Overexpression of G_sα protein in the hearts of transgenic mice

Alterations in β-adrenergic receptor-G_s-adenylyl cyclase coupling underlie the reduced catecholamine responsiveness that is a hallmark of human and animal models of heart failure. To study the effect of altered expression of G_sα, we overexpressed the short isoform of G_sα in the hearts of transgenic mice, using a rat α-myosin heavy chain promoter. G_sα mRNA levels were increased selectively in the hearts of transgenic mice, with a level 38 times the control. Despite this marked increase in mRNA, Western blotting identified only a 2.8-fold increase in the content of the G_sα short isoform, whereas G_s activity was increased by 88%. The discrepancy between G_sα mRNA and G_sα protein levels suggests that the membrane content of G_sα is posttranscriptionally regulated. The steady-state adenylyl cyclase catalytic activity was not altered under either basal or stimulated conditions (GTP + isoproterenol, GTPγS, NaF, or forskolin). However, progress curve studies did show a significant decrease in the lag period necessary for GppNHp to stimulate adenylyl cyclase activity. Furthermore, the relative number of β-adrenergic receptors binding agonist with high affinity was significantly increased. Our data demonstrate that a relatively small increase in the amount of the coupling protein G_sα can modify the rate of catalyst activation and the formation of agonist high affinity receptors.