Effects of exercise and its cessation on the heart and its blood supply.

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Abstract
Studies were carried out in rats on effects of daily and intermittent exercise and its cessation on heart size and on cross-sectional luminal areas of extracoronary collateral (ECA) and coronary arteries (CA) and on capillary-ventricular muscle fiber ratios (CFR). For 10 weeks young male rats swam for 1-hr. periods daily (I) or twice weekly (n). These animals and unexercised controls were sacrificed at the end of 10 weeks exercise or 14-42 days following cessation of exercise. Ventricular dry weights were determined, ventricular body weight ratios calculated, and histological sections prepared. At the end of the exercise period, body weights in both I and II were below controls. Cardiac hypertrophy with an associated increase in CA area was observed only in I. ECA areas and CFR were significantly increased over control levels in both groups. After cessation of exercise, ventricular weights regressed rapidly, dipping transiently below control levels at 14-28 days, while body weights increased markedly. Following this initial drop, ventricular weight increased along with body weight. Although CA area regressed at about the same rate as ventricular weight, ECA area and CFR remained significantly greater than controls in I at 42 days postexercise.