Effects of bilirubin on transepithelial transport of sodium, water, and urea

Urinary concentrating defects and renal salt wasting have been described in the hyperbilirubinemic Gunn strain strain of rat. Homozygous animals demonstrate significant reductions in renal medullary urea and sodium ion concentrations. These observations are consistent with possible bilirubin associated disorders in the transepithelial transport of water and solute. To test this hypothesis, measurements of active sodium transport and passive water and urea fluxes were made in hemibladders isolated from the Dominican toad, Bufo marinus. Tissues were exposed to amphibian bicarbonate Ringer's solution containing 0.1 mM bilirubin with 0.05% bovine serum albumin (BSA) or BSA alone. Vasopressin-stimulated sodium transport, as reflected by short circuit current (SCC), was inhibited by 18 +/- 6% in the presence of bilirubin (N = 10; P less than 0.02). Cyclic AMP (p-Cl-phenylthio cAMP 10(-5) M) stimulated SCC was inhibited to a similar degree in the presence of bilirubin. The inhibition was noted only when bilirubin was in the serosal bath, and it could be abolished with BSA 0.5%. Bilirubin had no effect on the increase in SCC induced by higher concentrations of cyclic AMP (10(-4) M), aldosterone, or amphotericin B. Furthermore, bilirubin had no effect on the hydro-osmotic response to vasopressin and vasopressin-induced changes in urea permeability. These findings show that short-term exposure to bilirubin exerts a tissue-specific effect on the vasopressin-stimulated active transport of sodium but has no effect on the vasopressin-induced fluxes of water and urea.