Determinants of biliary secretion in isolated perfused skate liver

In the isolated perfused liver of the little skate, Raja erinacea, bile flow averaged 5.07 +/- 0.58 (mean +/- SE) microliters.h-1.g liver-1 in 21 experiments at a perfusion pressure of 5.0 cm Ringer compared to 3.79 +/- 0.32 in 38 experiments at 2.5 cm (P less than 0.05). [14C]inulin readily entered skate bile. Bile-to-plasma [14C]inulin ratios corrected for delay in transit time, averaged 0.46 +/- 0.07 at 1 h and rose to 0.74 +/- 0.06 by 4 h, although bile flow remained constant. In experiments in which [14C]inulin reached equilibrium between bile and plasma, the bile-to-plasma ratio conformed to the theoretical relationship between bile flow, solvent drag, and inert solute diffusion predicted at extremely low bile flows, but demonstrated that the skate biliary tree is more permeable to inulin than that of the rat. Electron microscopic studies demonstrated that ionic lanthanum could traverse the tight junctions. However, freeze-fracture studies of junction structure did not differ qualitatively from similar studies in the rat. Partial dependence of bile flow on perfusion pressure, high bile-to-plasma inulin ratios, and permeability of the canalicular tight junctions to ionic lanthanum all suggest that the paracellular pathway may be an important component of bile formation in the skate.