Ionic requirements for the active ileal bile salt transport system

Abstract

Taurocholate transport by everted ileal gut sacs was studied in physiological media containing graded amounts of sodium ions. Significant uphill transport of taurocholate was observed when the bulk of NaCl was replaced by osmotic equivalents of mannitol or choline chloride. Seventy-seven percent of control transport activity was observed when 36 milliequivalents per liter of Na were present in the incubation medium with mannitol acting as the isosmotic replacement, and 74f the control transport was retained when 31 milliequivalents per liter of Na were present in the incubation medium with choline chloride acting as the osmotic replacement. Lowering the Na concentration to 19 milliequivalents per liter (i.e., 84eplacement of Na ) still allowed for 69f the uphill transport observed in the control incubations. Taurodehydrocholate transport by ileal everted sacs was more sensitive to decreased Na concentrations; 29f control transport was observed at 31 milliequivalents per liter of Na . A kinetic analysis comparing the transport of taurocholate with taurodehydrocholate, the triketo analogue, at different concentrations of Na indicated that the apparent affinity of the transport system for Na is greater in the presence of taurocholate than in the presence of taurodehydrocholate. The ability of taurodehydrocholate to depress taurocholate transport is less in media of low Na concentration. Finally, in vivo intestinal perfusion studies demonstrated that the depression of taurocholate absorption, following Na removal, is reversible. These results are in agreement with the idea that Na has a physiological role in intestinal bile salt transport, and that the affinities of the anionic bile salt and the sodium cation for the transport system appear to be cooperative in that one enhances the binding of the other