Potentiation of the Osmosensitive Taurine Release and Cell Volume Regulation by Cytosolic Ca²⁺ Rise in Cultured Cerebellar Astrocytes

Hyposmolarity (-30%) in cultured cerebellar astrocytes raised cytosolic Ca²⁺ concentration ([Ca²⁺]_i) from 160 to 400 nM and activated the osmosensitive taurine release (OTR) pathway. Although OTR is essentially [Ca²⁺]_i-independent, further increase in [Ca²⁺]_i by ionomycin strongly enhanced OTR, with a more robust effect at low and mild osmolarity reductions. Ionomycin did not affect isosmotic taurine efflux. OTR was decreased by tyrphostin A25 and increased by ortho-vanadate, suggesting a modulation by tyrosine kinase or phosphorylation state. Inhibition of phosphatidylinositol-3-kinase activity by wortmannin markedly decreased OTR and the ionomycin increase. Conversely, OTR and the ionomycin effect were independent of ERK1/ERK2 activation. OTR and its potentiation by ionomycin differed in their sensitivity to CaM and CaMK blockers and in the requirement of an intact cytoskeleton for the ionomycin effect, but not for normal OTR. Changes in the actin cytoskeleton organization elicited by hyposmolarity were not observed in ionomycin-treated cells, which may permit the operation of CaM/CaMK pathways involved in the OTR potentiation by [Ca²⁺]_i rise. OTR potentiation by [Ca ²⁺]_i requires the previous or simultaneous activation/operation of the taurine release mechanism and is not modifying its set point, but rather increasing the effectiveness of the pathway, resulting in a more efficient volume regulation. This may have a beneficial effect in pathological situations with concurrent swelling and [Ca²⁺] _i elevation in astrocytes.