[Myogenic effects of cyclic guanosine monophosphate in smooth muscle cells. Role of protein kinase C].

Heterogeneity of the cGMP-dependent contractility effects of the smooth cells (SMC) was shown in guinea pig ureter by the methods of the double sucrose gap junction. Sodium nitroprusside (SN, 0.1-100 microM) relaxed the high-K+ depolarisationprecontracted SMC but strengthened the SMC constriction triggered by electrical stimulation. In taenia coli, SN or nitroglycerin in the same concentration ranges depressed the electrical or mechanical activity of the SMC and relaxed the SMC, precontracted by depolarization in high-K+ medium. The inhibitor of the phosphodiesterases vinpocetine (1 microM) contributed to the activating effect of SN; the inhibitor of the soluble guanilatcyclase Methilen Blue (10 microM) depressed it. Histamine and mesotone (1-10 microM) increased the action potential and constriction of the SMC triggered by electrical stimulation but decreased the effect of SN. The activator of the protein kinase C (PK-C) phorbol miristoyl-13-acetyl (0.5 microM) changed direction of the SN effects inhibiting both the parameters of an action potential and of the SMC constriction. The pre-treatment with the inhibitor of PK-C calphostin C (0.1 microM) additionally depressed the effects of SN, increasing SMC constriction, especially in the presence of histamine and mesatone. We suggest that c-GMP depressed activity of the PK-C by independent mechanisms operating in SMC.