Volume overload cardiac hypertrophy exhibits decreased expression of Gsα and not of Giα in heart

Abstract

We have recently reported enhanced levels of Giα proteins in genetic and other experimentally induced models of hypertension, whereas the levels of Gsα were decreased in hypertensive rats expressing cardiac hypertrophy. The present studies were undertaken to investigate whether the decreased levels of Gsα are associated with cardiac hypertrophy per se and used an aortocaval fistula (AV shunt; volume overload) rat model that exclusively expresses cardiac hypertrophy. Cardiac hypertrophy in Sprague-Dawley rats (200–250 g) was induced under anesthesia, and, after a period of 10 days, the hearts were used for adenylyl cyclase activity determination, protein quantification, and mRNA level determination. A temporal relationship between the expression of Gsα proteins and cardiac hypertrophy was also examined on days 2, 3, 7, and 10 after induction of AV shunt in the rat. The heart-to-body-weight ratio (mg/g) was significantly increased in AV shunt rats after 3, 7, and 10 days of induction of AV shunt compared with sham-operated controls, whereas arterial blood pressure was not different between the two groups. Guanosine 5′- O-(3-thiotriphosphate) (GTPγS) stimulated adenylyl cyclase activity in a concentration-dependent manner in heart membranes from both groups; however, the degree of stimulation was significantly decreased in AV shunt rats. In addition, the stimulatory effects of isoproterenol were also diminished in AV shunt rats compared with control rats, whereas glucagon-stimulated adenylyl cyclase activity was not different in the two groups. The inhibitory effects of oxotremorine (receptor-dependent Gi functions) and low concentrations of GTPγS on forskolin-stimulated adenylyl cyclase activity (receptor-independent Gi functions) were not different in the two groups. In addition forskolin and NaF also stimulated adenylyl cyclase activity to a lesser degree in AV shunt rats compared with control rats. The levels of Giα-2 and Giα-3 proteins and mRNA, as determined by immunoblotting and Northern blotting, respectively, were not different in both groups; however, the levels of Gsα45 and Gsα47, and not of Gsα52, proteins were significantly decreased in AV shunt rats by days 7 and 10 compared with control rats, whereas no change was observed on days 2 and 3 after induction of AV shunt. These results suggest that the decreased expression of Gsα proteins may not be the cause but the effect of hypertrophy and that the diminished responsiveness of adenylyl cyclase to GTPγS, isoproterenol, NaF, and forskolin in hearts from AV shunt rats may partly be due to the decreased expression of Gsα. It can be concluded from these studies that the decreased expression of Gsα may be associated with cardiac hypertrophy and not with arterial hypertension.