The effect of complete and partial deconditioning on exercise-induced cardiovascular changes in the rat.

Exercise conditioning of young male rats by swimming for 1 h, 5 times per week for 10 weeks results in a reduced gain in body weight, cardiac hypertrophy and hyperplasia, an increase in myocardial capillary density, and an increase in cross-sectional laminal areas of extracoronary collateral arteries. The effects of partial or complete deconditioning on these changes were studied by randomizing young male rats exercise-conditioned for 10 weeks to groups either kept sedentary or subjected to varying amounts of exercise for a second 10-week period. Their hearts were compared with those of unexercised controls using quantitative histologic measurements. During the second 10 weeks, exercise conditioning was found not to continue to suppress weight gain. However, frequent relative short-duration exercise sessions (as little as 15 min per session) frequently prevented an accelerated increase in body weight associated with deconditioning. A complete regression of exercise-induced cardiovascular adaptive changes occurred when wxercise was discontinued completely for 10 weeks or the duration of sessions was cut to 15 min. In order to maintain cardiac hypertrophy, exercise had to be continued for 1 h, 5 times per week while hyperplasia could be maintained with shorter duration, less frequent sessions (30 min, twice a week). The maintenance of increased myocardial capillary density required a similar amount of exercise, and at least 1 h of exercise once a week was necessary to maintain the enlargement of ECA.20