EFFECTS OF PINACIDIL ON CONTRACTILE PROTEINS IN HIGH K+-TREATED INTACT, AND IN BETA-ESCIN-TREATED SKINNED SMOOTH-MUSCLE OF THE RABBIT MESENTERIC-ARTERY

1 The effects of pinacidil were investigated on changes in cellular Ca2+ concentration ([Ca2+]i) and tension in intact and chemically skinned smooth muscle strips of the rabbit mesenteric artery. 2 High K+ (128 mM) produced a large phasic followed by a tonic increase in [Ca2+]i and tension in intact muscle strips. Pinacidil at 10-mu-M but not 1-mu-M, inhibited the phasic and tonic contractions induced by 128 mM K+ without a corresponding change in [Ca2+]i. 3 In beta-escin-treated skinned smooth muscle, the minimum Ca2+ concentration that produced contraction was 0.1-mu-M and the maximum contraction was obtained at 10-mu-M. Pinacidil at 10-mu-M but not 1-mu-M, shifted the pCa-tension relation curve to the right and also inhibited the maximum contraction induced by Ca2+. The concentrations of Ca2+ required for half maximal tension were 0.9-mu-M in control and 1.5-mu-M in the presence of 10-mu-M pinacidil. Calmodulin (2-mu-M) increased the contraction induced by 0.3-mu-M Ca2+ (but not by 10-mu-M Ca2+) in the skinned strips. Pinacidil (10-mu-M) inhibited the contraction induced by 0.3-mu-M or 10-mu-M Ca2+ in the presence of 2-mu-M calmodulin. 4 Noradrenaline (NA, 10-mu-M) with guanosine triphosphate (GTP, 3-mu-M), guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S, 3-mu-M) or 12-O-tetradecanoylphorbol-13-acetate (TPA, 0.1-mu-M) all enhanced the contraction induced by 0.3-mu-M Ca2+. Pinacidil (10-mu-M) inhibited the contraction induced by 0.3-mu-M Ca2+ more strongly in the presence of the above agents than in their absence. 5 Following application of 2mM adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S) with 0.3-mu-M Ca2+, 4mM MgATP produced contraction in skinned strips in Ca2+-free solution containing 4mM EGTA ('Ca2+-independent contraction'). The amplitude of the Ca2+-independent contraction was almost the same as that obtained with 10-mu-M Ca2+. Pinacidil (10-mu-M) had no effect on the amplitude of the Ca2+-independent contraction nor did it have any effect on the contraction induced by a solution containing no MgATP ('rigor contraction'). 6 It is concluded that pinacidil (10-mu-M) acts directly on the contractile apparatus to inhibit Ca2+-induced contraction in smooth muscle of the rabbit mesenteric artery. The site of action of pinacidil may be between Ca2+-calmodulin complex formation and phosphorylation of the myosin light chain.