Qualitative level of GTP-binding proteins in diabetic rat myocardium and its biological significance in alteration of beta-adrenoceptor mediated cellular response.

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This research project has aimed to clarify which of ADP-ribosylation method or immunoblotting method is adequate for estimating GTP binding proteins (G) in diabetic rat myocardium. When estimated by ADP-ribosylation method, we found increased level of both Gs and Gi, but immunoblotting with anti-Gs alpha-and Gi alpha-subunit antiserum indicated decreased level of Gi and unchanged level of Gs. In diabetic rat myocardium, we observed : 1. augmented activation of adenylate cyclase (AC) , when stimulated by beta-adrenoceptors (R) , GppNHp, NaF and forskolin in comparison with the AC activities in control normal rat myocardium, 2. decreased level of GimRNA and unchanged level of Gs mRNA level. Under the hypothesis that the decreased level of Gi protein which gives tonic inhibitory influence on Gs-AC system, causes the alterations of AC activities in diabetic state, we examined influence of functional abolishment of Gi protein on R-Gs-AC system in PTX-treated rabbit myocardium. we found that PTX-treated myocardium produced augmentation of AC activities in response to beta-adrenoceptor stimulation and GppNHp, but, not to NaF and forskolin. Thus, even though there are a few discrepancy, the altered response of AC activities in diabetic myocardium is well explained by the decreased level of Gi. From these results, we conclude that G protein level estimated by immnoblotting method reflects well alterations in R-Gs-AC coupling in diabetic myocardium.