Effect of lithium on ionic balance and polyphosphoinositide metabolism during larval vegetalization of the sea urchin Paracentrotus lividus.

Treatment of Paracentrotus lividus embryos with lithium, which is known to block the polyphosphoinositide (PPI) cycle, induces their development into vegetalized larvae. We have investigated how ionic balance and PPI metabolism were affected in such vegetalized embryos. Exposure to 80 mM lithium for 3 1/2 hr at the morula stage induced an accumulation of lithium into the embryos, concomitant with a decrease in intracellular sodium and potassium contents. Intracellular lithium was totally expelled 6 hr after removal of this ion from the external medium. We found that intracellular levels of all PPI-cycle intermediates were altered after vegetalization: PIP2 and PIP increased, while PI decreased. Finally, vegetalized embryos showed a significant depression of IP3 mass. The degree of change in inositol phosphates increased as the intracellular accumulation of lithium increased. The change in morphology (vegetalization) also correlated well with the increase in lithium accumulation. This is consistent with the hypothesis that lithium alters the PPI cycle.