Enhanced Contractile Responses of Arteries from Diabetic Rats to α1-Adrenoceptor Stimulation in the Absence and Presence of Extracellular Calcium

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Abstract
The purpose of this study was to determine the receptor subtype mediating enhanced contractile responses of aortae and mesenteric arteries from diabetic rats to the .alpha.-adrenoceptor agonists, norepinephrine, clonidine, and methoxamine and to establish whether the enhanced responses are associated with increased release of intracellular Ca2+ or are dependent on the presence of extracellular Ca2+. The pA2 values calculated for the .alpha.1-selective antagonist prazosin were similar in arteries from control and diabetic rats and were 2.5-3 log units greater than pA2 values calculated for the .alpha.2-selective antagonist yohimbine. Contractile responses of aortae incubated in Ca2+-free Krebs'' solution containing 1 mM EGTA to maximal but not to ED50 concentrations of the .alpha.-adrenoceptor agonists were significantly greater in preparations from diabetic than from control rats. Following readdition of Ca2+, contractile responses of diabetic aortae to all concentrations of the agonists tested were significantly greater than control. Similar results were obtained in mesenteric arteries, except that no clonidine response could be detected in preparations from either diabetic or control rats in the absence of extracellular Ca2+. These data indicate that enhanced responses of arteries from diabetic rats to .alpha.-adrenoceptor agonists are mediated by .alpha.1 adrenoceptors, and are largely dependent on the presence of extracellular Ca2+. However, increased release of intracellular Ca2+ appears to contribute to enhanced responses to high concentrations of these agonists.