DIFFERENTIAL DISTRIBUTION OF POSTJUNCTIONAL ALPHA-2 ADRENOCEPTORS IN HUMAN OMENTAL SMALL ARTERIES

A pharmacologic characterization of the postjunctional α adrenoceptors of human omental small arteries was performed using a microvascular myograph. Both the α1-selective agonist phenylephrine in the presence of yohimbine (0.3 μM, α2-selective antagonist) and the α2-selective agonist B-HT 933 in the presence of prazosin (0.3 μM, α1-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 < 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 μM) were more marked in the smaller rather than the larger vessels (r = -0.71; p < 0.01), whereas the opposite was observed using 0.1 μM prazosin (r = 0.84; p < 0.001). These data suggest that both α1 and α2 adrenoceptors are present in the human omental small arteries, but whereas postjunctional α2 adrenoceptors appear to be located predominantly in the resistance arteries, postjunctional α1 adrenoceptors appear to be distributed evenly along the arterial tree.