AJP - Gastrointestinal and Liver Physiology, Vol 249, Issue 1 120-G124, Copyright © 1985 by American Physiological Society
Kinetic and energetic aspects of the inhibition of taurocholate uptake by Na+-dependent amino acids: studies in rat liver plasma membrane vesicles
B. L. Blitzer and R. L. Bueler
The kinetic and energetic aspects of the inhibition of taurocholate uptake
by the Na+-dependent amino acid L-alanine were studied in rat basolateral
liver plasma membrane vesicles. In the presence of an inwardly directed Na+
gradient, alanine (5 mM) reduced the initial velocity of taurocholate
uptake to 60% of control and virtually abolished the overshoot. In the
presence of a K+ gradient, the slow rate of Na+-independent taurocholate
uptake was similar in the presence or absence of the amino acid. Inhibition
of Na+-dependent taurocholate uptake increased nonlinearly with alanine
concentration (half-maximal inhibition at approximately 1 mM) and plateaued
at 5-10 mM. Kinetic studies showed that alanine significantly reduced the
Vmax for taurocholate uptake from 6.32 +/- 0.22 to 3.68 +/- 0.21 nmol X mg
prot-1 X min-1 but did not significantly affect taurocholate Km (38.4 +/-
3.6 vs. 29.0 +/- 4.9 microM). In contrast, the Na+-independent amino acid
2-aminobicyclo[2.2.1]heptane-2-carboxylic acid did not affect either the
initial velocity or peak uptake of taurocholate. The effects of alanine on
the driving forces for bile acid uptake were directly assessed by measuring
vesicle uptake of 22Na. At early time points, 22Na uptake was faster in the
presence of alanine than under control conditions. These findings provide
further evidence that Na+-dependent amino acids noncompetitively inhibit
Na+-dependent bile acid uptake in association with accelerated dissipation
of the transmembrane Na+ gradient and extend previous observations of this
phenomenon made in isolated rat hepatocytes [Am. J. Physiol. 245
(Gastrointest. Liver Physiol. 8): G399-G403, 1983].
