α₂-Adrenoceptor-mediated vasoconstriction requires a tyrosine kinase

α2-adrenoceptor-mediated contractions of the rabbit saphenous vein were previously found to be inhibited by wortmannin, a protein kinase inhibitor which blocks receptor-dependent phospholipase D activation. Since other studies have indicated that receptor-dependent phospholipase D activation required activity of a tyrosine kinase, we examined the influence of several tyrosine kinase inhibitors on both α2-adrenoceptor-mediated contractions of rabbit saphenous vein and α1-adrenoceptor-mediated contractions of rabbit aorta. Methyl 2,5-dihydroxycinnamate, genistein and erbstatin each caused non-competitive inhibition of rabbit saphenous vein contractions elicited by the α2-adrenoceptor-selective agonist 5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline (UK14304), yielding complete inhibition at 100 μM and IC50 values of 15, 35 and 40 μM respectively. By contrast, phenylephrine-induced dose-response curves in rabbit aorta were largely unaffected by tyrosine kinase inhibitors at 50 μM. In a separate analysis of intracellular Ca2+-dependent and extracellular Ca2+-dependent α1-adrenoceptor responses of rabbit aorta, genistein (50 μM) did partially reduce the initial intracellular Ca2+-dependent response, but did not reduce maximal response. Methyl 2,5-dihydroxycinnamate (25 μM) had no effect on intracellular or extracellular Ca2+ responses in rabbit aorta. High K+-induced contractions of both rabbit saphenous vein and aorta were unaffected by up to 100 μM of the tyrosine kinase inhibitors. These results indicate an obligatory requirement for tyrosine kinase activity in α2-adrenoceptor-mediated but not α1-adrenoceptor-mediated vasoconstriction.