Role of Acetylstrophanthidin in Augmenting Myocardial Oxygen Consumption

Transcription

1 Role of Acetylstrophanthidin in Augmenting Myocardial Oxygen Consumption RELATION OF INCREASED O, CONSUMPTION TO CHANGES IN VELOCITY OF CONTRACTION By Henry Neal Coleman, III, M.D. ABSTRACT A polarographic method was used to determine the effects of acetylstrophanthidin on myocardial oxygen consumption (MVO 2 ) of 19 cat papillary muscles contracting under both afterloaded and isometric conditions. Under afterloaded conditions, acetylstrophanthidin shifted the force-velocity relation to the right and produced increments in both the extent and velocity of shortening at constant levels of developed force. These changes in myocardial mechanical behavior after strophanthidin were always associated with an increased MVO. Since both the extent and velocity of shortening increased following augmentation of the contractile state with glycoside, consideration of the individual effect of these variables in myocardial mechanical behavior on MVO was precluded. Therefore, experiments were performed on isometrically contracting muscle to evaluate the effect of changes in the velocity of contraction on MVO;> independent of changes in contractile element shortening. Comparison of the MVO 2 of isometric contractions at equal levels of developed tension (and thus equivalent amounts of internal contractile element shortening) was accomplished by decreasing initial muscle length after augmentation of the contractile state by acetylstrophanthidin. Under these conditions, augmentation of the contractile state, characterized by an increase in the velocity of contraction, was associated with an increased MVO. Thus it is concluded that the effect of acetylstrophanthidin is to increase MVO 2 at a constant load and that this augmentation of MVO 2 can be related to the change in contractile state of the muscle. ADDITIONAL KEY WORDS contractile state papillary muscle glycosides myocardial mechanics force-velocity relations tension development inotropic state external work Although it has been demonstrated that cardiac glycosides augment the force of contraction of both the failing (1) and nonfailing myocardium (2), the effects of glycosides on myocardial oxygen consumption (MVO 2 ) have been a matter of controversy. Glycosides have been reported to increase MVO2 in some investigations (3), but in other studies in man (4) and in experimental animals in which the hemodynamic conditions were controlled (5), the contractile state of the heart was augmented without alterations in MVO 2. From the Cardiology Branch, National Heart Institute, Bethesda, Maryland Accepted for publication August 28, However, recent studies have emphasized that alterations in the mechanical behavior of the myocardium such as developed tension in the ventricular wall (6) and changes in the velocity of contraction (7, 8) are important determinants of MVO-j. Correlation of these changes with changes in MVO 2 has often been difficult because of the complex architecture of the intact heart. The study of the isolated papillary muscle with its simple geometry allows direct measurement of myocardial mechanics, while the use of the polarographic O2 electrode permits simultaneous determination of its O2 consumption. With recent descriptions of myocardial mechanics as a background (9, 10), the present CircmUtion Rts rcb, Vol. XXI Oaobtr

2 488 COLEMAN investigation was designed to examine the effects of acetylstrophanthidin on myocardial mechanical behavior and MVO2 of both isotonically and isometrically contracting papillary muscles. This study presents evidence in support of the view that alterations in the intrinsic velocity of contraction (V n un), independent of the effects of tension development and contractile element shortening, modify MVO 2. Methods 1. Measurement of Mechanical Activity of Cat Papillary Muscle. Papillary muscles rapidly excised from the right ventricle of cats anesthetized with sodium pentobarbital (30 nig/kg ip) were put in a Petri dish containing an oxygenated Krebs-Ringer's solution at 29 C for the placement of 4-0 noncapillary silk ties. The muscle was then mounted in the central tubular muscle chamber of a ludte bath (Fig. 1). Load extension curves of the silk thread were determined because, with very heavy loads, silk has considerable compliance, which partly depends on whether the silk is wet or dry (11). Since the maximal extension of the thread (88 mm long) with a 5-g load was 0.18 mm or 0.20% when dry and 0.22 mm or 0.26% of its length when wet, no corrections were made when muscle length was altered. The muscle was fixed at its base to the rod entering the bottom of the bath through a mercury seal which prevented fluid or air leaks, but because of friction, also precluded the determination of tension from below the bath, Its upper end was attached either to a Schilling isotonic transducer (12) or a Statham force transducer (Model G-l- 1000) mounted on Palmer adjustable screw stands. The use of a lever allowed examination of afterloaded contractions when (1) preload or initial tension was the determinant of initial fiber length; (2) afterload was the amount of tension that the muscle must develop before shortening could occur; and (3) total load equaled preload plus afterload and was the load at which both SAMPLING LINE RECORDER INFLOW DtAJH CENTRAL TUBULAR MUSCLE CHAMBER STIM, FIGURE 1 Schematic illustration of the experimental apparatus. B = electrode controls for position and polarizing voltage; C == condenser column; E = electrode; M = lever stop; L = lever; R = pressure regulator. CircuUuoa R.smtcb, Vol. XXI, Octob* 1967

3 STROPHANTHIDIN AND MYOCARDIAL OXYGEN CONSUMPTION 489 shortening and the velocity of shortening were determined. A Grass S4E stimulator coupled with a Tektronix wave form generator provided a supramaximal square wave stimulus (3 to 6 v) for 2.5 msec at a rate of 30/min for all experiments. The temperature of the condenser column, muscle bath, and electrode was maintained at 29 C with a Haake constant temperature batli. All tracings were recorded on a multichannel oscillographic recorder. The velocity of muscle shortening, expressed in millimeters per second, was determined from the tangent to the shortening trace at each load, while the rate of force development (df/dt), expressed in grams per second, was obtained in a similar fashion from the isometric twitch. An R-C differentiator (time constant, 0.001) was used to locate the tangent at the point of peak velocity for each trace. 2. Measurement of Oxygen Consumption. The bath and apparatus in Figure 1, modified from the apparatus used by McDonald (13), contained a modified Krebs-Ringer's solution (Na +, 152; K +, 3.6; Cl- 135; HCO,-, 25; Mg2 +, 1.2; H 2 PO 4 ", 1.3; SO 2 4 -, 1.2; and Ca 2 +, 5.0 meq/liter; glucose, 5.6 mmnle/liter) which had a ph of 7.4 when bubbled with 5% CO 2. The solution was circulated at 6.7 ml/min from a reservoir (not shown) by a Technicon Model I proportioning pump to column C, where it was equilibrated with 95% O 2-5% CO 2 before passing by gravity through the inflow line to the upper chamber (rectangular dimensions, 40 X 5 X 25 mm; volume, 5 ml) of the muscle bath. The fluid in the upper chamber in excess of that withdrawn past the muscle was removed and returned to the reservoir through the overflow line while the atmosphere above the solution was displaced through an 8-mm opening in the top of the bath by a continuous flow of 95% O 2-5% CO 2. The solution was withdrawn at a constant rate of 3.5 ml/hr down the central tubular muscle chamber (length, 41 mm; diameter, 3.5 ml; volume, 0.4 ml) into a sampling line containing the O 2 electrode, E. This sampling capillary had a diameter of 0.8 mm and length of 39 mm, dimensions adequate to insure uniform diffusion of O 2 across the capillary at the electrode site (14). Oxygen tension was polarographically measured with O 2 electrodes of the Clark type 1 (15). The electrode was constructed of lucite with a glass-embedded platinum cathode (0.5-mm diameter) and a silver-silver chloride anode reference. The electrode was filled with 0.1 N KCl, and a 1-mil Teflon membrane was placed Constructed and polished in this laboratory by Mrs. Zena Taylor McCallum. Circulation Kuurcb, Vol. XXI, October 1967 over its end. Plateau voltages were determined for each electrode, and a constant 0.68-v potential was placed across the electrode. The current from the electrode, proportional to the O 2 tension, was amplified by a Kintel electronic galvanometer and recorded on a Brown-Honeywell 12- inch chart recorder. Each electrode was tested for linear response by evaluating the current produced in O concentrations of 100%, 90%, 20.9% (room air), and 0* (nitrogen). Thereafter the electrode was calibrated in place during each experiment by changing the gas equilibrating the solution in column C from 95% O 2-5% CO 2 to 90% O 2-5% CO 2-5% No. Gas mixtures were analyzed either with a Haldane apparatus by the method of Peters and Van Slyke (16) or by gas chromatography. Myocardial oxygen consumption was calculated by use of the calibration curve, solubility of O 2 at 29 C, and the deflection in the O 2 record produced by each intervention at a constant flow past the muscle and electrode. The deflection from the 95% O 2 level due to resting MV0 2 was directly measured once a steady state had been reestablished following installation or removal of the muscle in the bath or bodi. The deflection for MVO 2 associated with activity was determined for each load by stimulating the muscle for 10 min and integration of tie curve for decreased O 2 tension with respect to the duration of stimulation. This procedure, previously used by McDonald (13), resulted in reproducible values for MV0 2 with an average variation of 5%. The results are expressed as [jmler/mg dry tissue per hr for resting MV0 2 and /Jiter/mg dry tissue per beat X 10" 3 for MVO 2 associated with activity. At the conclusion of each experiment, the muscle's length and diameter were determined with a caliper after a weight equal to the preload had been attached to the base of the muscle. Diameters determined by this method were found not to vary by amounts greater than 0.1 mm from diameters calculated from the length and weight, assuming the muscle to be cylindrical, a quantity which is probably within the error inherent in either method. The muscles were dried for 24 hr at 110 C and weighed on a precision balance accurate to ± 0.01 mg. 3. Selection of Muscles. No general agreement exists in the literature (17, 18) as to the critical diameter of papillary muscles for adequate oxygenation by diffusion from an external source. The selection of muscles for this study was based on the observation that the resting MVO 2 of muscles under 1 mm in diameter did not increase when stretched by increments in preload from 0.1 to 1.0 g and that the resting

4 490 COLEMAN MVO 2 of muscles with diameters greater than 1 mm increased when stretched by addition of preload. Since an increase in resting MVO 2 with increased length, and thus decreased diameter, might be attributed to oxygenation of a central core of muscle previously made hypoxic by diffusion limitations, it was decided to study only those muscles whose resting MV0 2 did not change with changes in length. The 19 muscles selected for study ranged from 0.7 to 1.0 mm in diameter (average 0.86) and 5.8 to 10 mm in length. 4. Types of Experiments. Two types of experiments were performed. In 13 muscles the velocity of shortening, extent of shortening, and MVO;, were measured for isotonic contractions before and after addition of sufficient acetylstrophanthidin to the Krebs-Ringer's solution in the reservoir to produce final concentrations from 0.01 to 0.20 /xg/ml. Six additional experiments were done to compare MVOo of isometric contractions at a constant level of developed tension before and during the effect of acetylstrophanthi LOAD IN GRAMS FIGURE 2 Length 6 5 mm Diameter 0 8 mm Temp. 29*C Effect of 0.15 ng/ml acetylstrophanthidin on forcevelocity relations and MVO t. Ordinates: A = velocity of shortening; B = shortening; C = oxygen consumption. Abscissa = total load. din (0.20 /ig/ml). In the latter experiments, peak developed tension was maintained constant for each point after administration of acetylstrophanthidin by decreasing initial muscle length. Results 1. Oxygen Consumption during Rest. The resting MV0 2 was satisfactorily determined in 9 muscles before, and in 7 muscles following, addition of acetylstrophanthidin in a dose sufficient to augment the contractile state. MV0 2 averaged 2.48 ± 0.24 SE /xliter/mg per hr, while the MVO 2 of the muscles later perfused with acetylstrophanthidin (0.15 to 0.20 /ig/ml) averaged 2.97 ± 0.33 fditer/mg per hr. This difference in resting MVO 2 between the two groups was not significant. 2. Oxygen Consumption during Activity. The effects of acetylstrophanthidin on forcevelocity relations and MVO 2 were determined in 13 papillary muscle preparations. The results from a representative experiment are depicted in Figure 2. In A it is evident that the administration of acetylstrophanthidin (0.15 /ig/ml) shifted the force-velocity relation to the right with augmentation of both the intrinsic velocity of shortening (V mbi ) and the maximum force of isometric contraction. Values for both the extent and velocity of shortening at comparable loads (A and B) were thus increased after the addition of acetylstrophanthidin to the perfusate. Further, it may be noted in C that changes in forcevelocity relations, produced by augmentation of the inotropic state with the glycoside, are associated with an increased MVO 2 over the entire range of loads that were evaluated. Similar results were obtained in 10 additional experiments for concentrations of acetylstrophanthidin between and 0.20 /xg/ml. In 2 experiments, a concentration of /ig/ml produced no alterations in myocardial mechanical behavior or MV0 2. The relationship between the change in MVO 2 and the change in velocity of shortening resulting from administration of acetylstrophanthidin for all experiments in which the muscle was allowed to shorten is depicted in Figure 3. A general re- CirctiUtion Rtsurcb, Vol. XXI, Oaobtr 1967

5 s z 5 z o n 3D o O Effact of Acetylstrophanthidin on Oxygen Consumption of TABLE 1 Isometric Contractions at Constant Levels of Tension Development Expt. no. 3 4 * Initial tension (*) Developed tension (g) Rate of force development (/iliter/mj consumption per he.t V 10-=l _, Chango in O3 consumption (/iliter/ mg per beat X 10-) (%) Acetylstrophanthidin, 0.2 of Krebs-Ringer's solution in each instance.

6 492 COLEMAN II IBO MO SO ?* -20 -an 1 i K)0 E0 140 BO 180 # '.. * \ * / / ys * k CHANGE IN VELOCITY OF SHORTENING AFTER ACETYLSTROPHANTHICHN FIGURE 3 Relationship between change in velocity of shortening and change in MVO S after acetylstrophanthidin for 13 experiments in which afterloaded contractions were examined. Least squares regression line is shown with equation y = (11.5 ± SE 5.6) + (0.63 ± SE 0.1)X. Estimate of slope is significantly different from zero, while estimate of y intercept is significantly different from zero with values of P <.001 and P <.05 respectively. lationship between the changes in MVO2 and changes in velocity of shortening can be seen, although there is considerable variation, due in part to the variation of dose levels of acetylstrophanthidin. In the experiments cited above, augmentation of the inotropic state with acetylstrophanthidin resulted in increments in the velocity of shortening for a given load, the extent of shortening (and thus the work done), in V m,,x, and in the MVO 2. To minimize any effects of changes in contractile element shortening, eliminate the effect of external work, and evaluate the effects of alterations in velocity of contraction and inotropic state on MV0 2) 6 experiments were performed to evaluate isometric contractions before and after administration of acetylstrophanthidin. The results of experiments of this type are shown in Table 1. In each experiment, initial tension (muscle length) was decreased following acetylstrophanthidin in order to achieve cornparable levels of tension development for both the control and augmented contractile state. While tension was maintained constant following augmentation of the contractile state, the rate of force development (df/dt) was increased in each experiment. The effect on MVO2 of these alterations in contractile state following addition of the glycoside in experiment 4 is graphically depicted in Figure 4. At each of three levels of isometric tension, augmentation of the inotropic state with 0.2 /tg/ml acetylstrophanthidin was associated with an increased MVO 2. Similar results were observed in the 5 other experiments of this type (Table 1). Discussion The premise that glycosides can alter the performance of the myocardium without altering MVOo (19) would appear to conflict with the data from this investigation which show that augmentation of the contractile state, characterized by an increased V mai, resulted in an increased MVO 2 when the muscle was contracting both isotonically and isometrically. The results of this investigation are, however, in accord with the findings of Covell et al. (20) who showed in the intact canine heart, that strophanthidin produced 3.0rs. f- g ISOMETRIC CONTRACTION o Points AcetytstrophenthMin CL2/ig/ml I I I I I " \ 5 4_i_ PEAK DEVELOPED TENSION (qromi) FIGURE 4 Effect of 0.20 fig/ml acetylstrophanthidin on of isometric contractions at constant levels of developed tension. Ordinate = oxygen consumption. Abscissa peak developed tension. 5 0 Circulation Rtitmrcb, Vol. XXI, Octobtr 1967

7 STROPHANTHIDIN AND MYOCARDIAL OXYGEN CONSUMPTION 493 increments in MVOo that correlated directionally with changes in the extent and velocity of shortening. In recent studies, alterations in mechanical behavior of the myocardium such as developed tension (13) and the velocity of contraction (7) have been shown to be important determinants of MVO 2. Consideration of these variables offers a possible explanation for the lack of similarity between the results from this investigation and the studies in which hemodynamic variables were either controlled or evaluated for their effects on MVO 2 (4, 5, 21, 22). In those investigations in which glycosides improved contractility without altering MVO2, end-diastolic pressure, and thus developed tension in the ventricular wall, declined, a factor which may be expected to decrease MVO 2. That this, as well as the increased rate of pressure development, might have influenced the MVO2 was commented on by Sarnoff et al. (5). Detailed examination of this problem by Covell et al. (20) demonstrated that the reduction in wall tension could mask the effect of alterations in the extent and velocity of shortening on MVO 2. Other support for this explanation can be found in the work of Peters and Visscher (3), who noted increased MVO 2 after glycoside administration in the heart-lung preparation when end-diastolic volume was maintained constant. Not explained by this mechanism are the results of Lee (23) who demonstrated in the cat papillary muscle that ouabain produced an initial augmentation of contractility without a change in MVO2 followed by further increments of both contractility and MVO 2. However, as the time lag between the effects on contractility and MVO2 was found to depend on whether manometric (23) or polarographic (24) techniques were used, this asynchrony may well be due to technical problems. Analysis of the present data in terms of mechanical behavior reveals that acetylstrophanthidin exerted a positive inotropic effect with increments in V mai and the extent of shortening and external work in muscles developing a constant level of tension - In Circulation RiiMrcb, Vol. XXI, O CM her 1967 each experiment, these alterations in mechanical behavior were associated with an increased MVO2. However, in these studies as well as other investigations of the effects of inotropic agents on MV0 2 (7, 20), both the velocity of shortening and the extent of shortening were increased by the intervention. Further, while the extent of shortening was increased in the experiments cited, the effect of inotropic interventions on external work, a factor recently suggested as a determinant of myocardial energy utilization (25), was not determined. Thus, in these studies individual consideration of the effects of these mechanical variables on MVO 2 was precluded. On the other hand, when the effects of strophanthidin on the MVO 2 of isometric contractions was measured at equal levels of developed tension, the factors of external contractile element shortening and external work were eliminated. The small remaining shortening of the contractile element is produced by the extension of the series elastic component by the contractile element and, although the stiffness of the series elastic element is somewhat decreased by the changes in initial length (26), contractile element shortening for contractions at a given developed force can be assumed to be of similar magnitude. Further, the amount of contractile element shortening in the isometric experiments is minimal relative to the total contractile element shortening in the isotonic experiments. Thus, the increments in MVO^ in Figure 4 following glycoside administration can be associated with augmentation of the contractile state as characterized by an increased V mai independent of changes in tension development or contractile element shortening. These data indicate that changes in contractile state are, in addition to the substantial effect described for tension development (6, 13), important determinants of MVO?. Although the change in intrinsic velocity was not directly determined in this series of isometric experiments, the observed increments in the rate of force development (df/ dt) following acetylstrophanthidin indicate that augmentation of V moi occurred. How-

8 494 COLEMAN ever, these increments in df/dt do not indicate changes of the same magnitude in V,MX under the conditions of the experiments. The rate of force development (df/dt) is determined by the interaction of the rate of contractile element shortening (dl/dt) and the stiffness of the series elasticity (df/dl). Thus df/dt = dl/dt df/dl (26). Since the stiffness of the series elastic element decreases with decreasing muscle length (26), the increments in df/dt determined after glycoside administration and after muscle length was decreased to equalize tension would underestimate the increase in contractile element velocity and V mo,. Thus while the observed changes in df/dt after acetylstrophanthidin indicate concomitant changes of the similar direction in V ma i, df/dt could not be used for quantitative evaluation of the relationship between changes in contractile state and alterations in MVO. Although the MVO 2 during rest, 2.48 filter/mg per hr in this study, is similar to the results reported by McDonald (13), as well as the figure of 2.58 /xl i ter/mg per hr determined for 18 muscles in a separate study (unpublished observations), the difference between the control value and the average value for 7 muscles after acetylstrophanthidin was not statistically significant. The results of this investigation are pertinent to the administration of glycosides to man. Glycosides have been demonstrated to increase peripheral resistance in normal subjects (2) and to augment the velocity of myocardial contraction both in patients with normal hearts and in those with failing hearts (27). These alterations could result, in a patient with coronary artery disease without elevated left ventricular end-diastolic pressure, in the augmentation of O 2 requirements in excess of available O 2 delivery and in the production of myocardial ischemia. Acknowledgment The author expresses his thanks to Dr. Eugene Braunwald for his advice in the course of these experiments and aid in preparation of the manuscript. The technical assistance of Mrs. Zena T. MeCallum is gratefully acknowledged. References 1. BLOOMFTELD, R. A., RAPOPORT, B., MILNOB, J. P., LONG, W. K., MEBANE, J. C, AND ELUS, L. B.: Effects of cardiac glycosides upon dynamics of circulation in congestive heart failure. J. Clin. Invest. 27: 588, BRAUNWALD, E., BLOODWELL, R. D., GOLDBERG, L. I., AND MORHOW, A. G.: Studies on digitalis: IV: Observations on the contractility of the nonfailing heart and on total vascular resistance. J. Clin. Invest. 40: 52, PETERS, H. C., AND VISSCHER, M. B.: Energy metabolism of the heart and the influence of drugs upon it. Am. Heart J. 11: 273, BING, R. J., MARAIST, F. M., DAMMAN, J. F., JR., DRAPER, A., JR., HETMBECKER, R., DALEY, R., GERARD, R., AND CALAZEL, P.: Effect of strophanthus on coronary blood flow and cardiac oxygen consumotion in normal and failing human hearts. Circulation 2: 513, SABNOFF, S. J., GILMORE, J. P., WALLACE, A. G., SKINNER, N. S., JR., MITCHELL, J., AND DAC- GETT, \V. M.: Effect of aeeryl strophanthidin therapy on cardiac dynamics, oxygen consumption and efficiency in isolated heart with and without hypoxia. Am. J. Med. 37: 3, MCDONALD, R. H., JB., TAYLOR, R. R., AND CIN- COLANI, H. E.: Measurement of myocardial developed tension and its relation to oxygen consumption. Am. J. Physiol 211: 669, SONNENBUCK, E. H., Ross, J., JR., COVELL, J. W., KAISER, G. A., AND BRAUNWALD, E.: Velocity of contraction as a determinant of myocardial oxygen consumption. Am. J. Physiol. 209: 919, KRASNOW, N., ROLETT, E. L., YURCHAK, P. M., HOOD, W. B., JH., AND GOBXTN, R.: Isoproterenol and cardiovascular performance. Am. J. Med. 37: 514, ABBOTT, B. C., AND MOMMAERTS, W. F. H. M.: A study of inotropic mechanisms in the papillary muscle preparation. J. Gen. Physiol. 42; 533, SONNENBLICK, E. H.: Force-velocity relations in mammalian heart muscle. Am. J. Physiol. 202: 931, JEWEL, B. R., AND WILKTE, D. R.: An analysis of the mechanical components in frog's striated muscle. J. Physiol. 143: 515, SCHILLING, M. D.: Capacitance transducers for muscle research. Rev. Sci. Inst 31: 1215, MCDONALD, R. H., JR.: Developed tension: A major determinant of myocardial oxygen consumption. Am. J. Physiol. 210: 351, CARLSON, F. D., BRINK, F., JR., AND BRONK, D. W.: A continuous flow respirometer utilizing Circulation Rtsetrcb, Vol. XXI, October 1967

9 STROPHANTHIDIN AND MYOCARDIAL OXYGEN CONSUMPTION 495 the oxygen cathode. Rev. Sci. Inst 21: 923, CLARK, L. C, JR., WOLF, R., GRANGER, D., AND TAYLOR, Z.: Continuoiis recording of blood oxygen tensions by polarography. J. Appl. Physiol. 6: 189, PETERS, J. P., AND VAN SLYKE, D. D.: Quantitative Clinical Chemistry. Baltimore, Williams & Wilkins Co., 1932, vol. 2, p CRANEFTELD, P. F., AND GREENSPAN, K.: Rate of oxygen uptake of quiescent cardiac muscle. J. Gen. Physiol. 44: 235, KOCH-WESEH, J.: Effect of rate changes on strength and time course of contraction of papillary muscle. Am. J. Physiol. 204: 451, MODELL, W.: Pharmaeologic basis for the use of digitalis in congestive heart failure. Physiol. Pharmacol. Physicians 1: 1, COVELL, J. W., BRAUNWALD, E., ROSS, J., JR., AND SONNENBLICK, E.: Studies on digitalis: XVI: Effects on myocardial oxygen consumption. J. Clin. Invest. 45: 1535, OLSON, R. E., RAUSH, G., AND LJANC, M. M. L.: Effect of acetylstrophanthidin upon myocardial metabolism and cardiac work of normal dogs and dogs with congestive heart failure. Circulation 12: 755, GOLLWITZER-MEIER, AND KBUCER, E.: Herzenergitik und Strophanthin Wirkung bei verschiedenen Formen des experimentillin Herzinsuffizienz. Pfliiger's Arch. Ges. Physiol. 238: 251, LEE, K. S.: A new technique for the simultaneous recording of oxygen consumption and contraction of muscle: The effect of ouabain on cat papillary muscle. J. Pharmacol. Exptl. Therap. 109: 304, LEE, K. S., YU, D. H., AND BURSTETN, R.: Effect of ouabain on the oxygen consumption, the high energy phosphates and the contractility of the cat papillary muscle. J. Pharmacol. Exptl. Therap. 129: 115, BRITMAN, N. A., AND LEVTNE, H. J.: Contractile element work: A major determinant of myocardial oxygen consumption. J. Clin. Invest. 43: 1397, PAHMLEY, W. W., AND SONNENBLJCC, E. H.: Series elasticity in heart muscle: Its relation to contractile element velocity and proposed muscle models. Circulation Res. 20: 112, MASON, D. T., AND BHAUNWALD, E.: Studies on digitalis: IV: Effects of ouabain on the nonfailing human heart. J. Clin. Invest. 42: 1105, CirculdUou Rutarch, Vol. XXI, Octobtr 1967