Differential distribution of postjunctional alpha 2 adrenoceptors in human omental small arteries.

A pharmacologic characterization of the postjunctional alpha adrenoceptors of human omental small arteries was performed using a microvascular myograph. Both the alpha 1-selective agonist phenylephrine in the presence of yohimbine (0.3 microM, alpha 2-selective antagonist) and the alpha 2-selective agonist B-HT 933 in the presence of prazosin (0.3 microM, alpha 1-selective antagonist) evoked strong vasoconstriction. The responses to phenylephrine were competitively antagonized by prazosin, whereas the responses to B-HT 933 were competitively antagonized by yohimbine; the pA2-values were 9.24 and 8.34, respectively. An inverse relationship of the maximum response to B-HT 933 (r = -0.53; p less than 0.001), but not to phenylephrine or to norepinephrine (NE), against artery caliber was observed. The shifts of the NE concentration-response curves caused by yohimbine (0.1 microM) were more marked in the smaller rather than the larger vessels (r = -0.71; p less than 0.01), whereas the opposite was observed using 0.1 microM prazosin (r = 0.84; p less than 0.001). These data suggest that both alpha 1 and alpha 2 adrenoceptors are present in the human omental small arteries, but whereas postjunctional alpha 2 adrenoceptors appear to be located predominantly in the resistance arteries, postjunctional alpha 1 adrenoceptors appear to be distributed evenly along the arterial tree.