Chronic exercise alters contractility and morphology of isolated rat cardiac myocytes

Chronic exercise training elicits positive adaptations in cardiac contractile function and ventricular dimension. The potential contribution of single myocyte morphological and functional adaptations to these global responses to training was determined in this study. Left ventricular cardiac myocytes were isolated from the hearts of sedentary control (Sed) or exercise-trained (TR) rats. Training elicited an ~5% increase in resting myocyte length (Sed, 121.0 ± 2.0 vs. TR, 126.7 ± 2.0 μm; P < 0.05), whereas resting sarcomere length and midpoint cell width were unaffected. These data suggest that longitudinal myocyte growth contributes to the training-induced increase in end-diastolic dimension. Single myocytes (28°C) were stimulated at 0.067 and 0.2 Hz and shortening dynamics assessed at extracellular Ca²⁺ concentrations ([Ca²⁺](o)) of 0.6, 1.1, and 2.0 mM. In both groups, maximal extent of myocyte shortening (ES(max)) increased as [Ca²⁺](o) increased and decreased as contraction frequency increased. TR myocytes were more strongly influenced by the effects of [Ca²⁺](o) and frequency. At 0.067 Hz and 2.0 mM, ES(max) was greater in TR than in Sed myocytes. The magnitude of this difference decreased as [Ca²⁺](o) was reduced. At 0.2 Hz, ES(max) was similar in Sed and TR myocytes at 2.0 mM [Ca²⁺](o). As [Ca²⁺](o) was reduced, ES(max) decreased more rapidly in TR than in Sed myocytes; at 0.6 mM, ES(max) was greater in Sed than in TR myocytes. Our data indicate that chronic exercise influences cardiac contractile function at the single myocyte level. This study also provides evidence in support of the hypothesis that chronic exercise influences myocyte Ca²⁺ influx and efflux pathways. Studies of fura-2-loaded cells indicate that under conditions in which TR and Sed myocytes contract to a similar extent, the amplitude of the intracellular [Ca²⁺] transient is lower in TR myocytes. This suggests that training sensitizes the contractile element to activation by Ca²⁺.