EFFECTS OF HYPOSMOTIC SHOCK ON TAURINE TRANSPORT IN ISOLATED TROUT HEPATOCYTES

Isolated trout hepatocytes exposed to hypotonic medium undergo a regul atory 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)(-1)). The osmolarity-sensitive release of amin o acids was studied using [H-3]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-m ) of 122 mu M and a maximum velocity (V-max) of 31.2 pmol (mg protein) (-)1 min(-1). This second way of uptake was also chloride dependent an d indicated an apparent coupling ratio Na+:Cl-:taurine of 2:1:1. The l atter component and the taurine efflux were stimulated during RVD, lea ding to intracellular amino acid loss. Taurine efflux activation durin g volume recovery was transient and also dependent on the presence of both Na+ and Cl- in the extracellular medium. Furthermore, taurine rel ease and RVD were slowed down when Ca2+ was omitted from the medium. T hese results suggested two distinct and complementary mechanisms for v olume regulation in trout hepatocytes during hypotonic conditions.