Sphingosylphosphorylcholine Activates G sub(q), G sub(i-2), and G sub(i-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 [ super(35)S]GTP gamma S-binding assay that sphingosylphosphorylcholine (SPC), which binds to both Edg-1 and EDG-5, activates G sub(q), G sub(i-2), and G sub(i-3) proteins. SPC potently increases intracellular free calcium concentrations ([Ca super(2+)] sub(i)). This effect is mediated through both G sub(q) and G sub(i) proteins, as the mobilization of sequestered calcium was insensitive to pertussis toxin (i.e., mediated by G sub(q)), while the SPC-evoked calcium entry was inhibited by pretreatment with pertussis toxin (i.e., mediated by G sub(i)). Furthermore, SPC in a concentration-dependent manner increases intracellular pH in acidified cells via a Na super(+)-H super(+) exchange mechanism. The enhanced activation of Na super(+)-H super(+) exchange is independent of both an increase in [Ca super(2+)] sub(i) and an activation of protein kinase C. The effect of SPC on Na super(+)-H super(+) exchange is insensitive to pertussis toxin, suggesting an effect mediated via G sub(q).