Volume-sensitive taurine transport in fish erythrocytes

Abstract

Taurine plays an important role in cell volume regulation in both vertebrates and invertebrates. Erythrocytes from two euryhaline fish species, the eel (Anguilla japonica) and the starry flounder (Platichthys stellatus) were found to contain high intracellular concentrations of this amino acid (≃ 30 mmol per liter of cell water). Kinetic studies established that the cells possessed a saturable high-affinity Na⁺-dependent β-amino-acid transport system which also required Cl⁻ for activity (apparentK _m (taurine) 75 and 80 μm;V _max 0.85 and 0.29 μmol/g Hb per hr for eel (20°C) and flounder cells (10°C), respectively. This β-system operated with an apparent Na⁺/Cl⁻/taurine coupling ratio of 2∶1∶1. A reduction in extracellular osmolarity, leading to an increase in cell volume, reversibly decreased the activity of the transporter. In contrast, low medium osmolarity stimulated the activity of a Na⁺-independent nonsaturable transport route selective for taurine, γ-amino-n-butyric acid and small neutral amino acids, producing a net efflux of taurine from the cells. Neither component of taurine transport was detected in human erythrocytes. It is suggested that these functionally distinct transport routes participate in the osmotic regulation of intracellular taurine levels and hence contribute to the homeostatic regulation of cell volume. Volume-induced increases in Na⁺-independent taurine transport activity were suppressed by noradrenaline and 8-bromoadenosine-3′, 5′-cyclic monophosphate, but unaffected by the anticalmodulin drug, pimozide.