Effect of isoproterenol on myocardial mechanical function and cyclic AMP content in the fetal rabbit

The effect of isoproterenol on mechanical function was studied in the isolated arterially perfused heart of the fetal (21st and 28th day of gestation) and newborn rabbits. The inotropic effect of isoproterenol in the fetus was less than in the newborn. In contrast, myocardial cyclic AMP levels after isoproterenol infusion in the fetus were greater than in the newborn. The inotropic effects of dibutyryl cyclic AMP and of calcium in the fetus were less than in the newborn. These data suggest that the process from beta-receptor to cyclic AMP in the fetus was equally or even more responsive to isoproterenol than in the newborn. The diminished inotropic effect of isoproterenol in the fetus may be due, at least in part, to the decreased inotropic effect of calcium.