Impaired cyclic nucleotide-dependent vasorelaxation in human umbilical artery smooth muscle

Activation of either the adenylate cyclase pathway with forskolin or the guanylate cyclase pathway with sodium nitroprusside fails to induce active relaxation of serotonin-precontracted human umbilical artery smooth muscle (HUASM) but causes active relaxation of serotonin-precontracted bovine carotid artery smooth muscle (BCASM). This difference in response appears to be unique to HUASM in that all other vascular muscles exhibit relaxation in response to these substances. Forskolin and sodium nitroprusside stimulation leads to respective increases in intracellular adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) concentrations in HUASM and BCASM. cAMP- and cGMP-dependent protein kinases are both present in HUASM and can be activated in homogenates of HUASM by the addition of exogenous cAMP and cGMP, respectively. When either forskolin or nitroprusside acts in BCASM, two low-molecular-weight proteins display an increase in the extent of phosphorylation. Neither protein shows such an increase when HUASM is treated with either agent. Thus the inability of HUASM to display active relaxation appears to be secondary to impaired activation of cyclic nucleotide-dependent protein kinases. The refractoriness to active relaxation may contribute to the vasospasm that occurs in the umbilical vasculature with parturition.