Ontogeny of G-protein expression: control by β-adrenoceptors

Abstract

Cardiac cell homeostasis is maintained in the face of excessive β-adrenoceptor stimulation through the process of desensitization. Desensitization is not an inherent property of these cells but rather is acquired during development; neonates given β-agonists actually show heterologous sensitization, involving changes in the expression and catalytic activity of adenylyl cyclase (AC) as well as an increased receptor/G-protein coupling. The current study examines the role of specific G-protein components, G sα and G iα, in the ontogeny of β-adrenoceptor responses and in the transition from agonist-induced sensitization to desensitization. Between postnatal days (PN) 6 and 15 there was a significant decrease in the 52 kDa isoform of G sα with no accompanying change of the 45 kDa form; over the same period, G iα3 also declined substantially. In contrast, the 45 kDa isoform of G sα and G iα1,2 remained fairly constant over the same period and fluoride-stimulated AC activity increased. Treatment with isoproterenol on PN2–5 did not result in any significant changes in G sα expression but robustly decreased G iα1,2. These changes were accompanied by heterologous sensitization of AC activity at the level of AC itself, evidenced by equivalent increases in the enzymatic response to fluoride and forskolin–Mn 2+. Isoproterenol given to older animals (PN11–14) also caused specific loss of G i protein, in this case targeting G iα3, whereas G sα again was unchanged; in contrast to the younger group, the older animals displayed heterologous desensitization of AC at the level of G-protein function (specific loss of the fluoride response). These results indicate that the normal ontogenetic increase of cardiac β-adrenoceptor coupling to AC is not dependent on the absolute amount of G-proteins, nor on the relative balance of stimulatory (G s) and inhibitory (G i) subunits. However, the ability of receptor stimulation to downregulate G iα1,2, an event which is specific to immature cardiac cells, is likely to be an important component of the resistance of the fetal/neonatal heart to agonist-induced desensitization and hypertrophy. The maintenance of cardiac β-adrenoceptor signaling in the face of intense stimulation is likely to play an important role in the physiologic adaptations necessary to the perinatal transition.