Dependence of soluble guanylyl cyclase activity on calcium signaling in pituitary cells

The role of nitric oxide (NO) in the stimulation of soluble guanylyl cyclase (sGC) is well established, but the mechanism by which the enzyme is inactivated during the prolonged NO stimulation has not been characterized. In this paper we studied the interactions between NO and intracellular Ca2+ in the control of sGC in rat anterior pituitary cells. Experiments were done in cultured cells, which expressed neuronal and endothelial NO synthases, and in cells with elevated NO levels induced by the expression of inducible NO synthase and by the addition of several NO donors. Basal sGC-dependent cGMP production was stimulated by the increase in NO levels in a time-dependent manner. In contrast, depolarization of cells by high K+ and Bay K 8644, an L-type Ca2+ channel agonist, inhibited sGC activity. Depolarization-induced down-regulation of sGC activity was also observed in cells with inhibited cGMP-dependent phosphodiesterases but not in cells bathed in Ca2+ deficient medium. This inhibition was independent from the pattern of Ca2+ signaling (oscillatory versus nonoscillatory) and NO levels, and was determined by averaged concentration of intracellular Ca2+. These results indicate that inactivation of sGC by intracellular Ca2+ serves as a negative feedback to break the stimulatory action of NO on enzyme activity in intact pituitary cells.