Relaxation of rat aorta by adenosine in diabetes with and without hypertension: role of endothelium.

Effects of diabetes on the responses of aortic rings of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rat to adenosine analogues were examined. Streptozotocin-induced diabetes caused an increase in blood glucose and plasma levels of cholesterol and triglycerides in normotensive (diabetic-WKY) as well as hypertensive (diabetic-SHR) rats. In diabetic-SHR group, the body weight was significantly low (50%) as compared to SHR (non-diabetic). Diabetic-SHR group showed the largest heart weight-to-body weight ratio indicating cardiac enlargement. The relaxation responses to adenosine analogues were obtained in endothelium-intact and -denuded aortic rings precontracted with phenylephrine. The IC(50) values of adenosine analogues were lower in endothelium-intact aortic rings of WKY as compared to diabetic-WKY and -SHR. Aortic rings from diabetic-SHR showed the greatest attenuation in adenosine analogue-mediated relaxation. Removal of endothelium from the aortic rings inhibited the relaxant response of adenosine analogues and abolished the differences among the groups. Nitric oxide (NO) synthase inhibitor L-monomethylarginine (L-NMMA) caused a significant rightward shift in the concentration-response curves in WKY and diabetic-WKY groups, only a small shift in SHR and no change in diabetic-SHR group indicating that it is primarily the inhibition of NO release which is responsible for attenuation of adenosine receptor responses in SHR and diabetic-WKY and there was absence of NO release in diabetic-SHR. Forskolin and sodium nitroprusside equally relaxed the aortic rings in all the groups. This suggested that there was no abnormality in the relaxant property of vascular smooth muscle due to hypertension and/or diabetes. Therefore, it is concluded that streptozotocin-induced diabetes in SHR aggravates the severity of vascular endothelial dysfunction which led to impairment in adenosine receptor-mediated vascular responses.