SOLVENT ISOTOPE EFFECT ON BILE FORMATION IN THE RAT

(H2O)-H-2 affects many membrane transport processes by solvent and kin etic isotope effects. Since bile formation is a process of osmotic fil tration where such effects could be important, we investigated the eff ects of (H2O)-H-2 on bile formation in the in situ perfused rat liver. Dose finding experiments showed that at high concentrations, (H2O)-H- 2 increased vascular resistance and induced cholestasis; at 60% (H2O)- H-2 however, a clear dissociation between the vascular and biliary eff ects was observed. Therefore, further experiments were carried out at this concentration. The main finding was a reduction in bile salt-inde pendent bile flow from 0.99+/-0.04 to 0.66+/-0.04 mu l . min(-1). g(-1 ) (P < 0.001). This was associated with a 40% reduction in biliary bic arbonate concentration (P < 0.001). Choleretic response to neither tau rocholate nor ursodeoxycholate was altered by (H2O)-H-2; in particular , there was a similar stimulation of bicarbonate secretion by ursodeox ycholate in the presence of 60% (H2O)-H-2. To further elucidate this p henomenon, the effect of (H2O)-H-2 on three proteins potentially invol ved in biliary bicarbonate secretion was studied in vitro. (H2O)-H-2 s lightly inhibited cytosolic carboanhydrase and leukocyte Na+/H+-exchan ge, these effects reached statistical significance at 100% 2H(2)O only , however. In contrast, Cl-/HCO3--exchange in canalicular membrane ves icles was already inhibited by 50% (P < 0.001) at 60% 2H(2)O. Finally, there was a slight reduction in biliary glutathione secretion while t hat of the disulphide was not affected. Our results are compatible wit h an inhibition of canalicular Cl-/HCO3--exchange by (H2O)-H-2. Whethe r this is due to altered hydration of the exchanger and/or of the tran sported bicarbonate remains to be determined.