Na/Ca Exchange Facilitates Ca-dependent Activation of Endothelial Nitric-oxide Synthase*
نویسندگان
چکیده
Recent evidence suggests the expression of a Na/ Ca exchanger (NCX) in vascular endothelial cells. To elucidate the functional role of endothelial NCX, we studied Ca signaling and Ca-dependent activation of endothelial nitric-oxide synthase (eNOS) at normal, physiological Na gradients and after loading of endothelial cells with Na ions using the ionophore monensin. Monensin-induced Na loading markedly reduced Ca entry and, thus, steady-state levels of intracellular free Ca ([Ca]i) in thapsigargin-stimulated endothelial cells due to membrane depolarization. Despite this reduction of overall [Ca]i, Ca -dependent activation of eNOS was facilitated as indicated by a pronounced leftward shift of the Ca concentration response curve in monensin-treated cells. This facilitation of Ca-dependent activation of eNOS was strictly dependent on the presence of Na ions during treatment of the cells with monensin. Na-induced facilitation of eNOS activation was not due to a direct effect of Na ions on the Ca sensitivity of the enzyme. Moreover, the effect of Na was not related to Na entry-induced membrane depolarization or suppression of Ca entry, since neither elevation of extracellular K nor the Ca entry blocker 1-(b-[3-(4-methoxyphenyl)-propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SK&F 96365) mimicked the effects of Na loading. The effects of monensin were completely blocked by 3*,4*-dichlorobenzamil, a potent and selective inhibitor of NCX, whereas the structural analog amiloride, which barely affects Na/Ca exchange, was ineffective. Consistent with a pivotal role of Na/Ca exchange in Ca-dependent activation of eNOS, an NCX protein was detected in caveolin-rich membrane fractions containing both eNOS and caveolin-1. These results demonstrate for the first time a crucial role of cellular Na gradients in regulation of eNOS activity and suggest that a tight functional interaction between endothelial NCX and eNOS may take place in caveolae.
منابع مشابه
Na(+)/Ca(2+) exchange facilitates Ca(2+)-dependent activation of endothelial nitric-oxide synthase.
Recent evidence suggests the expression of a Na(+)/Ca(2+) exchanger (NCX) in vascular endothelial cells. To elucidate the functional role of endothelial NCX, we studied Ca(2+) signaling and Ca(2+)-dependent activation of endothelial nitric-oxide synthase (eNOS) at normal, physiological Na(+) gradients and after loading of endothelial cells with Na(+) ions using the ionophore monensin. Monensin-...
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