Aims: Exercise started early after myocardial infarction (MI) improves in vivo cardiac function and myofilament responsiveness to Ca(2+). We investigated whether this represents partial or complete reversal of cellular remodelling.
Methods and results: Mice with MI following left coronary ligation were given free access to a running wheel (MI(EXE), N = 22) or housed sedentary (MI(SED), N = 18) for 8 weeks and compared with sedentary sham-operated animals (SHAM, N = 11). Myocytes were enzymatically isolated from the non-infarcted left ventricle. Myocytes in MI were significantly longer and even more so with exercise (165 +/- 3 microm in MI(EXE) vs. 148 +/- 3 microm in MI(SED) and 136 +/- 2 microm in SHAM; P < 0.05, mean +/- SEM); cell width was not different. Contraction was measured during electrical field stimulation at 1, 2, and 4 Hz. Unloaded cell shortening was significantly reduced in MI(SED) (at 1 Hz, L/L(0)=4.4 +/- 0.3% vs. 6.7 +/- 0.4% in SHAM; P < 0.05, also at 2 and 4 Hz). Exercise restored cell shortening to SHAM values (MI(EXE), L/L(0)=6.4 +/- 0.5%). Membrane currents and [Ca(2+)](i) were measured via whole-cell patch clamping, with Fluo-3 as Ca(2+) indicator, all at 30 degrees C. Ca(2+) transient amplitude, I(CaL) and sarcoplasmic reticulum Ca(2+) content were not different between the three groups. Diastolic Ca(2+) levels at 4 Hz were significantly elevated in MI(SED) only, with a trend to increased spontaneous Ca(2+) release events (sparks). Action potential duration was increased and transient outward K(+) currents significantly reduced after MI; this was unaffected by exercise.
Conclusions: Early voluntary exercise training after MI restores cell contraction to normal values predominantly because of changes in the myofilament Ca(2+) response and has a beneficial effect on diastolic Ca(2+) handling. However, the beneficial effect is not a complete reversal of remodelling as hypertrophy and loss of repolarizing K(+) currents are not affected.