We assessed the role of protein kinase C (PKC) in the mechanism responsible for the potentiation of UK-14304-induced contractions produced when isolated dog mesenteric vascular rings were pretreated with threshold concentrations of 12-O-tetradecanoyl-phorbol-13-acetate (TPA), KCl, or endothelin-1 (ET-1). In dog mesenteric artery. UK-14304 produced a biphasic concentration-response curve in the presence of TPA, KCl, or ET-1, with the curve portion at lower concentrations being alpha 2-adrenoceptor dependent and the portion at higher concentrations being alpha 1-adrenoceptor dependent. Calphostin C (10(-6)M), a PKC inhibitor, abolished amplified UK-14304-induced contraction in the TPA-pretreated tissues. In the KCl- and ET-1-pretreated tissues. 10(-6)M calphostin C antagonized amplified UK-14304-induced contractions by approximately 20% in both parts of the concentration-response curve. In contrast, in dog mesenteric vein, amplified UK-14304-induced contractions by TPA, KCl, and ET-1 were entirely dependent on alpha 2-adrenoceptors. Calphostin C (10(-6)M), which in control experiments had no effect on KCl-induced contraction and antagonized responses to TPA by 60.1%, inhibited UK-14304-induced contraction by 18.3%. Amplified UK-14304-induced contraction was antagonized by 10(-6)M calphostin C by 21.8% in KCl-precontracted tissues, 58.1% in ET-1-precontracted tissues, and 66.3% in TPA-precontracted tissues. In the ET-1- and TPA-pretreated dog mesenteric veins, 10(-6)M calphostin C decreased maximal tensions of enhanced UK-14304-induced contractions to the same level as the UK-14304-induced maximal tension inhibited by 10(-6)M calphostin C in untreated dog mesenteric vein. Therefore, TPA can be a precontracting agent that amplifies UK-14304-induced contractions through PKC activation in both dog mesenteric artery and vein. PKC predominantly mediates the contraction amplification mechanisms after exposure to ET-1 in dog mesenteric vein and does not play a major role in the amplification of UK-14304-induced contraction by KCl in both dog mesenteric artery and vein. These data show that a common mechanism need not underlie amplification of adrenergic responses in mesenteric artery and vein.