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SUMMARYIsolated trout hepatocytes exposed to hypotonic medium undergo a regulatory volume decrease (RVD) that occurs via two separate routes, K+-Cl-cotransport and amino acid release, the ion efflux accounting for 70 % of the total osmolyte loss. Taurine, glutamine and glutamic acid are the most important and represent 73 % of the total amino acid content (53 mmol (1 cell water)-').The osmolarity-sensitive release of amino acids was studied using [3H]taurine. Kinetic studies indicated two components for taurine influx: a linear Na+-independent transport and a saturable Na+-dependent system with a Michaelis-Menten constant (K.) of 122 fSM and a maximum velocity (Vmax) of 31-2 pmol (mg protein)-1 min-'. This second way of uptake was also chloride dependent and indicated an apparent coupling ratio Na+: Cl-: taurine of 2:1:1. The latter component and the taurine efflux were stimulated during RVD, leading to intracellular amino acid loss. Taurine efflux activation during volume recovery was transient and also dependent on the presence of both Na+ and Cl-in the extracellular medium. Furthernore, taurine release and RVD were slowed down when Ca2l was omitted from the medium. These results suggested two distinct and complementary mechanisms for volume regulation in trout hepatocytes during hypotonic conditions.

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