IS RECEPTOR CROSS-REGULATION IN HUMAN HEART CAUSED BY ALTERATIONS IN CARDIAC GUANINE-NUCLEOTIDE-BINDING PROTEINS

1. Guanine nucleotide-binding proteins (G-proteins) play a central role in signal transduction between a wide variety of cell-surface receptors and intracellular second messenger systems. Recently, we and others have demonstrated that cross-regulation can occur between a variety of G-protein-linked receptors in human heart. Chronic beta1-adrenoceptor blockade gives rise to sensitization of beta2-adrenoceptor and of 5HT4-receptor responses, both of which are mediated via stimulation of adenylate cyclase through stimulatory G-proteins (G(s)), and also gives rise to desensitization of muscarinic M2-receptor responses, which inhibit adenylate cyclase through inhibitory G-proteins (G(i)). 2. In order to investigate whether these effects are due to quantitative changes in cardiac G-protein isoforms, we measured their abundance in right atrial appendage from patients taking or not taking beta1-adrenoceptor antagonists, by immunoblotting. 3. Samples of right atrial appendage homogenate were subjected to SDS/PAGE, and proteins were electroblotted on to nitrocellulose membranes. These were then probed with specific anti-G protein antisera, and binding was revealed by means of a secondary antibody labelled with alkaline phosphatase and using a chromogenic substrate. The resulting bands were quantified by laser densitometry. 4. No quantitative differences were detected, between these two groups of patients, in the amounts of alpha-subunit of 'long' or 'short' G(s) isoforms (G(s)alphaL and G(s)alphaS), or in the amounts of G(i)1+2 alpha-subunit (G(i)alpha1+2). Nor was any difference found in the abundance of the beta-subunit of G-proteins. No 'other' G-protein (G(o)) was detectable in these samples by immunoblotting. 5. We conclude that the phenomenon of receptor cross-regulation which we have previously observed in human right atrial appendage is unlikely to be explained by quantitative changes at the G-protein level.