Permeability characteristics of bile duct in the rat

To determine the permeability of the bile duct epithelium of the rat to several hydrophilic nonelectrolytes, isolated segments of rat bile duct (1-2 cm) were perfused with Ringer solution in situ. [14C]urea, [14C]erythritol, [14C]sucrose, or [3H]inulin was administered intravenously, and drops of duct perfusate were collected. Perfusate-to-plasma ratios for each solute were inversely related to their molecular weight (MW): inulin [0.13 +/- 0.06 (SD)], sucrose (0.20 +/- 0.08), erythritol (0.32 +/- 0.11), and urea (0.34 +/- 0.13). Permeability coefficients (P) expressed in cm.s-1 X 10(5) were determined for inulin (4.5 +/- 1.5), sucrose (8.5 +/- 2.6), erythritol (12.6 +/- 3.9), and urea (15.8 +/- 1.6). These values vary directly with their respective nonaqueous-aqueous partition coefficients (K) and inversely with their MW. Although P is directly related to K in both olive oil and isobutanol systems, the bile duct behaves as if its permeability barrier is more hydrophilic than isobutanol. These results indicate that the inert solutes urea, erythritol, sucrose, and inulin cross the epithelium of the rat bile duct by a polar route, possibly via intercellular junctions, and that erythritol may not be an ideal marker of canalicular bile flow.