Sphingosine releases Ca2+ from intracellular stores via the ryanodine receptor in sea urchin egg homogenates

Various reports have demonstrated that the sphingolipids sphingosine and sphingosine-1-phosphate are able to induce Ca2+ release from intracellular stores in a similar way to second messengers. Here, we have used the sea urchin egg homogenate, a model system for the study of intracellular Ca2+ release mechanisms, to investigate the effect of these sphingolipids. While ceramide and sphingosine-1-phosphate did not display the ability to release Ca2+, sphingosine stimulated transient Ca2+ release from thapsigargin-sensitive intracellular stores. This release was inhibited by ryanodine receptor blockers (high concentrations of ryanodine, Mg2+, and procaine) but not by pre-treatment of homogenates with cADPR, 8-bromo-cADPR or blockers of other intracellular Ca2+ channels. However, sphingosine rendered the ryanodine receptor refractory to cADPR. We propose that, in the sea urchin egg, sphingosine is able to activate the ryanodine receptor via a mechanism distinct from that used by cADPR.