It has been shown that the sensitivity to isoproterenol of myocardial inotropic response increases with growth in rabbits. In the present study we attempted to delineate the biochemical mechanisms involved in this maturation-related change. We investigated cyclic AMP generation and protein kinase activation by isoproterenol in the ventricular myocardium of rabbits aged 1 day, 1 week, and 1 month. In contrast to an increase in inotropic response, both the sensitivity to isoproterenol of cyclic AMP generation and the sensitivity to exogenous cyclic AMP of protein kinase activation decreased with maturation from 1 day to 1 month of age. ED50 of isoproterenol (mean +/- SE) on cyclic AMP generation in myocardial slices was 1.83 +/- 0.42 x 10(-7) M for 1-day-old, 3.70 +/- 0.61 x 10(-7) M for 1-week-old, and 8.32 +/- 1.20 x 10(-7) M for 1-month-old rabbits. Likewise, the ED50 of isoproterenol on adenylate cyclase activation was 0.65 +/- 0.11 x 10(-7) M, 2.04 +/- 0.32 x 10(-7), and 15.2 +/- 2.5 x 10(-7) M in 1-day, 1-week- and 1-month-old rabbits, respectively. The ED50 of cyclic AMP on protein kinase activation was 1.50 +/- 0.64 x 10(-8) M for 1-day-old and 7.08 +/- 1.52 x 10(-8) M for 1-month-old rabbits. This apparent discrepancy between inotropic response and biochemical response in the sensitivity to isoproterenol indicates that the biochemical mechanism of the maturation-related change in inotropic response was at a step or steps distal to protein kinase activation.