Sphingosylphosphorylcholine Activates Gq, Gi-2, and Gi-3in 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 [35S]GTPγS-binding assay that sphingosylphosphorylcholine (SPC), which binds to both Edg-1 and EDG-5, activates Gq, Gi-2, and Gi-3proteins. SPC potently increases intracellular free calcium concentrations ([Ca2+]i). This effect is mediated through both Gqand Giproteins, as the mobilization of sequestered calcium was insensitive to pertussis toxin (i.e., mediated by Gq), while the SPC-evoked calcium entry was inhibited by pretreatment with pertussis toxin (i.e., mediated by Gi). 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+]iand an activation of protein kinase C. The effect of SPC on Na+-H+exchange is insensitive to pertussis toxin, suggesting an effect mediated via Gq.