Regional effects of voluntary exercise on cell size and contraction-frequency responses in rat cardiac myocytes

A model of voluntary exercise, in which rats are given free access to a run ning wheel over a l l-week period, led to left ventricular hypertrophy. To test whether the hypertrophic response to exercise was uniformly distribute d across the ventricular wall, single ventricular myocytes mere isolated fr om the sub-epicardium (EPI) and sub-endocardium (ENDO) of exercised rats an d from sedentary rats for comparison. Cellular hypertrophy (approximately 2 0% greater cell volume) was seen in ENDO cells from exercised animals, but no significant changes were observed in EPI cells when compared with sedent ary controls, This regional effect of exercise may be a response to transmu ral changes in ventricular wall stress and/or strain. Cell contraction was measured as cell shortening in ENDO and EPI cells at s timulation frequencies between 1 and 9 Hz at 37 degreesC. Exercise training had no effect on cell shortening. Positive and negative contraction-freque ncy relationships (CFRs) were found in both EPI and ENDO cells between 1 an d 5 Hz; at higher frequencies (5-9 Hz), ah myocytes displayed a negative CF R, The CFR of a myocyte was, therefore, independent of regional origin and unaffected by exercise, These results suggest that, in vivo, the rat heart displays a negative CFR. We conclude that increased cell size map be a more important adaptive response to exercise than a modification of excitation- contraction coupling.