Taurine efflux is a cell volume regulatory process in proximal renal tubules from the teleost Carassius auratus

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Abstract
The potential role of taurine transport associated with volume regulation in renal tissue and isolated proximal renal tubules was studied in the teleost Carassius auratus (goldfish). The cellular taurine content in renal tissue fragments incubated in isosmotic solution (290 mOsm) (7.8 +/- 0.9 (SD) micromol g wet wt(-1)) decreased by 60% following exposure to hyposmotic medium (100 mOsm). The rate coefficient for [14C]taurine efflux in renal tissue and in isolated proximal renal tubules was strongly stimulated following hyposmotically or urea-activated cellular swelling. The stimulated basolateral taurine efflux pathway exhibited channel-like functional characteristics since (a) [14C]taurine influx was stimulated in parallel with the osmolality-dependent taurine efflux and (b) this efflux could not be stimulated by high medium taurine concentrations (40 mM) applied 10 min following the osmolality reduction (trans-stimulation test). Administration of the 5-lipoxygenase inhibitor ETH 615-139 (20 microM) during hyposmotic stimulation inhibited regulatory volume decreases but had no effect on taurine efflux. In addition, hyposmotically induced taurine efflux was slightly but significantly inhibited by the cyclooxygenase inhibitor indomethacin (10 microM). The taurine efflux was also dependent on both extra- and intracellular Ca2+. It is concluded that taurine is likely to coparticipate with KCl as an osmoeffector during RVD in Carassius proximal renal tubule cells. Cellular swelling seems to activate a basolateral taurine transport pathway with functional properties of a channel. This efflux mechanism appears to be partly regulated by Ca2+. Such a transport pathway could play a role in the cell volume regulatory mechanisms participating during transepithelial solute and water transport.