Sphingosylphosphorylcholine activates an amiloride-insensitive Na+-H+-exchange mechanism in GH4C1 cells.

The effect of sphingosylphosphorylcholine (SphPCho) on the intracellular pH (pHi) in GH4C1 cells was investigated. SphPCho evoked a very slow increase in basal pHi. In cells acidified with nigericin, SphPCho induced a rapid alkalinization of the cells. The effect was inhibited in a Na+-free buffer solution, but was insensitive to ethylisopropyl amiloride, a potent inhibitor of Na+-H+ exchangers (NHE). Reverse transcription and PCR showed that the predominant isoform of the antiport expressed in GH4C1 cells is NHE-1. The rate of alkalinization after stimulation with propionate, and after addition of Na+ to cells acidified with NH4Cl, was enhanced in cells treated with SphPCho. The initial rate of alkalinization after addition of Na+ to acidified cells treated with SphPCho gave an apparent Km value of 15 +/- 2 mM for Na+. The Vmax value was 9 +/- 2 mM H+/min. The effect was insensitive to ouabain, staurosporine and bafilomycin A. However, the SphPCho-evoked alkalinization was abolished in cells treated with 2-deoxy-D-glucose. The effect was not due to the charge of the molecule, as stearylamine increased pHi in Na+-containing and Na+-free buffer. The results show that SphPCho may activate Na+-H+ exchange, and that this effect is mediated via an amiloride-insensitive exchange mechanism.