Altered cross‐bridge characteristics following haemodynamic overload in rabbit hearts expressing V₃ myosin

Abstract

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...

Two-State Model of Acto-Myosin Attachment-Detachment Predicts C-Process of Sinusoidal Analysis

“…In addition, the relaxation rate constant, k e /k v , in Eq. 18A is equivalent to the reciprocal of the mean time attached and is consistent with the interpretation that the exponential rate of force relaxation, due to cross-bridge cycling, mirrors the rate of cross-bridge detachment (12,50,51,60). The equivalent spring-dashpot ( Fig.…”

“…Huxley and Simmons and others (13)(14)(15) argue that phase 2 after a release reflects the rate of force regeneration by cross-bridges transitioning from weakly to strongly bound states, and that the rate of cross-bridge detachment is too slow to contribute to the observed time course of phase 2 (13). The current analysis and that of others (12,60) conversely suggest that crossbridge detachment constitutes a significant portion of the force response under normal experimental conditions. Our interpretation that 2pc of the C-process reflects the rate of cross-bridge detachment is supported by the results of previous studies.…”

RNA asymmetric distribution and daughter/mother differentiation in yeast

“…Motion driven from a single myosin V would be inefficient, due to the poor processivity of this type of motor. Redundancy of ASH1 mRNA zipcodes is therefore needed to provide a persistent motion of the cargo [23]. This is consistent with the presence of four localization elements in ASH1 mRNA, each presumably binding one myosin molecule (Figure 2a).…”

Effect of Cardiac Myosin Binding Protein-C on Mechanoenergetics in Mouse Myocardium

“…Although cMyBP-C deficiency has been shown to reduce 19 or enhance 9,16 myofilament calcium sensitivity, we propose that these reports may reflect differences in thin filament protein content or isoform, such as skeletal troponin-C used to replace cardiac 16 or variable troponin-T isoforms as occur in heart failure. 31,34 The 6-week PTU treatment used for both the t/t PTU and ϩ/ϩ PTU mice would be expected to induce similar protein isoform profiles in both the thick and thin filaments. 27,34 The strikingly similar calcium sensitivities in the t/t PTU and ϩ/ϩ PTU we report here may therefore indicate that cMyBP-C does not significantly affect calcium sensitivity of isometric tension, just as cMyBP-C phosphorylation also does not affect calcium sensitivity.…”

“…31,34 The 6-week PTU treatment used for both the t/t PTU and ϩ/ϩ PTU mice would be expected to induce similar protein isoform profiles in both the thick and thin filaments. 27,34 The strikingly similar calcium sensitivities in the t/t PTU and ϩ/ϩ PTU we report here may therefore indicate that cMyBP-C does not significantly affect calcium sensitivity of isometric tension, just as cMyBP-C phosphorylation also does not affect calcium sensitivity. 17,18 A reduction in isometric tension with increasing [P i ], as observed in the myofilaments lacking cMyBP-C, can be explained by a higher probability of reversal of the forceproducing power stroke of the acto-myosin crossbridge, which would occur with a higher probability of a backreaction of the phosphate-release step.…”