Cell volume regulation: the role of taurine loss in maintaining membrane potential and cell pH

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Abstract
In response to a hyposmotic stress cells undergo a regulatory volume decrease (RVD) by losing osmotically active solutes and obliged water. During RVD, trout red cells lost taurine, K+ and Cl− but gained Na+ and Cl−. Over the full time course of RVD the chloride concentration in the cell water remained remarkably constant. Thus membrane potential and cell pH, which depends on the ratio of internal to external chloride concentration ([Cl−]i:[Cl−]o), remained fixed. When cell volume decreases it is only possible to keep the chloride concentration in the cell water constant if an equal percentage of the cell chloride pool and of the cell water pool are lost simultaneously. Quantitative analysis of our data showed that this requirement was fulfilled because, over the full time course of RVD, cells lost osmotically active solutes with a constant stoichiometry: 1 Cl−:1 positive charge:2.35 taurine. Any change in taurine permeability, by modifying the stoichiometric relationship, would affect the amount of water lost and consequently cell chloride concentration. Experiments carried out with different cations as substitutes for external Na+ suggest that the constancy of the chloride concentration is not finely tuned by some mechanism able to modulate the channel transport capacity, but results in part from the fact that the swelling-dependent channel constitutively possesses an adequately fixed relative permeability for cations and taurine. However, as a significant fraction of K+ and Cl− loss occurs via a KCl cotransporter, the contribution of the cotransport to the stochiometric relationship remains to be defined. The large amount of taurine released during RVD (50 % of all solutes) was shown to be transported as an electroneutral zwitterion and not as an anion. How the channel can accommodate the zwitterionic form of taurine, which possesses a high electrical dipole, is considered.