Phorbol ester-induced contraction in chemically skinned vascular smooth muscle

We studied the contractile response to phorbol esters and its relationship to myosin light chain phosphorylation in intact and Triton X-100-skinned porcine carotid preparations. Muscle contraction was activated by phorbol 12,13-dibutyrate (PDBu) and phorbol 12,13-didecanoate (PDD). Dose-dependent contractions to PDBu were obtained both in the intact and skinned preparations. The maximal values of stress in response to PDBu were 1.11 +/- 0.10 X 10(5) N/m2 (n = 7) in the intact and 5.72 +/- 0.59 X 10(4) N/m2 (n = 10) in the skinned muscles. The skinned tissues responded to PDD, which has been shown to activate protein kinase C, but not to the inactive isomer 4 alpha-PDD, thus ruling out nonspecific phorbol effects. The phorbol ester response exhibited a Ca2+ dependence. High stresses in the skinned muscles (5.53 +/- 0.69 X 10(4) N/m2, n = 8) were associated with low values of myosin light chain phosphorylation (0.18 +/- 0.01 mol Pi/mol light chain, n = 8). Thus phorbol esters can contract vascular smooth muscle by a mechanism that is not proportional to myosin light chain phosphorylation and that may involve activation of protein kinase C.