Taurine transport associated with cell volume regulation in flounder erythrocytes under anisosmotic conditions.

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Abstract
The taurine transport of flounder erythrocytes is associated with a cell volume regulation in anisosmotic media. An osmolality reduction leads to a cell volume increase, which is followed by a volume readajustment towards the original level. A 75 mosM reduction is accompanied by a 33 mumol g dry wt.‐1 reduction in the cellular taurine content. The reduction in osmolality activates the taurine release mechanism by transiently increasing the rate coefficient for taurine efflux. The rate coefficient for taurine influx is similarly stimulated. This influx is mediated by a Na+‐independent transport system. The concomitant activation of influx and efflux suggests a coupling between these two systems. Higher taurine efflux and influx rate coefficients which decayed more slowly with time were measured in cells suspended in Na+‐free (choline replacement) media than in the presence of Na+. This suggests that Na+ may play a role in the taurine release mechanism. Noradrenaline induced a cellular swelling at normal osmolality (330 mosM), but had only a minor effect on the taurine efflux and influx and the cellular taurine content. Urea‐induced cellular swelling at normal osmolality initiated a volume regulatory process and activated the taurine release mechanism, similarly to an osmolality reduction. These results show that osmolality reduction and cellular swelling are no prerequisites for the activation of the taurine release mechanism and the cell volume readajustment. It is suggested that the dimension of an intracellular solute compartment determines the activation level of this mechanism.