Hyposmolarity‐induced taurine release in cerebellar granule cells is associated with diffusion and not with high‐affinity transport

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Abstract
The effects of hyposmotic conditions on taurine uptake and release were studied in mice cultured cerebellar granule cells. The effect of DIDS (4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonate) and of the divalent cations Mg++ and Mn++ on the hyposmolarity‐induced changes in these parameters was investigated. Mg++ (20 mM) and Mn++ (5mM) inhibited by 25% and 41%, respectively, the release of taurine observed in 30% hyposmolar media. DIDS (100 μM) inhibited this release by 46%. Taurine efflux evoked by 50% hyposmolar solutions was reduced about 40% by Mg++ and 55% by Mn++. Taurine uptake into the granule cells could be resolved into a high‐affinity carrier‐mediated component plus a nonsaturable diffusion component. The kinetic constants (Km and Vmax) for the high‐affinity uptake were unaffected by a 50% decrease in the osmolarity. The diffusion constant for the nonsaturable taurine uptake was increased from 1.5 × 10−4 in isosmotic media to 4.6 × 10−4 ml × min−1 mg−1 in hyposmotic (50%) media. This increase in the diffusional component of taurine uptake elicited by the hyposmotic condition was inhibited approximately 25% by either 100 μM DIDS or 5 mM Mn++. These results strongly suggest that the increase in taurine release induced by swelling under hyposmotic conditions is mediated by a diffusional process and not by a reversal of the high‐affinity taurine carrier.