Background. In congestive heart failure (idiopathic dilated cardiomyopathy), exercise is accompanied by a smaller-than-normal decrease in end-diastolic left ventricular volume, depressed peak rates of left

To prevent dissection injury when cutting strip preparations from human left ventricular papillary muscle tissue, dissections were carried out with 2,3-butanedione monoxime (30 mM) added to Krebs-Ringer solution and followed by washout with normal solution. Eleven muscle strip preparations were dissected from left ventricular papillary muscle tissue of five patients undergoing mitral valve replacement surgery. The average muscle strip length was 6.8 +/- 1.4 mm, and cross-section area was 0.49 +/- 0.16 mm2. Peak twitch tension was 2.02 +/- 1.33 g/mm2 and ranged from 0.67 to 5.5 g/mm2 at an extracellular calcium concentration of 2.5 mM (21 degrees C, 0.16 Hz). In one muscle strip, which was stored in Krebs-Ringer plus 2,3-butanedione monoxime solution for 20 hours, peak twitch tension in normal Krebs-Ringer solution was 1.85 g/mm2. When temperature was increased from 21 degrees C, there was a continuous increase in peak twitch tension (by 38%) up to about 28 degrees C; then peak twitch tension decreased so that at 37 degrees C (n = 3) average peak twitch tension was lower than at 21 degrees C by 47%. The force-frequency relation exhibited a broad force plateau between 40 and 120 beats/min at 37 degrees C. The plateau was markedly narrowed at 30 degrees C and 24 degrees C. Thermopile heat measurements revealed appropriate waveform characteristics in high-resolution single-beat heat records indicating minimal surface cell damage. Thus, cardioplegia with 2,3-butanedione monoxime protects human left ventricular myocardium from dissection injury facilitating dissection and preservation of strip preparations with extraordinarily low cross-sectional areas and high peak twitch tensions. These preparations are suitable for myothermal and mechanical measurements.

The force-velocity relation has been studied in sixteen single fibres from frog semitendinosus muscle with particular attention to the high-force portion of the curve. The force-velocity curve was hyperbolic except for a reversal of curvature near 80% measured isometric tension (PO). Rectangular hyperbolas fitted (linear, least-squares method) these data well only when values below 0.78 PO were considered. Extrapolation of these hyperbolas above 0.78 PO gave predicted isometric tensions (P*O) which averaged 32+/-6% above the measured PO values. Hill's constants (1.84 degrees C) for these hyperbolas were: a/P*O=0.177+/-0.021, b=0.329+/- 0.035 M.L./sec, Vmax=1.91+/-0.074 M.L./sec. The reversal of curvature persisted when force-velocity data were obtained using: 1 or 60 min response intervals, afterloaded isotonic responses, grid stimulation, electrically induced contractures and bundles of fibres. The reversal of curvature diminished when force-velocity data were obtained from slightly stretched fibres (about 2.3 mum sarcomere length as compared to 2.1 mum in the control). The results indicate that sarcomere length redistributions probably do not account for the non-hyperbolic force-velocity relation. An explanation for the behavior based on the geometry of the contractile filament lattice is discussed.

Alteration in crossbridge behavior and myocardial performance have been associated with myosin isoenzyme composition in animal models of myocardial hypertrophy or atrophy. In the hypertrophied human heart, myocardial performance is altered without significant changes in myosin isoenzymes. To better understand this discrepancy, isometric heat and force measurements were carried out in 1) control and volume-overload human myocardium, 2) control, pressure-overload, and hyperthyroid rabbit myocardium, and 3) control and hypothyroid rat myocardium. In control human myocardium, peak isometric twitch tension was 44.0 +/- 11.7 mN/mm2, and maximum rate of tension rise was 69.2 +/- 21.0 mN/sec.mm2. In volume-overload human myocardium, peak twitch tension and maximum rate of tension rise were reduced by 55% (p less than 0.05) and 65% (p less than 0.05), respectively. The average force-time integral of the individual crossbridge cycle, calculated by myothermal techniques, was increased by 85% (p less than 0.005) in volume-overload human myocardium. In control and hormonally altered myocardium, both across and within species (control human, control rat, control rabbit, hypothyroid rat, and hyperthyroid rabbit), there was a close relation between the crossbridge force-time integral and the percentage of V3-type myosin isoenzyme in the myocardium. However, hemodynamically altered (volume-overload human and pressure-overload rabbit) myocardium did not follow this relation. Across and within species, there were significant correlations between maximum rate of tension rise and average tension-dependent heat rate (r = 0.97, p less than 0.001) and between maximum rate of tension fall and average tension-independent heat rate (r = 0.82; p less than 0.025). Furthermore, there were close inverse relations between these heat rates and the crossbridge force-time integral. In addition, there was an inverse relation between tension-independent heat and the crossbridge force-time integral. Across and within species total myocardial energy turnover was significantly correlated with the crossbridge force-time integral (relative total heat, r = -0.84, p less than 0.02; relative total-activity related heat, r = -0.88, p less than 0.01). The present findings indicate that 1) factors separate from myosin isoenzymes account for the altered crossbridge cycle in volume-overload human and pressure-overload rabbit myocardium, 2) changes in excitation-contraction coupling processes accompany changes in the crossbridge cycle within and across species, and 3) the force-time integral of the crossbridge cycle is a major determinant of total myocardial energy turnover.

Hypothyroidism was induced in Wistar-Kyoto rats by adding propylthiouracil to the drinking water (0.8 mg/ml). Initial heat, total activity-related heat, and resting heat rate were measured in left ventricular papillary muscle preparations of propylthiouracil-treated and control rats contracting isometrically at 12 beats/min (21 degrees C), using Hill type, planar vacuum-deposited bismuth and antimony thermopiles. In the propylthiouracil preparations, relative to control, time-to-peak tension increased from 288 +/- 27 (mean +/- SD) to 411 +/- 25 msec (P less than 0.001), dp/dtmax decreased from 38.3 +/- 9.5 to 20.4 +/- 3.5 g X mm-2/sec (P less than 0.001), and peak developed tension decreased from 6.11 +/- 1.75 to 4.64 +/- 0.89 g X mm-2 (P less than 0.05). In the propylthiouracil preparations, initial heat was significantly (P less than 0.001) reduced by 27 or 43% when normalized to peak twitch tension or tension-time integral, respectively. In experiments where the papillary muscles were tetanized, the slope of the linear function of total activity-related heat versus tension-time integral was decreased by 43% (P less than 0.001) in the propylthiouracil preparations, indicating an improved economy of isometric tension maintenance. The predominant myosin isoenzyme of the left ventricular wall, as well as the papillary muscle myocardium, was the V3 variety in the propylthiouracil animals, in contrast to V1 in the controls. Myofibrillar actomyosin calcium-magnesium-stimulated adenosine triphosphatase activity was significantly (P less than 0.02) decreased from 55 +/- 18 (control) to 31 +/- 8 nmol inorganic phosphate ion/mg X min (propylthiouracil).(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial failure in dilated cardiomyopathy may result from subcellular alterations in contractile protein function, excitation-contraction coupling processes, or recovery metabolism. We used isometric force and heat measurements to quantitatively investigate these subcellular systems in intact left ventricular muscle strips from nonfailing human hearts (n=14) and from hearts with end-stage failing dilated cardiomyopathy (n = 13). In the failing myocardium, peak isometric twitch tension, maximum rate of tension rise, and maximum rate of relaxation were reduced by 46%o (p =0.013), 51% (p =0.003), and 46% (p=0.018), respectively (37°C, 60 beats per minute). Tension-dependent heat, reflecting the number of crossbridge interactions during the isometric twitch, was reduced by 61% in the failing myocardium (p=0.006). In terms of the individual crossbridge cycle, the average crossbridge force-time integral was increased by 33% (p=0.04) in the failing myocardium. In the nonfailing myocardium, the crossbridge force-time integral was positively correlated with the patient's age (r=0.86, p<0.02), whereas there was no significant correlation with age in the failing group. The amount and rate of excitation-contraction coupling-related heat evolution (tension-independent heat) were reduced by 69%1 (p=0.024) and 71% (p=0.028), respectively, in the failing myocardium, reflecting a considerable decrease in the amount of calcium released and in the rate of calcium removal. The efficiency of the metabolic recovery process, as assessed by the ratio of initial heat to total activity-related heat, was similar in failing and nonfailing myocardium (0.54±0.03 versus 0.50±0.02, p =0.23). Thus, in failing dilated cardiomyopathy, contractile protein function is altered with an increased force-time integral of the individual crossbridge cycle. However, this alteration does not explain failure since the same changes are present in nonfailing myocardium from older patients. The findings suggest that reduced tension generation in failing dilated cardiomyopathy primarily results from disturbed excitation-contraction coupling processes with a reduced amount of calcium released and a reduced rate of calcium removal. (Circulation Research 1992;70:1225-1232 The recent development of a method for dissecting thin viable muscle strips from larger pieces of human myocardium13 makes it possible to investigate these subcellular systems in the intact muscle by means of a myothermal method14 under physiological conditions (37°C, 60 beats per minute). Heat evolution represents the entire metabolic energy turnover during the isometric twitch. Total activity-related heat can be partitioned into the heat evolution of the contractile proteins, the excitation-contraction coupling system, and the recovery system. These measurements in conjunction with the mechanical performance provide quantitative information on the extent and rate of the reactions involved

SUMMARY. Initial, recovery, and resting heat were measured in normal and hypertrophied papillary muscles in order to monitor the energetic consequences of the subcellular changes accompanying hypertrophy. The pulmonary artery was constricted in rabbits 4 weeks prior to measurements. Right ventricular papillary muscles were stimulated at 0.2 Hz and 21 °C in Krebs-Ringer solution under isometric conditions at optimum length. Peak twitch tension was 5.90 ± 0.25 g/mm 2 (SEM) in normal muscle (N) and 5.11 ± 0.47 g/mm 2 (NS) in pressure overload muscle (P). The maximal rate of tension generation decreased 26% (P < 0.02) from 15.9 ± 0.85 g/mm 2 sec (N) to 11.7 ± 1.37 g/mm 2 sec (P). Time-to-peak tension increased 30% (P < 0.001) from 627 ± 20 msec (N) to 816 ± 21 msec (P). The total activity related heat production per beat decreased 36% (P < 0.001) from 3.92 ± 0.26 mcal/g (N) to 2.51 ± 0.29 mcal/g (P). Initial heat was reduced 37% (P < 0.001) from 1.66 ± 0.10 mcal/g (N) to 1.04 ± 0.12 mcal/g (P). The isometric heat coefficient increased 43% (P < 0.005) from 8.76 ± 0.54 (N) to 12.5 ± 1 (P) showing increased economy in hypertrophy. There was an early fast phase (1.29 ± 0.12 mcal/g per sec) of initial heat lasting 396 ± 25 msec which was related to tension build-up. A slow phase (1.05 ± 0.07 mcal/g per sec) accompanied relaxation. In hypertrophy, the fast phase was 32% (P < 0.05) slower than normal and lasted 27% (P < 0.02) longer; the slow phase was 48% (P < 0.001) slower than normal. The ratio of recovery to initial heat (1.37 ± 0.09) was not different in N and P muscles. Resting heat was 2.08 ± 0.35 mcal/g per beat in N and 1.35 ± 0.23 mcal/g per beat in P muscles. Our present results and previous enzymatic and mechanical studies suggest that the relation between the compensated pressure overload hypertrophied and normal hearts is similar to the relation between slow and fast skeletal muscle. The changes in the heart that undergoes hypertrophy secondary to pressure overload are beneficial, since they meet the new hemodynamic demands with increased economy of force production. (Circ Res 50: 491-500, 1982)THIS investigation was undertaken to provide in vivo monitoring of subcellular changes induced by pressure overload hypertrophy in rabbit heart. The deposited film thermopile that we recently developed (Mulieri et al., 1977) is ideally suited to this study, since its low thermal capacitance, high time resolution, and sensitivity allow temperature changes accompanying an individual twitch to be correlated with the simultaneously recorded myogram. This offers the possibility of monitoring the time course of initial and recovery heat in an individual twitch response and correlating these with the intracellular mechanical and enzymatic processes known to accompany different portions of the twitch myogram.In addition, it is also of interest to compare myothermically derived values of total isometric energy liberation with values derived from oxygen consumption determinations, since the latter measurements are not in agreeme...

Abstract-Mitral regurgitation (MR) causes ventricular dilation, a blunted myocardial force-frequency relation, and increased crossbridge force-time integral (FTI). The mechanism of FTI increase was investigated using sinusoidal length perturbation analysis to compare crossbridge function in skinned left ventricular (LV) epicardial muscle strips from 5 MR and 5 nonfailing (NF) control hearts. Myocardial dynamic stiffness was modeled as 3 parallel viscoelastic processes. Two processes characterize intermediate crossbridge cycle transitions, B (work producing) and C (work absorbing) with Q 10 s of 4 to 5. No significant differences in moduli or kinetic constants of these processes were observed between MR and NF. The third process, A, characterizes a nonenzymatic (Q 10 ϭ0.9) work-absorbing viscoelasticity, whose modulus increases sigmoidally with [Ca 2ϩ ]. Effects of temperature, crossbridge inhibition, or variation in [MgATP] support associating the calcium-dependent portion of A with the structural "backbone" of the myosin crossbridge. Extension of the conventional sinusoidal length perturbation analysis allowed using the A modulus to index the lifetime of the prerigor, AMADP crossbridge. This index was 75% greater in MR than in NF (Pϭ0.02), suggesting a mechanism for the previously observed increase in crossbridge FTI. Notably, the A-process modulus was inversely correlated (r 2 ϭ0.84, Pϭ0.03) with in vivo LV ejection fraction in MR patients. The longer prerigor dwell time in MR may be clinically relevant not only for its potential role as a compensatory mechanism (increased economy of tension maintenance and increased resistance to ventricular dilation) but also for a potentially deleterious effect (reduced elastance and ejection fraction). Key Words: mitral regurgitation Ⅲ heart failure Ⅲ myocardial stiffness Ⅲ crossbridge function Ⅲ prerigor dwell time H eart failure in mitral regurgitation (MR) is accompanied by impaired ventricular function and altered myocardial function involving defects in excitation-contraction coupling 1,2 and in crossbridge function. The latter is evidenced by a 50% lower myofibrillar Ca 2ϩ -activated myofibrillar ATPase 3,4 and an 85% increased crossbridge force-time integral (FTI). 5 Reduced ATPase suggests slowing of the rate-limiting prerigor step of the crossbridge cycle (ie, ADP release from AMADP 6 ) or slowing of an earlier step (eg, the intermediate phosphate-release step from AMADPPi 6 ). Consequently, we first assessed crossbridge intermediate reaction kinetics using sinusoidal length perturbation analysis in skinned MR and NF myocardium. Because preliminary experiments showed no alterations in these kinetics, the present study repeated the sinusoidal analyses in preparations that were conditioned by regular twitch activity immediately before skinning. This better complies with conditions present in our previous FTI measurements 5 because phosphorylation levels within previously stimulated skinned strips better comply with intracellular phosphorylation levels in our previ...