Correlation between myofilament response to Ca2+ and altered dynamics of contraction and relaxation in transgenic cardiac cells that express beta-tropomyosin

We compared the dynamics of the contraction and relaxation of single myocyt es isolated from nontransgenic (NTG) mouse hearts and from transgenic (TC-b eta-Tm) mouse hearts that overexpress the skeletal isoform of tropomyosin ( Tm), Compared with NTG controls, TG-beta-Tm myocytes showed significantly r educed maximal rates of-contraction and relaxation with no change in the ex tent of shortening, This result indicated that the depression in contractio n dynamics determined in TG-P-Tm isolated hearts is intrinsic to the cells. To further investigate the effect of Tm isoform switching on myofilament a ctivity and regulation, we measured myofilament force and ATPase rate as fu nctions of pCa (-log of [Ca2+]). Compared with controls, force generated by myofilaments from TG-beta-Tm hearts and myofibrillar ATPase activity were both more sensitive to Ca2+, However, the shift in pCa,, (half-maximally ac tivating pCa) caused by changing sarcomere length from 1.8 to 2.4 mu m was not significantly different between NTG and TG-beta-Tm fiber preparations, To test directly whether isoform switching affected the economy of contract ion, force versus ATPase rate relationships were measured in detergent-extr acted fiber bundles. In both NTG and TG-beta-Tm preparations, force and ATP ase rate were linear and identically correlated, which indicated that cross bridge turnover was unaffected by Tm isoform switching. However, detergent extracted fibers from TG-beta-Tm demonstrated significantly less maximum te nsion and ATPase activity than NTG controls. Our results provide the first evidence that the Tm isoform population modulates the dynamics of contracti on and relaxation of single myocytes by a mechanism that does not alter the rate-limiting step of crossbridge detachment. Our results also indicate th at differences in sarcomere-length dependence of activation between cardiac and skeletal muscle are not likely due to differences in the isoform popul ation of Tm.