[Pharmacological studies on alterations in myocardial beta-adrenoceptors and their intracellular signal transduction in experimental diabetic rats]

Abstract

The present study was undertaken to determine abnormalities which are responsible for the diminished functional responsiveness to beta-adrenoceptor stimulation in experimental diabetic rats. Rats were given an intravenous injection of 45mg/kg streptozotocin and hearts were removed from 4 to 6 weeks later. The positive inotropic effects of isoproterenol, norepinephrine and epinephrine were markedly depressed in diabetic rat papillary muscles. Diabetic cardiac muscles also exhibited a reduced maximum contractile responses to forskolin, 3-isobutyl-1-methylxanthine and dibutylic cyclic AMP. The density of beta-adrenoceptors in membranes prepared from diabetic hearts was decreased by 40%, with uniform decreases in the beta 1- and beta 2-subtypes. The affinity of the receptor for the radioligand antagonist [125I]-iodocyanopindolol remained unchanged. Competition binding studies with isoproterenol did not reveal a difference in the fraction of beta-adrenoceptors with high-affinity binding between control and diabetic cardiac membranes, suggesting that interaction between beta-adrenoceptors and Gs may be preserved well in the diabetic state. All measures of adenylate cyclase activity showed that beta-adrenoceptor-dependent and Gs-dependent cyclic AMP productions are well maintained in membranes from diabetic hearts. Thus, these data suggest that the decreased positive inotropic response to beta-adrenoceptor stimulation in diabetic hearts is not attributable to changes in beta-adrenoceptor-Gs-adenylate cyclase system but rather may be due to an alteration in cellular function beyond the level of cAMP generation. To test this hypothesis, incorporation of [32P]-inorganic phosphate into phospholamban in sarcoplasmic reticulum was examined in Langendorff-perfused hearts. Isoproterenol (100nM) increased phosphorylation of phospholamban threefold in control hearts, but did not cause a significant change in the phosphorylation state in diabetic hearts. From these findings it is concluded that the decreased functional responses to cyclic AMP-increasing agents like beta-adrenoceptor agonists in diabetic hearts may be associated with impaired phosphorylation of cardiac regulatory phosphoproteins including phospholamban.