AJP - Gastrointestinal and Liver Physiology, Vol 249, Issue 1 34-G38, Copyright © 1985 by American Physiological Society
Enhancement of Na+-dependent bile acid uptake by albumin: direct demonstration in rat basolateral liver plasma membrane vesicles
B. L. Blitzer and L. Lyons
The effects of albumin on taurocholate uptake by rat basolateral liver
plasma membrane vesicles were examined. With this system direct effects on
carrier-mediated Na+-dependent bile acid uptake may be distinguished from
nonspecific alterations in carrier-independent diffusion. Bovine serum
albumin increased both the initial velocity and peak uptake of taurocholate
in the presence of an Na+ gradient but did not enhance Na+-independent bile
acid uptake. The effects of albumin were strikingly dependent on albumin
concentration. Maximal enhancement of bile acid uptake was observed at
albumin concentrations of 0.125 g/dl (18 microM) to 0.25 g/dl (37 microM),
which are similar to reported values for the Kd for albumin binding to the
plasma membrane. At high albumin concentrations (2.5 g/dl, 367 microM),
uptake was reduced but to a lesser extent than the expected fall in free
bile acid concentration. Kinetic studies showed that albumin (0.5 g/dl, 74
microM) reduced the taurocholate Km from 36.5 to 16.1 microM but did not
affect Vmax, suggesting that albumin binding may increase the affinity of
the bile acid receptor for taurocholate. Bovine gamma-globulin, chicken
ovalbumin, and human transferrin did not enhance taurocholate uptake.
Control experiments demonstrated that the mechanism of the albumin effect
was not dependent on residual albumin-bound calcium and did not involve
alteration of the underlying driving forces for bile acid uptake (Na+
gradient). These studies have "unmasked" an enhancing effect on
taurocholate uptake by low concentrations of albumin that would not have
been readily detected at higher concentrations.
