Mechanisms of impaired β-adrenergic receptor signaling in G(αq)- mediated cardiac hypertrophy and ventricular dysfunction

Targeted cardiac overexpression of the α-subunit of the heterotrimeric G protein G(q) in transgenic mice evokes hypertrophy and depressed stimulation of cardiac inotropy and chronotropy by β-adrenergic receptor (βAR) agonists in vivo, which is a hallmark of many forms of experimental and human heart failure. The molecular basis of this βAR dysfunction was explored in transgenic mice overexpressing G(αq) ~5-fold over background. Isoproterenol-stimulated adenylyl cyclase activities in myocardial membranes were significantly depressed in G(αq) mice compared with nontransgenic controls (19.7 ± 2.6 versus 43.7 ± 5.6 pmol/min/mg) without a decrease in βAR expression levels. Functional coupling of both βAR subtypes was impaired. Similarly, in whole-cell patch-clamp studies, βAR stimulation of L-type Ca²⁺ channel currents was depressed ~75% in the G(αq) mice. Cardiac βAR from these mice showed decreased formation of the active high- affinity conformation (R(H) = 29% versus 62% for nontransgenic littermates), confirming a receptor-G(s)-coupling defect. Of the three candidate kinases that might impose this uncoupling by receptor phosphorylation (protein kinase A, βAR kinase, protein kinase C), only protein kinase C activity was elevated in G(αq) mouse hearts. Type V adenylyl cyclase was decreased ~45% in these mice, consistent with decreased basal, NaF, and forskolin-stimulated enzyme activities. Although cellular G(s) levels were unaltered, G(i2) and G(i3) were increased in G(αq) mice. Pertussis toxin treatment of isolated G(αq) myocytes resulted in an improvement in βAR, but not that of forskolin or NaF, stimulation of adenylyl cyclase. This three distinct mechanisms contribute to impaired βAR function by in vivo G(q) signaling cross-talk in myocytes. Because many elements of hypertrophy and/or failure in cellular and animal models can be initiated by increased G(αq) signaling, the current work may be broadly applicable to interfaces whereby modification of heart failure might be considered.