SPHINGOSINE 1-PHOSPHATE STIMULATES NA+ H+ EXCHANGE IN THYROID FRTL-5 CELLS/

Sphingosine derivatives are potent mitogens in several cell types. Man y mitogens activate the Na+/H+ exchange, although the interrelationshi ps between Na+/H+ exchange and mitogenesis are unclear. The present in vestigation in thyroid FRTL-5 cells shows that sphingosine l-phosphate (SPP) activates Na+/H+ exchange in a dose-dependent manner in acid-lo aded cells. The effect of SPP was abolished in a Nat-free buffer and b y pretreatment of the cells with ethylisopropylamiloride. SPP did not affect basal intracellular pH (pH(i)). SPP stimulated the release of s equestered Ca2+ and a substantial entry of Ca2+ The effect of SPP on p H(i) was abolished in cells incubated in a Ca2+-free buffer, and in ce lls loaded with the intracellular Ca2+ chelator ,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid. Furthermore, the effect of SPP was abolished in pertussis toxin (PTX)-treated cells. PTX decreased Ca2+ e ntry only, without affecting the release from intracellular stores. Ph osphatidic acid (PA) did not activate Na+/H+ ex change, suggesting tha t the effect of SPP was not mediated via activation of phospholipase D and the production of PA. Thus one mechanism of action of SPP in FRTL -5 cells appears to be to activate Na+/H+ exchange. This action is med iated via a G protein-dependent mechanism and requires an increase in intracellular free Ca2+.