OSMOREGULATED TAURINE TRANSPORT IN H4IIE HEPATOMA-CELLS AND PERFUSED-RAT-LIVER

The effects of anise-osmotic exposure on taurine transport were studie d in H4IIE rat hepatoma cells. Hyperosmotic (405 mosmol/l) exposure of H4IIE cells stimulated Na+-dependent taurine uptake and led to an inc rease in taurine transporter (TAUT) mRNA levels, whereas hypo-osmotic (205 mosmol/l) exposure diminished both taurine uptake and TAUT mRNA l evels when compared with normo-osmotic (305 mosmol/l) control incubati ons. Taurine uptake increased 30-40-fold upon raising the ambient osmo larity from 205 to 405 mosmol/l. When H4IIE cells and perfused livers were preloaded with taurine, hypo-osmotic cell swelling led to a rapid release of taurine from the cells. The taurine efflux, but not taurin e uptake, was sensitive to 4,4'-di-isothiocyanatostilbene-2,2'-disulph onic acid (DIDS), suggestive of an involvement of DIDS-sensitive chann els in mediating volume-regulatory taurine efflux. Whereas in both H4I IE rat hepatoma cells and primary hepatocytes TAUT mRNA levels were st rongly dependent upon ambient osmolarity, mRNAs for other osmolyte tra nsporters, i.e. the betaine transporter BGT-1 and the Na+/myo-inositol transporter SMIT, were not detectable. In line with this, myo-inosito l uptake by H4IIE hepatoma cells was low and was not stimulated by hyp erosmolarity. However, despite the absence of BGT-1 mRNA, a slight osm osensitive uptake of betaine was observed, but the rate was less than 10% of that of taurine transport. This study identifies a constitutive ly expressed and, osmosensitive TAUT in H4IIE cells and the use of tau rine as a main osmolyte, whereas betaine and myo-inositol play little or no role in the osmolyte strategy in these cells. This is in contras t with rat liver macrophages, in which betaine has been shown to be a major osmolyte.