Characteristics of L-alanine uptake in freshly isolated hepatocytes of elasmobranch Raja erinacea.

The specific transport mechanisms that mediate the hepatic uptake of L-[3H]alanine and of an unnatural homologue, alpha-[14C]methylaminoisobutyric acid (MeAIB), were analyzed in hepatocyte suspensions from Raja erinacea. Aminooxyacetate, an inhibitor of aminotransferase activity was used to prevent alanine metabolism. After 3 h of incubation with either 0.5 mM alanine or MeAIB, hepatic concentrations of these amino acids were significantly higher in the presence than absence of Na+ (8 vs. 1 and 1 vs. 0.1 mM, respectively). Kinetic studies indicated that both alanine and MeAIB transport occurred via sodium-dependent saturable mechanisms. [14C]MeAIB uptake was completely inhibited by excess L-alanine. Uptake of [3H]alanine was inhibited by a 40-fold excess of serine and cysteine (53-54%), by MeAIB and methylalanine (26-31%), and by leucine (14%), whereas D-alanine, beta-alanine, taurine, and glutamate had no effect. Insulin and glucagon were unable to stimulate [3H]alanine uptake. Glucose release from hepatocytes was unaffected by 10 mM alanine or 2 mM aminooxyacetate, indicating that alanine is not a major gluconeogenic precursor in this marine elasmobranch. These results suggest that uptake of L-alanine by skate hepatocytes occurs predominantly via a sodium-dependent system, with properties similar to those exhibited by the ASC neutral amino acid transport system previously characterized in Ehrlich ascites tumor cells and rat hepatocytes.