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Activation of hepatic stellate cells (HSCs) results in multiple alterations of cell function, but nothing is known about organic osmolytes in these cells. Organic osmolyte transport and transporter messenger RNA (mRNA) expression was studied in quiescent rat HSCs and after their transformation into alpha1-smooth muscle actin-positive myofibroblastlike cells. Quiescent stellate cells expressed in an osmosensitive manner the mRNA levels of the transporters for taurine (TAUT) and myoinositol (SMIT), whereas that for betaine was not detectable. However, these cells showed osmosensitive uptake not only of taurine and myoinositol but also of betaine. Osmosensitive betaine uptake was mediated by amino acid transport system A. After transformation into myofibroblasts, taurine and myoinositol uptake increased 5.5-fold and 4.5-fold, respectively, together with the respective transporter mRNA levels. Betaine uptake increased twofold because of osmosensitive induction of BGT1 expression. In both quiescent and activated HSCs, hypoosmotic cell swelling induced a rapid and 4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid-sensitive osmolyte efflux. In quiescent HSCs, hyperosmotic exposure increased the messenger RNA (mRNA) level of cyclooxygenase-2, which was counteracted by taurine but not by betaine or myoinositol. The study identifies taurine, myoinositol, and betaine as osmolytes in HSCs. Transformation of HSCs is accompanied by enhanced osmolyte transport activity and induction of the BGT1 transporter, which may be another activation marker of HSCs.

Compatible organic osmolytes, such as betaine and taurine are involved in the regulation of Kupffer cell (KC) function, but nothing is known about osmolytes in liver endothelial cells. This was investigated here by studying the effect of aniso-osmotic exposure of rat liver sinusoidal endothelial cells (SEC) on osmolyte transport and the messenger RNA (mRNA) levels for the transport systems for betaine (BGT1), taurine (TAUT), and myo-inositol (SMIT). Compared with normo-osmotic exposure (305 mosmol/L), hyperosmotic exposure (405 mosmol/L) of SEC led to an increase in the mRNA levels for these transport systems and simultaneously to a stimulation of betaine, taurine, and myo-inositol uptake, which led to an increase of cell volume. Conversely, hypo-osmotic exposure decreased osmolyte uptake. When hyperosmotically pre-exposed SEC were loaded with betaine, taurine, or myoinositol, hypo-osmotic stress stimulated the efflux of these osmolytes from the cells. Studies on osmolyte tissue levels revealed that taurine was an important compatible organic osmolyte under normo-osmotic conditions and predominantly released following hypo-osmotic stress. Conversely, following hyperosmotic exposure, the increase in cellular betaine and myo-inositol exceeded that of taurine. In lipopolysaccharide (LPS)-treated SEC, hyperosmotic exposure markedly raised the mRNA levels for cyclo-oxygenase-2 (COX-2), but not for inducible nitric oxide synthase (iNOS). The increase of COX-2 mRNA levels was counteracted by betaine and taurine and, to a lesser extent, by myo-inositol. The findings indicate that SEC use taurine, betaine, and myo-inositol as compatible organic osmolytes.