title: Calcium dependence of phorbol 12,13-dibutyrate-induced force and myosin light chain phosphorylation in arterial smooth muscle.
abstract: Phorbol dibutyrate (PDB) is an activator of protein kinase C and has been observed to cause a slow developing contraction in vascular smooth muscle. The mechanism of phorbol ester-induced contraction is unknown. We studied the Ca++-dependence of, and the degree of myosin light chain phosphorylation (MLC-P), during PDB-induced contractions in rabbit aortic rings. PDB elicited concentration-dependent contractions (3 X 10(-8) to 10(-6) M) in rabbit aortic rings incubated in normal (1.6 mM Ca++) physiologic salt solution (PSS). Addition of the Ca++-channel blocker nifedipine (0.1 microM) to PSS or removal or Ca++ from PSS significantly reduced the contractile responses to PDB. Depletion of Ca++ by repeated washes in O Ca++-PSS containing 10(-3) M ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid reduced, but did not eliminate, the responses to PDB. In PSS, PDB significantly increased the fraction of phosphorylated MLC/total MLC to 0.33 from a resting value of 0.20. Ca++ depletion reduced the resting fraction (MLC-P/MLC) to 0.14. PDB-stimulated contractions in Ca++-depleted tissues occurred in the absence of significant increases in MLC-P. Sodium nitroprusside partially relaxed PDB-induced contractions by approximately 50% whether elicited in the presence of 1.6 mM Ca++ or after Ca++ depletion. In both cases relaxation occurred in the absence of statistically significant decreases in MLC phosphorylation. Ca++-dependent MLC phosphorylation may account for a component of the PDB contractile response in rabbit aorta. Studies in the absence of Ca++ suggest that PDB may activate contraction without concomitant MLC-P.
mesh_terms: D000818:Animals; D001158:Arteries; D002118:Calcium / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000378:metabolism; D009219:Myosin-Light-Chain Kinase / Q000032:analysis; D009218:Myosins / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D009599:Nitroprusside / Q000494:pharmacology; D015240:Phorbol 12,13-Dibutyrate; D010703:Phorbol Esters / Q000494:pharmacology*; D010766:Phosphorylation; D011817:Rabbits; D014661:Vasoconstriction / Q000187:drug effects*
chemical_list: D010703:Phorbol Esters; D009599:Nitroprusside; D015240:Phorbol 12,13-Dibutyrate; D009219:Myosin-Light-Chain Kinase; D009218:Myosins; D009543:Nifedipine; D002118:Calcium