AJP - Heart and Circulatory Physiology, Vol 242, Issue 1 13-H18, Copyright © 1982 by American Physiological Society
Comparative response of the developing canine myocardium to inotropic agents
I. S. Park, L. H. Michael and D. J. Driscoll
Developmental changes in cardiac muscle function were assessed in isolated
dog ventricular muscle strips obtained from dogs of three age groups: 2
day, 4 wk, and adult. Active tension (AT) and maximum rate of contraction
and relaxation (+dT/dt, -dT/dt) increased significantly with age. Total
contraction time increased with age. Time to peak tension increased between
4 wk and adulthood, and relaxation time increased between 2 day and 4 wk (P
less than 0.05). The degree of post-"extra stimulus" potentiation in
response to a single premature extra stimulus (SPES) or to sustained paired
stimulation (SPS) was similar in all age groups. However, a longer period
of SPS was necessary to reach peak AT in young animals than in adults. AT,
+dT/dt, -dT/dt, and (-dT/dt)/T increased significantly in all age groups in
response to increasing concentrations of isoproterenol. However, while
(+dT/dt)/T increased significantly in the 4-wk and adult groups, it
remained unchanged from control in the 2-day group with increasing
concentration of isoproterenol. The ED50 of isoproterenol was significantly
greater in the 2-day and 4-wk groups than in the adult group. AT increased
significantly in young muscles but not in adult muscles when additional
calcium was added to the bath containing the highest concentration of
isoproterenol. In adult muscles SPS produced the greatest AT but not in the
2-day group, which required increased calcium concentration as well as
isoproterenol. Maturation of several membrane systems is implicated.
