Alpha-adrenoceptors in human resistance arteries from colon, pericardial fat, and skeletal muscle.

Human resistance arteries (144-332 microns diam) from colon, pericardial fat, and skeletal muscle were mounted in a myograph for measurements of isometric contractions under conditions of partial depolarization by potassium chloride. In all preparations, both phenylephrine (alpha 1-selective agonist) and B-HT 933 (alpha 2-selective agonist) evoked concentration-dependent contractions that were antagonized by the alpha 1-selective antagonist prazosin (10(-8) M) and the alpha 2-selective antagonist yohimbine (10(-7) M), respectively. The affinities (expressed as pKB values) of prazosin for the receptor mediating the responses to phenylephrine were 8.88-9.41, whereas the affinities of yohimbine for the receptor mediating the responses to B-HT 933 were 7.71-7.97. Norepinephrine (mixed alpha 1-agonist/alpha 2-agonist) also elicited concentration-dependent responses that were modestly, but significantly, antagonized by prazosin alone and yohimbine alone at the above-mentioned concentrations. The two antagonists in combination, however, effectively antagonized the responses to this agonist. These findings strongly suggest the presence of functional, postjunctional alpha 1- and alpha 2-adrenoceptors in isolated human resistance arteries from colon, pericardial fat, and skeletal muscle and that responses to norepinephrine in these vessels are mediated by both alpha-adrenoceptor subtypes.