title: Studies that question the existence of alpha-2 adrenoceptors in tail arteries of normotensive Sprague-Dawley rats.
abstract: Many pharmacological studies have demonstrated two distinct types of alpha adrenoceptor in the vasculature; these receptors have been named alpha-1 and alpha-2. In the present study, using isolated perfused tail arteries from normotensive Sprague-Dawley rats, we have demonstrated two types of alpha adrenoceptor but neither of these could be classified as an alpha-2 adrenoceptor. Dose-dependent contraction of rat tail arteries was produced by the following alpha adrenoceptor agonists or agonist-antagonist combination: phenylephrine (PE alpha-1), clonidine (alpha-1 and alpha-2), clonidine in the presence of 10(-7) M prazosin (alpha-2) and BHT-920 (alpha-2). The ED50 values for PE and clonidine were four orders of magnitude lower than those for clonidine plus prazosin and BHT-920. In addition, the action of PE was faster in onset than that of BHT-920, reached a higher maximum (5-fold) and attenuated more rapidly than that of BHT-920. The specific alpha-2 adrenoceptor antagonist yohimbine, in concentrations as high as 10(-6) M, did not antagonize arterial responses to BHT-920. However, responses to BHT-920 were antagonized by the alpha-1 adrenoceptor antagonist prazosin, in concentrations as low as 10(-10) M, and by the serotonin/alpha-1 adrenoceptor antagonist ketanserin (10(-7) M). These results suggest that the two alpha-adrenoceptor types in isolated rat tail arteries are both of the alpha-1 type. We also found that whereas responses to PE were stable and reproducible between 2 and 5 hr of arterial perfusion, responses to BHT-920 increased progressively over 5 hr. The latter effect probably resulted from a gradual disappearance of the arterial endothelium.
mesh_terms: D000818:Animals; D001158:Arteries / Q000032:analysis*; D001381:Azepines / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains / Q000033:anatomy & histology*; D011942:Receptors, Adrenergic, alpha / Q000032:analysis*; D013623:Tail / Q000098:blood supply*; D014661:Vasoconstriction / Q000187:drug effects; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D001381:Azepines; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D015016:Yohimbine; C024763:talipexole; D003000:Clonidine; D011224:Prazosin