Effect of sodium fluoride on cytosolic free Ca2(+)-concentrations and cGMP-levels in endothelial cells.

Sodium fluoride was used to investigate a possible involvement of G-proteins in the regulation of endothelial calcium channels. Incubation of cultured porcine aortic endothelial cells with sodium fluoride produced a dose-dependent increase in intracellular free calcium (EC50 approximately 5 mM). The effect strictly depended on the presence of extracellular CaCl2, indicating an enhanced influx of extracellular Ca2+ rather than a release of Ca2+ from intracellular stores. The Al3+ chelator deferoxamine abolished the stimulatory effect of sodium fluoride but did not interfere with the stimulatory effect of bradykinin. These data confirm the current hypothesis that the complex AlF-4 and not the fluoride anion activates G-proteins and exclude a direct inhibitory effect of deferoxamine on Ca2(+)-uptake. In contrast to isoproterenol and 5'-N-ethylcarboxamido-adenosine (NECA), which elevated endothelial cAMP-levels without affecting intracellular Ca2(+)-concentrations, sodium fluoride was not able to increase endothelial cAMP. This indicates that the effect of sodium fluoride on endothelial Ca2(+)-levels is not due to stimulation of a Gs-protein. Similar to its effect on cytoplasmic Ca2+, sodium fluoride also increased endothelial cGMP-levels which has recently been suggested to serve as biochemical marker for the formation of endothelium derived relaxing factor (EDRF). Thus, similar to the activation of receptor operated calcium channels, direct stimulation of a G-protein by sodium fluoride results in an increase of cytoplasmic Ca2+ and the formation of EDRF.