IMPAIRED CONTRACTILE RESPONSE TO BETA-ADRENOCEPTOR STIMULATION IN DIABETIC RAT HEARTS - ALTERATIONS IN BETA-ADRENOCEPTORS G-PROTEIN ADENYLATE-CYCLASE SYSTEM AND PHOSPHOLAMBAN PHOSPHORYLATION

The aim of this study was to explore the cellular mechanisms underlyin g the impaired contractile response to beta adrenoceptor stimulation i n diabetic hearts. Chronic diabetes was induced in rats by a streptozo tocin injection. Four to six weeks later, papillary muscles isolated f rom diabetic hearts exhibited marked reductions in the positive inotro pic responses to isoproterenol, norepinephrine and epinephrine. The co ntractile responses to forskolin, 3-isobutyl-1-methylxanthine and dibu tylic cyclic AMP were also prominently depressed. The density of beta adrenoceptors was decreased by 50%. However, competitive binding studi es with isoproterenol showed no difference in the proportion of beta a drenoceptors with high-affinity binding between control and diabetic m yocardial membranes. Determination of the levels of the alpha subunits of G(s) and G(i) by immunoblotting revealed markedly less expression of G(i) in diabetic myocardium. The abilities of isoproterenol, sodium fluoride, 5'-guanylyl imidodiphosphate and forskolin to stimulate ade nylate cyclase were preserved well in membranes prepared from diabetic hearts. Nevertheless, neither stimulation of beta adrenoceptors with isoproterenol nor direct activation of adenylate cyclase with forskoli n evoked any significant increase in the degree of phosphorylation of phospholamban in diabetic hearts. These results suggest that impaired contractile response to beta adrenoceptor stimulation is not caused by an alteration in the beta adrenoceptors-G(s)-adenylate cyclase system , but is possibly caused by an alteration in cellular function beyond the step of adenylate cyclase activation.