THE PATTERN OF VASOPRESSIN-INDUCED REDUCTION IN BILIARY OUTPUT OF CHOLEPHILIC PROBES IN THE RAT CAN BE MIMICKED BY DIALYSIS

During a recent study using isolated perfused rat liver, we concluded that the effects of vasopressin on biliary excretion of several anionic and cationic cholephilic probes could best be explained by passive diffusion of these probes through a paracellular pathway with permeability increased by vasopressin. Replication of the results of that study in an in vitro dialysis system would support the passive nature of the selective efflux of these cholephilic probes from the rat biliary tree and render metabolic events an unlikely explanation for the biliary excretory patterns of the probes. We studied biliary excretion of two bile acids - taurodehydrocholate and taurocholate - and two cholephilic cations - propidium and tributylmethylammonium - perfused single-pass through the isolated rat liver. We collected bile and measured probe output before and during infusion of 10(-8) mol/L vasopressin. We also studied rates of diffusion of these probes in artificial solution and in whole rat bile through a 3,500 mol wt cut-off cellulose dialysis membrane. The relative order of probe efflux rates from the biliary tree with vasopressin administration paralleled their diffusion rates through the dialysis membrane. The only exception was that the biliary tree manifested charge selectivity. The data are compatible with the passive efflux of probe from the biliary tree, and metabolic events need not be invoked to explain their pattern of biliary excretion with vasopressin administration.