Sphingosylphosphorylcholine activates Gq, Gi-2, and Gi-3 in thyroid FRTL-5 cells:: Implications for the activation of calcium fluxes and NA+-H+ exchange

In the present investigation of rat thyroid FRTL-5 cells, we show using reverse-transcriptase PCR that these cells express both Edg-1 and Edg-5. We show using a [S-35]GTP gamma S-binding assay that sphingo-sylphosphorylcholine (SPC), which binds to both Edg-1 and EDG-5, activates G(q), G(i-2), and G(i-3), proteins. SPC potently increases intracellular free calcium concentrations ([Ca2+](i)). This effect is mediated through both G(q) and G(i) proteins, as the mobilization of sequestered calcium was insensitive to pertussis toxin (i.e., mediated by G(q)), while the SPC-evoked calcium entry was inhibited by pretreatment with pertussis toxin (i.e., mediated by G(i)). Furthermore, SPC in a concentration-dependent manner increases intracellular pH in acidified cells via a Na+-H+ exchange mechanism. The enhanced activation of Na+-H+ exchange is independent of both an increase in [Ca2+]i and an activation of protein kinase C. The effect of SPC on Na+-H+ exchange is insensitive to pertussis toxin, suggesting an effect mediated via G(q). (C) 1999 Academic Press.