Hormonal regulation of amino acid transport system N in primary cultures of rat hepatocytes

Abstract

The transport of histidine and glutamine via system N in cultured hepatocytes was found to be subject to hormonal control. This long‐term regulation showed the following characteristics. 1 The transport capacity for histidine and glutamine (system N) increased slowly in response to the combination of dexamethasone and insulin to about 4‐fold that of controls after 18–30 h. A similar time course was found for the stimulation of system N (2.5‐fold) by dexamethasone and glucagon. In contrast the uptake of α‐aminoisobutyric acid (system A) was rapidly stimulated 3‐fold by dexamethasone and insulin and 5‐fold by dexamethasone and glucagon within 3–6 h but decreased towards control rates after 24 h of cultivation in minimal essential medium. 2 Dexamethasone, insulin and glucagon each stimulated glutamine uptake about 2‐fold in cultures maintained in W/AB 77 medium, while the combination of dexamethasone with either glucagon or insulin resulted in a 3–4‐fold increase. 3 Dexamethasone was most effective at about 0.1 μM. Higher concentrations were less efficient. Insulin reached its optimal effect at concentrations above 1 μM. 4 Kinetic analysis revealed that the increased capacity of glutamine transport in response to hormones was due to an increase in V_max, while K_m was essentially unchanged. 5 The hormone‐induced stimulation of system N was prevented by cycloheximide. 6 The induced uptake of glutamine was inhibited by excess amounts of asparagine and histidine but not of α‐methylaminoisobutyric acid or cysteine. These results clearly differentiate the hormonal regulation of system N from that of system A.