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

The role of nitric oxide (NO) in the stimulation of soluble guanylyl cyclas e (sGC) is well established, but the mechanism by which the enzyme is inact ivated during the prolonged NO stimulation has not been characterized. In t his paper we studied the interactions between NO and intracellular Ca2+ in the control of sGC in rat anterior pituitary cells. Experiments were done i n cultured cells, which expressed neuronal and endothelial NO synthases, an d in cells with elevated NO levels induced by the expression of inducible N O synthase and by the addition of several NO donors. Basal sGC-dependent cG MP production was stimulated by the increase in NO levels in a time-depende nt manner. In contrast, depolarization of cells by high K+ and Bay K 8644, an L-type Ca2+ channel agonist, inhibited sGC activity. Depolarization-indu ced down-regulation of sGC activity was also observed in cells with inhibit ed cGMP-dependent phosphodiesterases but not in cells bathed in Ca2+ defici ent medium. This inhibition was independent from the pattern of Ca2+ signal ing (oscillatory versus nonoscillatory) and NO levels, and was determined b y 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.