Aj. Natali et al., Regional effects of voluntary exercise on cell size and contraction-frequency responses in rat cardiac myocytes, J EXP BIOL, 204(6), 2001, pp. 1191-1199
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.