Phorbol ester (PMA) activates voltage-dependent Ca(2+)-channels in helical but not ring preparation of the rat aorta

Effects of phorbol 12-myristate 13-acetate (PMA) on the helical and ring preparations from rat thoracic aorta was investigated in the present study. In helical strips, PMA produced biphasic contraction, i.e., sustained contraction followed by a phasic response superimposed on the sustained contraction. The phasic contraction was completely inhibited by removal of external Ca2+, whereas sustained contraction was not affected by the treatment. In the ring preparation, PMA produced only sustained contraction, whereas phasic contraction was not induced. On the other hand, in the ring preparation which had been treated with 15 mM K+, PMA was able to produce phasic contraction. Contractile response of the aorta to norepinephrine (NE) or clonidine in the ring preparations was significantly weaker than that in the helical strips. In the presence of nifedipine, however, these different sensitivity to NE between ring and helical preparations was not shown. These results suggest that an activation of C-kinase by PMA is able to stimulate voltage-dependent Ca(2+)-channels in the helical strips but not ring preparation of the aorta. An increased contractile response of the aorta to agonists in helical strips may be responsible for an increased activities of voltage-dependent Ca(2+)-channels.