Regional Differences in the Regulation of Vascular Smooth Muscle Relaxation by the Endothelium in Health and in Diabetes

Abstract

1 Monash University, Melbourne, Australia. Correspondence should be addressed to M. Tare (e-mail: marianne.tare@med.monash.edu.au). The potent vasodilator influence of the vascular endothelium is compromised in diabetes, and this contributes to the increased vascular tone and reduced tissue perfusion characteristics of this disease. Regional differences exist in the extent of this vasodilator dysfunction, and in larger arteries the focus has been on nitric oxide (NO). In smaller arteries and arterioles, it is emerging that the actions of endothelium-derived hyperpolarizing factor (EDHF) are diminished in diabetes. In the mesenteric arteries of streptozotocin (STZ)-induced diabetic rats, smooth muscle (SM) hyperpolarization and relaxation attributed to EDHF is halved. It is unclear whether the dysfunction is due to changes in the generation of hyperpolarization in the endothelial cells (EC), or to spread of hyperpolarization to the SM via myoendothelial gap junctions (MEGJ), or in the SM response. In our study, we addressed this question. Responses were compared with those of the femoral artery where the entire endothelium-dependent relaxation is mediated by NO and is resistant to diabetes. Male Wistar rats were injected either with 60 mg/kg STZ dissolved in citrate buffer or with the buffer alone (control) into the tail vein, and 8 weeks after injection they were killed by exsanguination. Segments of the mesenteric and femoral arteries were isolated, secured to a recording chamber (or a myograph), and superfused with warmed oxygenated physiological saline. To isolate the EDHF response, 10 M N-nitro-L-arginine methyl ester and 10 M indomethacin were added to this saline. Intracellular glass microelectrodes with tips filled with 2% Lucifer Yellow CH were used to record the membrane potential of EC and SMC. Blood glucose levels were 6-fold higher in STZinjected rats. Acetylcholine evoked concentrationdependent hyperpolarization in the mesenteric artery EC and SMC of control rats. Hyperpolarization in EC was halved in diabetic rats, and this accounts for the reduced hyperpolarization and relaxation in SMC. Hyperpolarization, selectively evoked in the SMC by the ATP-sensitive potassium channel opener levcromakalim, spread to the EC via MEGJ, and this was not altered by diabetes. Femoral arteries lack MEGJ; the fact that the ability of EC to generate hyperpolarization is not influenced by diabetes is intriguing. Impaired generation of EC hyperpolarization accounts for the reduction in EDHF-mediated SMC hyperpolarization and relaxation in the mesenteric arteries in diabetes. The EC–SMC communication via MEGJ remains functional in diabetes. In contrast, EC hyperpolarization was preserved in the femoral arteries. Thus, there are regional differences in the impact of diabetes on EC hyperpolarization, and this may be related to the existence/absence of electrical and/or chemical contacts between EC and SMC.