SOLVENT ISOTOPE EFFECT ON BILE FORMATION IN THE RAT

(H2O)-H-2 affects many membrane transport processes by solvent and kinetic isotope effects. Since bile formation is a process of osmotic filtration where such effects could be important, we investigated the effects 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 effects was observed. Therefore, further experiments were carried out at this concentration. The main finding was a reduction in bile salt-independent 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 bicarbonate concentration (P < 0.001). Choleretic response to neither taurocholate nor ursodeoxycholate was altered by (H2O)-H-2; in particular, there was a similar stimulation of bicarbonate secretion by ursodeoxycholate in the presence of 60% (H2O)-H-2. To further elucidate this phenomenon, the effect of (H2O)-H-2 on three proteins potentially involved in biliary bicarbonate secretion was studied in vitro. (H2O)-H-2 slightly inhibited cytosolic carboanhydrase and leukocyte Na+/H+-exchange, these effects reached statistical significance at 100% 2H(2)O only, however. In contrast, Cl-/HCO3--exchange in canalicular membrane vesicles was already inhibited by 50% (P < 0.001) at 60% 2H(2)O. Finally, there was a slight reduction in biliary glutathione secretion while that of the disulphide was not affected. Our results are compatible with an inhibition of canalicular Cl-/HCO3--exchange by (H2O)-H-2. Whether this is due to altered hydration of the exchanger and/or of the transported bicarbonate remains to be determined.