INVOLVEMENT OF PROTEIN-KINASE-C ACTIVATION IN ALPHA(2)-ADRENOCEPTOR-MEDIATED CONTRACTIONS OF RABBIT SAPHENOUS-VEIN

The role of protein kinase C alpha(2)-adrenoceptor-induced contractions of rabbit saphenous vein was investigated. Contractions induced by the alpha(2)-adrenoceptor-selective agonist 5-bromo-6-[2-imidazolin-2-ylamino]-quinoline (UK14304) were inhibited by prior treatment with pertussis toxin and by Ca2+ removal, confirming a G(i)/G(o)-dependent coupling pathway which was highly dependent upon Ca2+ influx. Protein kinase C inhibitors calphostin-C and staurosporine each caused a non-competitive inhibition of UK14304 response. Down-regulation of protein kinase C by pretreatment with tetradecanoylphorbol acetate reduced UK14304 response by almost 90% with no effect on contractions induced by elevated KCl. The ineffectiveness of L-type Ca2+ channel blockers and the absence of stimulated Ca-45(2+) uptake or efflux by UK14304 indicated that phospholipid-derived products were most likely responsible for protein kinase C activation. alpha(2)-Adrenoceptor stimulation failed to increase [H-3]myoinositol phosphate formation, but caused a significant increase in the formation of both [P-32]phosphatidic acid and diacylglycerol, indicating the possible activation of phospholipase D activity. These results suggest that protein kinase C is important for the vasoconstriction induced by alpha(2)-adrenoceptors and that diacylglycerol derived from receptor-initiated phospholipase D activity may provide protein kinase C stimulation.