Influence of exercise on excitation-contraction coupling in rat myocardium.

The present studies were conducted to investigate further the mechanisms by which the myocardium adapts to exercise training. Sixty female rats were randomly divided into sedentary control (group C) and trained (group T) groups. Group T was progressively trained for 12 wk. After the rats were killed, left ventricular papillary muscles were mounted in a tissue bath for mechanical studies. Muscles from group T generated greater peak isometric twitch tension per unit cross-sectional area than muscles from group C with [Ca2+]o ranging from 0.25 to 3.5 mM. Analyses of these data indicated that the Km for Ca2+ was not different but that the predicted number of sarcolemmal Ca2+ binding sites was 63% higher in group T. The ATPase activity of the purified cardiac myofibrils was not different between the two groups in the pCa range of 8.53-4.42. Action potentials were recorded with microelectrodes impaled into left ventricular muscle fibers of the subendocardium. Although there was no difference in the resting membrane potential, overshoot, or 90% duration, there was a significant prolongation of the action potential at 0 mV (20.2 +/- 1.0 vs 30.0 +/- 1.3 ms) in group T. These data further support the hypothesis that treadmill exercise enhances cardiac performance by increasing Ca2+ availability to the contractile element. This adaptation is mediated, at least in part, by a sarcolemmal adaptation induced by the exercise paradigm.