Plasma membrane domain localization and transcytosis of the glucagon-induced hepatic system A carrier

Abstract

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.