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 underlying the impaired contractile response to beta adrenoceptor stimulation in diabetic hearts. Chronic diabetes was induced in rats by a streptozotocin injection. Four to six weeks later, papillary muscles isolated from diabetic hearts exhibited marked reductions in the positive inotropic responses to isoproterenol, norepinephrine and epinephrine. The contractile responses to forskolin, 3-isobutyl-1-methylxanthine and dibutylic cyclic AMP were also prominently depressed. The density of beta adrenoceptors was decreased by 50%. However, competitive binding studies with isoproterenol showed no difference in the proportion of beta adrenoceptors with high-affinity binding between control and diabetic myocardial membranes. Determination of the levels of the alpha subunits of Gs and Gi by immunoblotting revealed markedly less expression of Gi in diabetic myocardium. The abilities of isoproterenol, sodium fluoride, 5'-guanylyl imidodiphosphate and forskolin to stimulate adenylate 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 forskolin 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-Gs-adenylate cyclase system, but is possibly caused by an alteration in cellular function beyond the step of adenylate cyclase activation.