Interaction of phorbol esters with Ca2+ channels in smooth muscle

The phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA), a selective activator of protein kinase C, had no effect on the sensitivity to Ca2+ or verapamil of K+-depolarized taenia preparations from the guinea-pig caecum, despite the use of high concentrations (1 microM for 3 h); this preparation is sensitive to Ca2+ channel activators and antagonists. TPA (0.03-3 microM) caused a slow contraction of rat aorta preparations; the contractions were resistant to the calcium-antagonists nifedipine (0.01 microM), verapamil (10 microM), diltiazem (10 microM) and cinnarizine (10 microM), but were antagonized by N-(6-aminohexyl)-5-chloro-1-naphthalensulphonamide (W-7, 50-200 microM). Prolonged exposure to TPA (greater than 2 h) resulted in spontaneous contractions which were sensitive to verapamil (1 microM). Isoprenaline and sodium nitroprusside relaxed phenylephrine-induced contractions in rat aorta preparations. TPA (0.3 microM) blocked the maximal response to isoprenaline but not to sodium nitroprusside indicating that TPA did selectively activate protein kinase C under these experimental conditions. These findings indicate that protein kinase C activation does not result in direct effects on Ca2+ channel function, but may exert effects indirectly (e.g. by modifying intracellular sensitivity to Ca2+, Ca2+ extrusion, or cellular depolarization).