Gando S
Department of Pharmacology, Hokkaido University School of Medicine, Sapporo, Japan.
Hokkaido Igaku Zasshi. 1994 Sep;69(5):1140-53.
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.
本研究旨在确定导致实验性糖尿病大鼠对β-肾上腺素能受体刺激的功能反应性降低的异常情况。给大鼠静脉注射45mg/kg链脲佐菌素，4至6周后取出心脏。糖尿病大鼠乳头肌中异丙肾上腺素、去甲肾上腺素和肾上腺素的正性肌力作用明显降低。糖尿病心肌对福斯可林、3-异丁基-1-甲基黄嘌呤和二丁酰环磷酸腺苷的最大收缩反应也降低。糖尿病心脏制备的膜中β-肾上腺素能受体密度降低了40%，β1和β2亚型均均匀降低。受体对放射性配体拮抗剂[125I]-碘氰吲哚洛尔的亲和力保持不变。用异丙肾上腺素进行的竞争结合研究未发现对照和糖尿病心脏膜之间高亲和力结合的β-肾上腺素能受体分数有差异，这表明在糖尿病状态下β-肾上腺素能受体与Gs之间的相互作用可能保存良好。腺苷酸环化酶活性的所有测量结果表明，糖尿病心脏膜中β-肾上腺素能受体依赖性和Gs依赖性环磷酸腺苷生成保持良好。因此，这些数据表明，糖尿病心脏对β-肾上腺素能受体刺激的正性肌力反应降低并非归因于β-肾上腺素能受体-Gs-腺苷酸环化酶系统的变化，而是可能由于细胞功能在环磷酸腺苷生成水平之上发生了改变。为了验证这一假设，在Langendorff灌注心脏中检测了[32P]-无机磷酸掺入肌浆网中受磷蛋白的情况。异丙肾上腺素（100nM）使对照心脏中受磷蛋白的磷酸化增加了三倍，但在糖尿病心脏中未引起磷酸化状态的显著变化。从这些发现可以得出结论，糖尿病心脏对β-肾上腺素能受体激动剂等环磷酸腺苷增加剂的功能反应降低可能与包括受磷蛋白在内的心脏调节磷蛋白的磷酸化受损有关。
