Cellular mechanisms controlling EDRF/NO formation in endothelial cells

Summary

We investigated the molecular mechanisms whereby Ca²⁺ enters the endothelial cytosol and regulates endothelial nitric oxide synthesis L-arginine-dependent nitric oxide synthesis by isolated endothelial cytosol as quantified by activation of a purified soluble guanylate cyclase was concentration-dependently enhanced by free Ca²⁺ (EC₅₀ 0.3 μM). The Ca²⁺-dependent activation was inhibited by the calmodulin antagonists mastoparan, melittin, and calcineurin (IC⁵⁰ 450, 350, and 60 nM, respectively) in a calmodulin-reversible manner. After removal of endogenous calmodulin the Ca²⁺-dependency of endothelial NO synthase was lost, but could be reconstituted with exogenous calmodulin. The results indicate that Ca²⁺-calmodulin directly activates the endothelial nitric oxide synthase, thereby transducing agonist-induced increases in intracellular free Ca²⁺ concentration to nitric oxide formation from L-arginine, K⁺-induced depolarization of the endothelial cells markedly inhibited the sustained, but not initial phase of the intracellular Ca²⁺ response to bradykinin, indicating that K⁺-induced depolarization depresses the transmembrane Ca²⁺ influx. On the contrary, the K⁺ channel activator Hoe 234 which elicits hyperpolarization of the endothelial cell membrane, augmented the sustained phase of the agonist-induced intracellular Ca²⁺ signal, but not the resting intracellular Ca²⁺ level. The effects of K⁺ and Hoe 234 on the agonist-induced Ca²⁺-response were reflected by corresponding changes in agonist-induced EDRF/NO release. From these data, we suggest that the endothelial membrane potential may play an important role for the extent of agonist-induced Ca²⁺ influx and, thereby, the endothelial EDRF/NO synthesis.