Troponin T isoforms modulate calcium dependence of the kinetics of the cross-bridge cycle: studies using a transgenic mouse line.

Alternative splicing of troponin T (TnT) in striated muscle during development results in expression of different isoforms, with the splicing of a 5(') exon of TnT resulting in the expression of low-molecular-weight basic adult TnT isoforms and high-molecular-weight acidic embryonic TnT isoforms. Although other differences exist, the main differences between cardiac TnT (cTnT) and fast skeletal muscle TnT (fTnT) are in the NH(2) terminus, with fTnT being less acidic than cTnT. A transgenic mouse line expressing chicken fTnT in the heart was used to investigate the functional significance of TnT NH(2)-terminal charge differences on cardiac muscle contractility. The rates of force redevelopment (k(tr)) at four levels of Ca(2+) activation were recorded for skinned left ventricular trabeculae from control and transgenic mice. The k(tr) vs Ca(2+) relationship was different in control mice and transgenic mice, suggesting that the structure of TnT, and possibly the NH(2)-terminal region, is involved in determining the kinetics of cross-bridge cycle. These results suggest that isoform shifts in TnT may be an important molecular mechanism for determining the Ca(2+) dependence of cardiac muscle contractility.