Developmental aspects of beta-adrenergic receptors and catecholamine-sensitive adenylate cyclase in rat myocardium.

Abstract

Summary: The ontogeny of β-adrenergic receptors and the characteristics of catecholamine sensitive adenylate cyclase activity were demonstrated in rat ventricular myocardium. β-Adrenergic receptors were identified by the binding of (—)-[3H]dihydroalprenolol, [(—)-[3H]DHA|, and (±)-[125I]iodohydroxy-benzylpindolol, (±)-[125[I]HYP. Heart weight, protein, and the number of β-adrenergic receptors per cell increased with advancing age; however, binding capacity (Bmax) per nig protein was lower in membrane from the adult rat (220 g body weight) than in membrane from the fetal or neonatal rat, 36.8 ±4.1 and 58.3 ± 6.3 fmoles·mg−1 protein, respectively, mean ± SE, P < 0.01. The age-dependent increase in the numbers of receptors per heart correlated well with the increases in ventricular weight. In contrast, protein content, Na+, K+-ATPase, 5′-nucleotidase, and adenylate cyclase content increased more rapidly than ventricular weight, and the developmental increase in [3H]DHA binding sites (per heart) did not keep up with increases in ventricular protein. Although specific activity of (—)-[3H]DHA binding was lower in the adult ventricle, the characteristics of the β-receptor did not vary with age; the subtype was β1 determined by classical agonist competition in fetal and adult ventricular membranes and the β1- and β2-adrenergic receptor subtypes were present in the ratio of 75% β1 and 25% β2 in both fetal and adult samples. Guanine nucleotides decreased agonist affinity for the inhibition of (±)-[125I]HYP binding equally in adult and fetal samples and enhanced the activation of adenylate cyclase by catecholamine. Basal adenylate cyclase activity was significantly higher in weanling, 13.4 ± 1.42, and adult, 14.2 ± 5.8, than in 21-day fetal or newborn ventricular membrane, 8.18 ± 1.5 pmoles cyclic adenosine 3′:5′-monophosphate produced mg−1· min−1 (mean ± S.D.). The percent activation by catecholamine (with and without GTP) was also slightly higher in samples from the older rats, P < 0.01. In the adult and weanling heart, both guanine triphosphate (GTP) and guanyl-5′-yl-imidodiphosphate [Gpp(NH)p] were effective in enhancing catecholamine sensitive activity. Maximal catalytic activation of adenylate cyclase was observed in the presence of (—)-epinephrine and Gpp(NH)p and was significantly higher (fold stimulation) in fetal or neonatal ventricular samples as compared to adult or weanling ventricular samples, P < 0.01. Speculation: The decreased specific activity (per mg protein) of β-adrenergic receptors in adult as compared to fetal and neonatal myocardium may therefore be related to the decreasing proportion of sarcolemmal protein versus total and particulate cell protein which occurs during hypertrophic cardiac growth, supporting the hypothesis that sarcolemmal surface area is a major determinant of β-receptor number. In contrast, sarcolemmal activities which mediate cellular functions by regulating the concentration of various substances within the cell, such as Na+, K+-ATPase, and adenylate cyclase, increased to a greater extent than (—)-[3H]DHA binding, and were more directly proportional to cell protein. The function and affinity of these β-adrenergic receptors in the rat myocardium are mediated by guanine nucleotides early in development, and age-dependent differences in the activation of catecholamine sensitive adenylate cyclase by GTP and Gpp(NH)p may relate to developmental differences in the properties of guanine nucleotide-dependent factors which mediate receptor occupancy and cyclic adenosine 3′:5′-monophosphate formation.