Expression of cardiac troponin T with COOH-terminal truncation accelerates cross-bridge interaction kinetics in mouse myocardium

Transgenic mice expressing an allele of cardiac troponin T (cTnT) with a COOH-terminal truncation (cTnT(trunc)) exhibit severe diastolic and mild systolic dysfunction. We tested the hypothesis that contractile dysfunction in myocardium expressing low levels of cTnT(trunc) (i.e., <5%) is due to slowed cross-bridge kinetics and reduced thin filament activation as a consequence of reduced cross-bridge binding. We measured the Ca2+ sensitivity of force development [pCa for half-maximal tension generation (pCa(50))] and the rate constant of force redevelopment (k(tr)) in cTnT(trunc) and wild-type (WT) skinned myocardium both in the absence and in the presence of a strong-binding, non-force-generating derivative of myosin subfragment-1 (NEM-S1). Compared with WT mice, cTnT(trunc) mice exhibited greater pCa50, reduced steepness of the force-pCa relationship [Hill coefficient (n(H))], and faster k(tr) at submaximal Ca2+ concentration ([Ca2+]), i.e., reduced activation dependence of ktr. Treatment with NEM-S1 elicited similar increases in pCa(50) and similar reductions in nH in WT and cTnT(trunc) myocardium but elicited greater increases in ktr at submaximal activation in cTnT(trunc) myocardium. Contrary to our initial hypothesis, cTnT(trunc) appears to enhance thin filament activation in myocardium, which is manifested as significant increases in Ca2+-activated force and the rate of cross-bridge attachment at submaximal [Ca2+]. Although these mechanisms would not be expected to depress systolic function per se in cTnT(trunc) hearts, they would account for slowed rates of myocardial relaxation during early diastole.