Effect of maturation and aging on beta-adrenergic signal transduction in rat kidney and liver

The characteristics of the β-adrenergic signal transduction system were analyzed in kidney and liver membrane preparations from neonatal (2–3 days), mature (2 months), and old (2 years) rats. When comparing kidneys from adult to neonatal rats, we found a higher β-receptor density and a higher percentage of β₁-receptor subtype, lower immunoreactive G_s-protein, a lower ratio between the high and low molecular weight splice variant of G_s, lower immunoreactive G_i-protein, and lower basal adenylate cyclase activity. When comparing livers from adult to neonatal rats, we found lower β-receptor density and basal adenylate cyclase activity. Very few differences could be detected when comparing mature to old kidneys or livers. Stimulated adenosine 3′,5′-cyclic monophosphate (cAMP) synthesis was tissue- and age-dependent. In liver, G-protein- and β-receptor-stimulated cAMP synthesis mirrored basal adenylate cyclase activity and was highest in liver from neonatal animals. In contrast, cAMP synthesis was significantly more stimulated in kidneys from mature animals than from neonatal and senescent rats. We conclude that: (i) the stoichiometry of the components within the β-receptor/G-protein/adenylate cyclase complex is not fixed but is both tissue- and age-dependent; (ii) adenylate cyclase enzyme activity is possibly but not necessarily the rate-limiting step in the β-receptor-mediated synthesis of cAMP; and (iii) there is in vivo evidence for a preferential co-expression of the large splice variant of the G_s-protein and β₂-receptor subtype. It is speculated that this could have important physiological consequences for the development of the kidney.