ± ± 8 9 ± 2 10 ilg/min 126 ± 7 94 ± ± 4 9 ± 2 50 ilg/min 126 ± 6 89 ± ± 5 6 ± 1 Cntrl U ± 7 98 ± ± 5 8 ± ilg/kg per min 125 ± ± 4' 124 ± 7 8 ± ilg/kg per min 122 ± ± 5' 130 ± 8' 9 ± 1 'p< 0.05 versus cntrl value. Values are mean values ± SEM. LV = left ventricular. flw (p < 0.05). This decrease was similar at 2, 4 and 8 min f sertnin infusin. Increasing the sertnin infusin rate t 50 JLg/min significantly further decreased the retrgrade flw t 48 ± 11% belw the cntrl value (range - 5 t -80%; p < 0.01). The decrease in retrgrade flw was similar at 2, 4 and 8 min f infusin (Fig. 1). During baseline cnditins befre administratin f U46619 mean retrgrade bld flw was 32.2 ± 12.1 ml/min (range 10 t 88) (Table 2). There was n significant change in retrgrade bld flw during infusin f U46619 at a dse f JLg/kg per min. Hwever, retrgrade flw decreased significantly during infusin f U46619 at a dse f 0.01 JLg/kg per min, with a decrease f 38 ± 13% after 8 min f infusin f U46619 (range 0 t -80%; p < 0.05). The decrease in retrgrade flw prduced by U46619 was nt seen until 4 min after the start f infusin, and then remained stable between 4 and 8 min f drug infusin (Fig. 2). Cllateral zne. The shadw technique demnstrated sharp bundaries between cllateral-dependent and nrmal znes. Ttal mass f the cllateral-dependent regin ranged frm 2.42 t 29.6 g (mean 16 ± 4.3). The size f the cllateral-dependent regin crrespnded t the distance alng the crnary artery at which the intravascular plug was psitined. Ttal left ventricular mass was ± 5.6 g; cllateral-dependent tissue represented 15.3 ± 4.3% f the left ventricle. Tissue bld ftw. Mycardial tissue flw measured with micrspheres is shwn in Table 3. During cntrl cnditins with the crnary cannula clsed, there was n significant difference in bld flw t nrmally perfused and cllateraldependent mycardium. On pening the crnary cannula, mean mycardial tissue flw decreased t 55 ± 16% f nrmal zne flw (p < 0.01). The respnses f tissue bld flw during administratin f sertnin and U46619 were measured with the crnary cannula pen, indicating micrvascular flw nt diverted during the retrgrade flw cllectin. In the nrmal zne sertnin caused a 123 ± 19% increase in mean tissue flw with significant increases in flw t bth subendcardial and subepicardial muscle (p < 0.01). Sertnin infusin increased bld flw t the subepicardium f the cllateral-dependent regin (p < 0.05), but did

4 690 WRIGHT ET AL. COLLATERAL EFFECTS OF SEROTONIN AND THROMBOXANE A 2 JACC Vl. 19, N.3 Table 2. Retrgrade Bld Flw, Flw t the Cllateral-Dependent Mycardium Measured With Micrspheres, Ttal Cllateral Flw and Cllateral Vascular Resistance During Cntrl Cnditins and During Infusin f Sertnin and U46619 in Seven Dgs A. Retrgrade Bld Flw (mvmin) Sertnin study Cntrl 36.4 ± 12 Sertnin 24.6 ± 12.9* (50 ~min) U46619 study Cntrl 32.2 ± 12.1 U ± 13.4* (0.01 ~kg per min) B. Cllateral-Dependent Tissue Bld Flw Ttal Cllateral Bld Flw (A + B) (mvmin) ml/min ml/min per 100 g 11.9 ± ± ± ± ± ± 16.2* 45.8 ± ± 17.3* Values are mean values ± SEM. *p < 0.05 in cmparisn with cntrl cnditins. 413 ± ± 91* 339 ± ± 101* Mean Arterial Pressure (mm Hg) 98 ± 7 89 ± 6 98 ± ± 5* Cllateral Vascular Resistance (mm Hg. min. 100 glml) 24.2 ± ± 12* 28.9 ± ± 15.9* nt significantly alter bld flw t the subendcardium. U46619 did nt cause a significant change in nrmal zne flw. Althugh cllateral zne tissue flw tended t decrease during infusin f U46619, the reductin did nt achieve statistical significance. Ttal cllateral ftw. Ttal cllateral bld flw (the sum f retrgrade flw plus tissue flw in the cllateraldependent regin) and cllateral vascular resistance, expressed per g f cllateral-dependent mycardium, are shwn in Table 2. Infusin f sertnin (50 JLg/min) caused a 36 ± 10% decrease in ttal cllateral bld flw (p < 0.02), with a 54 ± 22% increase in cllateral vascular resistance (p < 0.05). Infusin f U46619 (0.01 JLg/min) caused a 34 ± 13% decrease in ttal cllateral bld flw (p < 0.05), with a prprtinate increase in cllateral vascular resistance (p < 0.01). Discussin Previus studies have demnstrated that thrmbxane Az and sertnin cause cnstrictin f large crnary arteries (10-12). The present study demnstrates that these plateletderived mediatrs als cause cnstrictin f well develped crnary cllateral vessels. The methdlgy used t measure cllateral bld flw and the ptential significance f these experimental findings will be discussed in detail. Methdlgic cnsideratins. Cllateral bld flw was determined as the sum f retrgrade flw frm the cannulated crnary artery and cntinuing tissue flw in the cllateral-dependent mycardium. This prcedure was fllwed because Dwney et al. (13) have dcumented the develpment f tw separate cmpnents f cllateral vasculature in respnse t crnary artery cclusin. Bld diverted int the crnary artery cannula is derived frm interarterial cllateral vessels that are principally epicardial in lcatin (13). This interarterial cmpnent represented apprximately 75% f ttal cllateral flw and was respnsible fr mst f the change in cllateral flw in respnse t the vasactive agents used in ur study. Cntinuing tissue flw measured with micrspheres during retrgrade flw diversin represents bld delivered int the recipient vascular system thrugh micrvascular cmmunicatins that Figure 1. Percent change in retrgrade bld flw frm the left anterir descending crnary artery cannula during intracrnary infusin f sertnin, 50 /Lg/min. *p < 0.05 in cmparisn with the cntrl value befre the infusin. ~ 25y , l:l lu 1:1 e c> e ii a::.!: lu c> g -25.r; l' I Figure 2. Percent change in retrgrade bld flw frm the left anterir descending crnary artery cannula during intracrnary infusin f U46619, 0.01!-LWkg per min. *p < 0.05 in cmparisn with the cntrl value befre the infusin. ~ 0 l:l lu 'tl e c> 2 ii a::.!: lu c> C.r; 0" ~ lu Minutes After Beginning Infusin f Sertnin... C lu 25r , ,_ l I Minutes After Beginning Infusin f U46619

5 JACC Vl. 19, N.3 March I, I99Z: WRIGHT ET AL. 691 COLLATERAL EFFECTS OF SEROTONIN AND THROMBOXANE A 2 Table 3. Mycardial Tissue Flw Measured With Micrspheres in Seven Dgs During Cntrl Cnditins and During Infusin f Sertnin Bld Flw (ml/min per g) Subendcardial Subepicardial Mean Mycardial Nrmal Cllateral Nrmal Cllateral Nrmal Cllateral Cntrl Cannula clsed 1.26 :!: :!: :!: :!: :!: :!: 0.21 Cannula pen 1.08 :!: :!: :!: :!: :!: :!: 1.48 Sertnin (50 p.g/min) 2.1 :!: 0.36* 0.62 :!: :!: 0.42* 1.10 :!: 0.22* 2.33 :!: 0.38* 0.89 :!: 0.16 U46619 (0.01 /Lg/kg per min) 1.23 :!: :!: :!: :!: :!: :!: 1.96 *p < 0.05 in cmparisn with the respective measurement during cntrl cnditins with the cannula pen. Cllateral = cllateral-dependent mycardium; Nrmal = nrmally perfused. are intramural in lcatin (13). Bth retrgrade flw and tissue flw int the ccluded vascular bed were measured in the present study, thereby accunting fr ttal cllateral bld flw. Vasactive agents were infused int the left main crnary artery. They were therefre delivered t cllateral vessels arising frm the left circumflex, septal and anterir descending artery prximal t the site f bstructin, but nt t cllateral vessels riginating frm the right crnary artery. Failure f vasactive agents t reach cllateral vessels frm the right crnary artery wuld cause underestimatin f the effect f these agents. Hwever, Scheel et al. (5) demnstrated that in the canine heart nly apprximately 10% f cllateral flw int the anterir descending artery arises frm the right crnary artery. This relatively small cntributin t cllateral bld flw frm the right crnary artery shuld have caused nly slight underestimatin f the effects f the agents tested n cllateral flw. The present study was perfrmed in anesthetized dgs with surgically placed instruments. General anesthesia and surgical preparatin cause neurendcrine activatin that increases the basal level f vascnstrictin in the crnary system (6). The presence f increased crnary vascnstrictr tne wuld impair the ability fr further cnstrictin in respnse t the agents tested. Fr this reasn, the respnses bserved in the present study may underestimate the respnses that wuld ccur in an intact awake animal. Sertnin. Althugh sertnin des nt nrmally circulate in sufficient cncentratins t cause crnary vasmtr activity, its release by aggregating platelets may result in lcal bld cncentratins that reach the micrmlar range (7). In the nrmal crnary artery circulatin sertnin exerts differing effects depending n vessel size (8). Measurements btained with quantitative angigraphy r with ultrasnic micrcrystals have demnstrated 00-12) that the large epicardial crnary arteries underg vascnstrictin in respnse t sertnin. Similarly, studies (9) f crnary artery ring preparatins have shwn vascnstrictin in respnse t sertnin. In cntrast t the epicardial crnary artery vascnstrictin, intraarterial administratin f sertnin has resulted in increased crnary bld flw, indicating vasdilatin f the crnary resistance vessels 00,12,18). In agreement with this finding, in the present study sertnin caused a 226% increase in mean bld flw t the nrmally perfused area. N previus data are available n the effects f sertnin n cllateral bld flw in viv. Akita et al. (20) examined the effect f sertnin n islated helical strips f epicardial crnary cllateral vessels and nrmal epicardial arteries frm dgs 2 t 4 mnths after placement f an amerid cnstrictr n the left circumflex crnary artery. Sertnin prduced cntractin f bth epicardial artery strips and cllateral vessel strips with a threshld dse f M. The present study extends these previus in vitr findings by demnstrating that sertnin als caused a decrease in retrgrade bld flw in the intact animal. This finding is cnsnant with the cncept that sertnin causes vascnstrictin f epicardial crnary cllateral vessels. Hwever, sertnin had an ppsite effect n micrvascular cllateral bld flw, causing an increase in subepicardial tissue flw in the cllateralized regin. This finding may be interpreted in light f the reprt f Lamping et al. (8) that in nrmal hearts sertnin caused cnstrictin f crnary artery vessels >90 /Lm in diameter, but dilatin f arteriles <90 /Lm in diameter. Epicardial interarterial cllateral vessels that develp in respnse t chrnic crnary cclusin are generally larger than 200 /Lm in diameter, whereas intramural interarterilar cmmunicatins are generally smaller than 80 /Lm in diameter 03,21). The present findings indicate that in the crnary cllateral circulatin, as amng ther crnary artery vessels, larger vessels underg vascnstrictin in respnse t sertnin, whereas the smaller interarterilar cmmunicatins underg vasdilatin. Thrmbxane A 2 Thrmbxane Az is a prduct f prstaglandin endperxide metablism liberated during platelet aggregatin (22). Because thrmbxane Az has a half-life f nly 30 s, the stable thrmbxane Az mimetic U46619 was used in the present study (23,24). Thrmbxane Az r its analgues have been shwn t prduce vascnstrictin f bth epicardial crnary arteries and crnary resistance

6 692 WRIGHT ET AL. COLLATERAL EFFECTS OF SEROTONIN AND THROMBOXANE Az JACC Vl. 19, N.3 vessels (22,24). The present data prvide the first evidence that thrmbxane Az als cnstricts crnary cllateral vessels. U46619 decreased retrgrade flw but did nt decrease cntinuing tissue bld flw t the cllateraldependent regin, indicating that thrmbxane Az causes cnstrictin f the interarterial cllateral vessels, which are principally epicardial in lcatin, but des nt cause cnstrictin f the small intramural cllateral vessels. U46619 caused a significant decrease in retrgrade bld flw at a dse that did nt alter tissue flw in either the nrmally perfused r cllateral-dependent regin, suggesting that the interarterial cllateral vessels are mre sensitive t the vascnstrictr effects f thrmbxane than are the crnary resistance vessels. Mehta et al. (25) reprted that U46619 administered intravenusly t nrmal dgs, in larger dgs than thse used in the present study, caused vascnstrictin f the crnary resistance vessels which was fllwed by vasdilatin. The delayed vasdilatin phase was mediated by endgenus prstacyclin (PGIz) and culd be blcked with cyclxygenase inhibitrs. In cntrast, U46619 did nt cause vasdilatin in either the nrmally perfused r the cllateral dependent mycardial regins in ur study. This difference may be related t the larger dses r the intravenus rute f administratin used by Mehta et al. (25), r t the use f blus injectins rather than a cntinuus infusin. In a recent study, Rucc et al. (26) fund that intraarterial infusin f U46619 in swine prduced vascnstrictin f crnary resistance vessels that decreased bld flw even in the presence f an arterial stensis. Bld cncentratins. It is imprtant t cnsider whether the dses f vasactive agents resulted in bld cncentratins that culd be achieved in patients with crnary artery disease. Because tissue flws t nrmally perfused and cllateral-dependent mycardial regins and retrgrade flw were directly measured, it is pssible t calculate ttal bld flw thrugh the left main crnary artery int which the drug was diluted. On the basis f these measurements infusin f sertnin at a dse f 10 ILg/min resulted in a cmputed crnary bld cncentratin f apprximately 6 x 10-6 M. In a previus study (27) f crnary artery thrmbus frmatin in a canine mdel, sertnin cncentratins were reprted t reach levels f 2 x 10-6 M, whereas in vitr studies (17) have shwn that sertnin released by aggregating platelets in numbers fund in circulating bld can result in lcal cncentratins as high as 6 x 10-4 M. Thus, cncentratins f sertnin similar t thse used in the present study culd be achieved during platelet aggregatin in viv. The calculated crnary artery bld cncentratin during infusin f U46619 at a dse f O.OllLg/kg per min was 6 x 10-9 M. Althugh direct measurements f thrmbxane Az cncentratins in crnary artery bld in viv are nt available, the cncentratin that caused cllateral vascnstrictin in the present study is less than that which prduces maximal crnary artery vascnstrictin in vitr (24). Clinical implicatins. Unstable mycardial ischemic syndrmes are frequently assciated with evidence fr intravascular platelet activatin (28-30). In additin t causing wrsening f stensis severity by increasing the degree f mechanical bstructin at the site f an athersclertic lesin, activated platelets liberate vasactive substances, including sertnin and thrmbxane Az, which may cause vascnstrictin f epicardial arteries and crnary resistance vessels (31). In patients with multivessel crnary artery disease, cllateral vessels may riginate distal t an athersclertic lesin. Platelet activatin ccurring at the site f such a lesin wuld result in liberatin f sertnin and thrmbxane Az, which culd cause cnstrictin nt nly f the epicardial artery segment and resistance vessels in the dependent vasculature, but als f cllateral vessels riginating distal t the stensis. Vascnstrictin f cllateral vessels in respnse t sertnin r thrmbxane Az culd further jepardize perfusin f the cllateraldependent mycardium. In autpsy studies f patients wh died with an acute mycardial ischemic syndrme, Blumgart et al. (32) described the ccurrence f "infarctin at a distance." In these patients the area f infarct was nt the regin directly perfused by a stentic crnary artery, but rather a distant mycardial regin that had been dependent n the stensed artery fr perfusin thrugh cllateral vessels. Althugh wrsening stensis severity resulting frm thrmbus frmatin at the site f a crnary atherma culd have jepardized perfusin f the cllateral-dependent mycardium, cllateral vessel cnstrictin prduced by prducts f platelet activatin culd have als cntributed t underperfusin f the dependent mycardium. We acknwledge the expert technical assistance prvided by Tdd Pavik, Melanie Cramptn and Paul Lindstrm. References 1. Schaper W. The Cllateral Circulatin f the Heart. Amsterdam: Nrth Hlland, 1971: Grregg DE. The natural histry f crnary cllateral develpment. Circ Res 1974:35: Fam WM, McGregr M. Effect f vasdilatr drugs n retrgrade flw in areas f chrnic mycardial ischemia. Circ Res 1964;15: Winbury MM. Hwe BB, Hefner MA. Effect f nitrates and ther crnary dilatrs n large and small crnary vessels: an hypthesis fr the mechanism f actin f nitrates. J Pharmacl Exp Ther 1969;168: Freman B, Dai X, Hman DC, Laxsn DD, Bache RJ. Effect f atrial natriuretic peptide n crnary cllateral bld flw. Circ Res 1989;65: Heusch G, Deussen A, Schipke J, Thamer V. Alpha\- and alphazadrenceptr-mediated vascnstrictin f large and small canine crnary arteries in viv. J Cardivasc PharmacI 1984;6:% Harrisn DG, Chilian WM, Marcus ML. Absence f functining alphaadrenergic receptrs in mature canine crnary cllaterals. Circ Res 1986;59: Hautamaa PV. Dai X, Hmans DC. Bache RJ. Vasmtr activity f the mderately well develped canine crnary cllateral circulatin. Am J PhysiI1989;256:H890-7.

7 JACC Vl. 19, N.3 March I. 1992: WRIGHT ET AL. COLLATERAL EFFECTS OF SEROTONIN AND THROMBOXANE A Katusic ZS, Shepherd JT, Vanhutte PM. Vaspressin causes endthelium dependent relaxatins f the canine basilar artery. Circ Res 1984;55: Bve AA, Dewey JD. Effects f sertnin and histamine n prximal and distal crnary vasculature in dgs: cmparisn with alpha-adrenergic stimulatin. Am J CardiI1983;52: II. Glin P, Ashtn JH, Buja M, et al. Lcal platelet activatin causes vascnstrictin f large epicardial canine crnary arteries in viv: thrmbxane A2and sertnin are pssible mediatrs. Circulatin 1989; 79: Chu A, Cbb FR. Vasactive effects f sertnin n prximal crnary arteries in awake dgs. Circ Res 1987;6I(supplll): Dwney HF, Crystal GJ, Bashur FA. Functinal significance f micrvascular cllateral anastmses after chrnic crnary artery cclusin. Micrvasc Res 1981;21: Pattersn RE, Kirk ES. Apparent imprvement in canine cllateral mycardial bld flw during vasdilatin depends n criteria used t identify ischemic mycardium. Circ Res 1980;47: Scheel KW, Wilsn JL, Ingram LA, McGehee L. The septal artery and its cllaterals in dgs with and withut circumflex cclusin. Am J Physil 1980;238:H Vatner SF, Braunwald E. Cardivascular cntrl mechanisms in the cnscius state. N Engl J Med 1975;293: Chen RA, Shepherd JT, Vanhutte PM. Inhibitry rle f the endthelium in the respnse f islated crnary arteries t platelets. Science 1983;221: Lamping KG, Kanatsuka H, Eastham CL, Chilian WM, Marcus ML. Nnunifrm vasmtr respnses f the crnary micrcirculatin t sertnin and vaspressin. Circ Res 1989;65: Chen RA. Cntractins f islated canine crnary arteries resistant t S2-sertnergic blckade. J Pharmacl Exp Ther 1986;237: Akita H, Ykyama M, Fukuzaki H. Cntractile respnses f canine crnary cllateral vessels. Jpn Heart J 1984;25:93-IOl 21. Scheel KW, Daulat G, Williams S. Functinal anatmical site f intramural cllaterals in dgs. Am J Physil 1990;259:H Ellis EF, Oelz 0, Rberts LJ, et al. Crnary arterial smth muscle cntractin by a substance released frm platelets: evidence that it is thrmbxane A2. Science 1976;193: l Bundy GL. The synthesis f prstaglandin endperxide analgs. Tetrahedrn Lett 1975;24: Burke SE, Lefer AM, Niclau KC, Smith GM, Smith JB. Respnsiveness f platelets and crnary arteries frm different species t synthetic thrmbxane and prstaglandin endperxide analgues. Br J Pharmacl 1983;78: Mehta J, Nichls WW, Gldman R. Prstacyclin release fllwing endperxide analgue infusin in the intact dg. Am J Physil 1984 ;246: R Rucc NA, Mst AS, Sasken H, Steiner M, Gewirtz H. Rle f endgenus prstacyclin in mycardial bld flw regulatin distal t a severe crnary stensis. Cardivasc Res 1988;22: Benedict CR, Mathew B, Rex KA, Cartwright J Jr, Srdahl LA. Crrelatin f plasma sertnin changes with platelet aggregatin in an in viv dg mdel f spntaneus cclusive crnary thrmbus frmatin. Circ Res 1986;58: Hirsh PD, Hillis LD, Campbell WB, Firth BG, Willersn JT. Release f prstaglandins and thrmbxane A 2 in the crnary circulatin in patients with ischemic heart disease. N Engl J Med 1981 ;304: Hamm CW, Lrenz RL, Bleifeld W, Kupper W, Wber W, Weber PC. Bichemical evidence f platelet activatin in patients with persistent unstable angina. J Am Cll CardiI1987;1O:998-IOOI. 30. van den Berg EK, Schmitz JM, Benedict CR, Mally CR, Willersn JT, Dehmer GJ. Transcardiac sertnin cncentratin is increased in selected patients with limiting angina and cmplex crnary lesin mrphlgy. Circulatin 1989;79: Willersn JT, Glin P, Eidt J, Campbell WB, Buja LM. Specific platelet mediatrs and unstable crnary artery lesins: experimental evidence and ptential clinical implicatins. Circulatin 1989;80: B1umgart HL, Schlesinger MJ, Davis D. Studies f the relatin f the clinical manifestatins f angina pectris, crnary thrmbsis and mycardial infarctin t the pathlgic findings. Am Heart J 1940;19:1-91.