We present a mechanical assay for estimating the time course of Ca2+ bound to low-affinity sites on troponin C (TnC) in twitching rabbit papillary muscle. The assay is based on a theoretical correlation between the rate of force redevelopment after detachment of all cross-bridges and the amount of Ca2+ bound to TnC. Experimentally, we applied length impulses at different times to detach all cross-bridges; the initial rate of force redevelopment after each impulse was taken as an index of bound Ca2+ at that time. Under control conditions, the magnitude of this index decreased to 10% of its maximum during early relaxation, when force had declined only slightly 78 +/- 12% of its peak isometric value. The time course of this index was examined after addition of either isoproterenol or ryanodine, which are known to shorten and prolong, respectively, the intracellular free Ca2+ transient. As expected, changes previously reported in the free Ca2+ time course were qualitatively reflected in the time course of the bound Ca2+ index. We conclude that this index constitutes a reasonable method for estimating the time course of bound Ca2+ and that bound Ca2+ declines well ahead of force in isometrically contracting rabbit myocardium at 24 degrees C.

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The time course of mechanical efficiency during working contractions in rabbit papillary muscle is presented. Efficiency is found to remain relatively constant during the working portion of the twitch, when the muscle is contracting against a constant load. As afterload was decreased, efficiency increased to 65 ± 11% (mean ± SE, n = 3) at 10% developed force at maximum length. This is in contrast to muscle work, which reached a peak of 3.0 ± 0.3 (n = 6) mJ/g at 50% developed force at maximum length. heart; initial heat; rabbit; working contractions

Cardiac performance in systole and diastole is critically dependent upon the underlying characteristics of the cross-bridge cycle. Myosin cross-bridges extending out from the thick filament produce muscle force and motion via repetitive interactions with actin-containing thin filaments, with the obligatory hydrolysis of ATP.The failing human heart exhibits a 100 % greater crossbridge force-time integral (FTI XBr ) than normal (Hasenfuss et al. 1991) and a decrease in myofibrillar ATPase activity (Alpert & Gordon, 1962;Pagani et al. 1988) with no change in the myosin heavy chain (MHC) isoform profile (100 % V 3 ; Schiaffino & Reggiani, 1996). The FTI XBr and ATPase activity are both accepted measures of cross-bridge performance, where the former is an index of mechanical response per molecule of ATP hydrolysed, while the latter is an index of interaction kinetics. The mechanisms underlying these mechanical and kinetic alterations in the human heart are unknown, though a shift in cardiac troponin T (cTnT) isoform expression has been observed to correlate with the decrease in myofibrillar ATPase activity (Anderson et al. 1991).In the rabbit heart, surgically induced right ventricular pressure overload (PO) results in a two-fold increase in the isometric FTI XBr (Hasenfuss et al. 1991), along with a reduction in shortening velocity (Hamrell & Dey, 1993) and ATPase activity (Maughan et al. 1979;Litten et al. 1982). These changes are accompanied by a relatively subtle myosin isoform shift from 88 % V 3 to 100 % V 3 (Litten et al. 1982Hasenfuss et al. 1991). This raises the possibility that, as in the failing human heart, modulatory mechanisms independent of the MHC composition may be important in the haemodynamically stressed rabbit. 1. Our goal in this study was to evaluate the effect of haemodynamic overload on cross-bridge (XBr) kinetics in the rabbit heart independently of myosin heavy chain (MHC) isoforms, which are known to modulate kinetics in small mammals. We applied a myothermal-mechanical protocol to isometrically contracting papillary muscles from two rabbit heart populations: (1) surgically induced right ventricular pressure overload (PO), and (2) sustained treatment with propylthiouracil (PTU). Both treatments resulted in a 100 % V 3 MHC profile.2. XBr force-time integral (FTI), evaluated during the peak of the twitch from muscle FTI and tension-dependent heat, was greater in the PO hearts (0.80 ± 0.10 versus 0.45 ± 0.05 pN s, means ± S.E.M., P = 0.01).3. Within the framework of a two-state XBr model, the PO XBr developed more force while attached (5.8 ± 0.9 versus 2.7 ± 0.3 pN), with a lower cycling rate (0.89 ± 0.10 versus 1.50 ± 0.14 s _1) and duty cycle (0.14 ± 0.03 versus 0.24 ± 0.02).4. Only the ventricular isoforms of myosin light chain 1 and 2 and cardiac troponin I (cTnI) were expressed, with no difference in cTnI phosphorylation between the PO and PTU samples. The troponin T (TnT) isoform compositions in the PO and PTU samples were significantly different (P = 0.001), with TnT 2 comprising 2.29...

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