Journal of Lipid Research, Vol 17, 572-577, Copyright © 1976 by Lipid Research, Inc.
Ionic requirements for the active ileal bile salt transport system
K Gallagher, J Mauskopf, JT Walker and L Lack
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 74% of 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., 84% replacement of Na+) still allowed for 69% of the
uphill transport observed in the control incubations. Taurodehydrocholate
transport by ileal everted sacs was more sensitive to decreased Na+
concentrations; 29% of 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.
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