Relative role of caloric restriction and exercise training upon susceptibility to isoproterenol-induced myocardial infarction in male rats.

To determine the relative effect of body weight and physical inactivity on susceptibility to drug-induced myocardial infarction, randomly selected groups of 100-day-old male Sprague-Dawley rats were subjected to a 10-wk program of exercise training and (or) to caloric restriction followed by two subcutaneous injections of L-isoproterenol. Two groups of rats were fed a restricted diet consisting of minimum calories to maintain body weight and were either exercised (R-Ex) or remained sedentary (R-C), one group was fed ad libitum and subjected to exercise (Ex), and one group remained sedentary (C), but was provided only enough food to maintain body weight in a range similar to Ex rats. Initially, there was no difference between group body weights, but Ex and C rats exhibited a significantly greater final body weight. All Ex, R-Ex, and R-C rats survived the isoproterenol injections, but 50% of C rats died. Group C rats exhibited significantly greater activity of total plasma lactate dehydrogenase (LDH), whereas R-Ex rats had the lowest total LDH activity (p less than 0.05). R-Ex and R-C rats had a significantly lower activity of plasma LDH-1, the heart isozyme, than did the heavier Ex and C rats. More specifically, R-C rats exhibited a significantly decreased amount of plasma LDH-1 activity when compared with Ex rats, indicating that smaller, untrained rats had less myocardial damage than the heavier, exercise-trained rats. These data suggest that either exercise or maintenance of body weight is beneficial toward prevention of the drug-induced myocardial infarction, but when weight maintenance is combined with exercise additional protection is provided.