METHODS

Male Wistar rats from the same shipments were assigned to four groups, treated as follows: 1) rats were made to swim 75 min twice daily, 5 days/wk for 8 wk (CH) as described previously (9); 2) rats were kept sedentary (normal cage activity) for 8 wk (SH); 3) rats were made to swim 75 min twice daily, 5 days/wk for 8 wk and then were kept at normal cage activity for the ensuing 2 wk (DC); and 4) rats were kept at normal cage activity for 10 wk (IX). The hearts were then perfused with a Krebs-Henseleit buffer containing 5.5 mM glucose at 37 C in an isolated working rat heart apparatus which included an aortic flowmeter, as described previously (2). The heart rates were maintained during the experiment by pacing at 340 beatslmin and ventricular performance was recorded at left atria1 perfusion pressures of 5, 10, and 20 cmH, O. The experimental protocol has been described previously (2, 10). At the end of the experiment, the heart was blotted and weighed, and a small portion of the left ventricle was removed for wet and dry weight determination. The remainder of the ventricles were prepared for studins of actomyosin ATPase activity as described previously (3) l

Data were analyzed for pressure, flow, positive and negative dP/dt, and phases of ejection (2). All results are expressed as a means t SE. Statistical significance of differences between SH and CH or between DS and DC was determined by the unpaired Student t-test.