Regulation of myocardial β-adrenergic receptor function in adult and neonatal rabbits

beta-Adrenergic receptor (beta AR) desensitization is the decrease in response following sustained agonist stimulation by catecholamines. While developmental changes in beta AR response have been well documented in the mammalian heart, much less is known regarding the regulation of beta AR function in immature hearts. The purpose of the present study was to determine whether there are developmental differences in myocardial beta AR desensitization. We used an isolated heart preparation to examine the beta AR-mediated inotropic response before and after sustained exposure to 1 mu M isoproterenol in adult and neonatal rabbits. We also assayed the adenylyl cyclase activity and performed radioligand-binding studies to determine beta AR characteristics in adult and neonatal ventricular tissues with and without exposure to isoproterenol. Both adult and neonatal rabbit hearts showed a concentration-dependent increase in systolic function, namely, isovolumic left ventricular developed pressure (LVDP) and maximal positive dP/dt of LVDP (dP/dt(max)) in response to isoproterenol. Adults, however, showed a significantly greater response than neonates. After sustained exposure to isoproterenol, the subsequent beta AR-mediated responses in LVDP and dP/dt(max) were significantly attenuated in adults, but much less so in neonates. The adenylyl cyclase activity in response to isoproterenol was significantly different between adult, but comparable in neonatal tissues both exposed or not to isoproterenol. The total beta AR density was higher in neonatal than in adult tissues without isoproterenol exposure, but there was no significant change in beta AR density in either group following isoproterenol exposure. In addition, isoproterenol exposure increased the amount of the inhibitory G protein in adult, but not neonatal tissues. Our results suggest that there were developmental differences in myocardial beta AR functional responses in beta AR desensitization.