Glucocorticoid and catecholamine stimulation of amino acid transport in rat hepatocytes. Synthesis of a high-affinity component.

The kinetic properties of glucocorticoid and catecholamine stimulation of amino acid transport in freshly isolated rat hepatocytes were investigated. In the basal state (i.e., with hepatocytes incubated for 2 h in the absence of glucocorticoid or catecholamine), the saturable transport of alpha-aminoisobutyric acid (AIB) was accounted for mainly by a low-affinity component (Km for AIB approximately 5 mM). Hepatocyte exposure to cortisol (or dexamethasone), or to epinephrine for isoproterenol), for 2 h resulted in a 3- to 4-fold increase in the Vmax of a high-affinity component (Km for AIB approximately 1 mM) which was only weakly expressed in the basal state. Neither glucocorticoids nor catecholamines exerted a detectable effect on the low-affinity transport component. Cycloheximide prevented the emergence of the high-affinity component in hepatocytes exposed to dexamethasone or epinephrine. The results suggest that the stmulatory effect of glucocorticoids and catecholamines on amino acid transport in hepatocytes results from the synthesis of a high-affinity transport component.