Pharmacological characterization of postjunctional alpha-adrenoceptors in isolated human omental arteries and veins.

The alpha-adrenoceptors in human omental arteries and veins were characterized and compared. In the arteries both prazosin (pA2 9.48) and rauwolscine (pA2 7.19) displaced the noradrenaline (NA) concentration-response (cr) curve towards higher concentrations without reduction of maximum. Neither clonidine, nor oxymetazoline had any consistent contractile effects. Phenylephrine had a lower potency than NA, but a similar intrinsic activity. In the veins, both prazosin (pA2 9.72) and rauwolscine (pA2 8.11) displaced the NA cr-curve towards higher concentrations, but also significantly depressed maximum. Clonidine and oxymetazoline contracted veins from 3 out of 7 and 4 out of 6 patients, respectively. Their pD2-values were similar to that of NA, but their intrinsic activities were significantly lower. NA was more potent than phenylephrine in these vessels, and there was no significant difference in intrinsic activity. The results suggest that in human omental arteries, the postjunctional alpha-adrenoceptors are mainly of the alpha 1-type, even if a small population of alpha 2-adrenoceptors cannot be excluded. In omental veins, there seems to be a functionally important population of postjunctional alpha 2-adrenoceptors occurring together with a population of alpha 1-adrenoceptors.