AJP - Endocrinology and Metabolism, Vol 263, Issue 6 E1021-E1028, Copyright © 1992 by American Physiological Society
Plasma membrane domain localization and transcytosis of the glucagon-induced hepatic system A carrier
R. Cariappa and M. S. Kilberg
Department of Biochemistry and Molecular Biology, J. Hillis Miller Health Center, University of Florida, Gainesville 32610.
We have utilized canalicular (cLPM) and basolateral (blLPM) liver plasma
membrane vesicles to investigate the domain localization of several
Na(+)-dependent amino acid transporters. Neutral amino acid transport by
systems N and ASC was measurable in both domains but was greater in blLPM
vesicles. Sodium-dependent glutamate uptake (system X-) was preferentially
localized to cLPM. The absolute activity and domain distribution of these
three carriers remained unchanged after treatment of rats with a
combination of glucagon and dexamethasone. A low level of basal system A
activity was present in both the blLPM and cLPM. Glucagon-induced system A
activity was first observed in blLPM vesicles approximately 60 min
posthormone treatment and, in cLPM vesicles, approximately 30 min later. In
situ perfusion of glucagon-treated rat liver with the membrane-impermeant
protein modification reagent glutathione maleimide specifically inactivated
blLPM system A activity and abolished the delayed arrival of
hormone-induced activity to the cLPM. Transient perfusion of the liver with
glutathione maleimide followed by a recovery period in vivo showed that the
reagent did not irreversibly inactivate the transcytotic process and also
provided an independent demonstration of the time delay between arrival of
the carrier activity at the two membrane surfaces. These results support
the concept of a transcytotic process in which the blLPM is the sorting
site for the hormone-induced system A carrier proteins that eventually
reach the cLPM.
