ACTIVATION OF G-PROTEIN EVOKES CA2+ INFLUX IN ENDOTHELIAL-CELLS WITHOUT CORRELATION TO INOSITOL PHOSPHATES

It is well established that the formation of endothelium-derived relaxing factor (EDRF) depends on an increase in intracellular free Ca2+ concentration ([Ca2+]i) in vascular endothelial cells. The aim of our study was to investigate whether the origin of this increase in intracellular free Ca2+ plays any role in this important physiological pathway. Therefore, we investigated the effects of bradykinin on [Ca2+]i in the presence and in the absence of external Ca2+ and correlated these effects with bradykinin-induced EDRF biosynthesis also in the presence and the absence of external Ca2+. Thereby, we found that the release of intracellularly stored Ca2+. on the one hand, and influx of external Ca2+, on the other hand, apparently differ in their effects on EDRF formation. Release of intracellularly stored Ca2+ by inositol-1,4,5-trisphosphate (IP3) results in a rapid increase in [Ca2+]i, and this seems to be necessary for a quick response for agonist-induced EDRF formation. However, Ca2+ influx, which is a sustained effect, seems to be important for a basal and long-lasting EDRF formation, which is involved in the regulation of vascular tone. Also, the mechanism of Ca2+ influx regulation in endothelial cells was investigated. Sodium fluoride, a known activator of G proteins, was used to investigate whether a G protein is involved in the mechanism of Ca2+ influx regulation. In our experiments, sodium fluoride increased [Ca2+]i and also induced an endothelium-dependent relaxation. Furthermore, the stimulation Of IP3 formation induced by sodium fluoride was measured. The results indicate that neither a correlation between Ca2+ influx and formation Of IP3 nor a depletion of intracellular Ca2+ stores appears to be involved in the mechanism of Ca2+ influx regulation. These findings may suggest the existence of a direct regulation of Ca2+ channels by a G protein.