Alterations of adenylyl cyclase and G proteins in aortocaval shunt-induced heart failure

Abstract

Unlike most other experimental models of congestive heart failure, the volume overload model induced by aortocaval shunt (AVS) in rats was found to exhibit enhanced β-adrenoceptor (β-AR) signaling. To study whether the adenylyl cyclase (AC)-G protein system is involved in such a change, we examined cardiac AC activity and protein content as well as Gsα and Giα activities, protein contents, and mRNA levels in both left (LV) and right (RV) ventricles at the failing stage (16 wk after surgery). Basal and forskolin-stimulated AC activities were significantly increased in both LV and RV from the failing hearts; this change was associated with an upregulation of type V/VI AC protein. In contrast to 5′-guanylyl imidodiphosphate and NaF, the stimulatory effect of isoproterenol on AC was increased in the failing heart. Although Gsα and Giα protein contents in the failing hearts were not altered, the mRNA level for Gsα was decreased by 20% and that for Giα was increased by 20%. In addition, the activity of Gsα, but not Giα, as assessed by toxin-catalyzed ADP ribosylation, was significantly decreased in the failing heart. Losartan and imidapril treatments improved cardiac function and attenuated alterations in mRNA levels for Gsα and Giα proteins, as well as Gsα activity, without affecting changes in AC protein content or activities in heart failure due to volume overload. These data suggest that increased AC activity may contribute to the enhanced β-AR signaling in the AVS model of heart failure, whereas alterations in gene expression for G proteins may be of an adaptive nature at this stage of heart failure.