Attenuated intracellular Ca 2+ response to beta-adrenoceptor stimulation is due to conversion of a 45 kDa Gs-alpha isoform to a 52 kDa Gs-alpha isoform in the heart of chronically hypoxic rat

Abstract

The cardiac responses to β-adrenoceptor stimulation are attenuated in hypertrophied and failing heart. In addition to down-regulation of the β-adrenoceptor receptor it is believed that the intracellular signaling mechanisms may also be altered. In the present study we determined firstly the electrically induced intracellular Ca2+ ([Ca2+]i) transient in response to activation of β-adrenoceptor with isoproterenol (Iso), Gs protein with cholera toxin (CTX) and adenylate cyclase (AC) with forskolin in isolated myocytes of the right hypertrophied rat induced by treatment with 10% oxygen for 4 weeks. The electrically induced intracellular [Ca2+]I transient was determined with a soectrofluorometric method using fura-2A as Ca2- indicator. 1µM Iso, 20µg/ml CTX for 6 h and 1µM forskolin increased the electrically induced [Ca2+]I transient in both types of rats. The response to Iso and CTX was significantly attenuated, ehile that to forskolin not changed, in the right ventricle of chronically hypoxic rats. Secondly we determined with an immunoblotting method the contents of the 45kDa and a 52kDa isoforms of Gsα in the right ventricle of normoxic and hypoxic rats. We found that the 45kDa isoform was significantly reduced, while the 52 kDa protein increased, in the ventricle of hypoxic rats. Since the 45kDa isoform is the active isoform, the attenuated Ca2+ response to activation of Gs-protein may be due to the conversion of the active insoform to the less active isoform in the heart of hypoxic rats. (The study was supported by a grant from the Institute of Cardiovascular Science and Medicine).