Concentrative uptake of alanine in hepatocytes from fed and fasted rats

The transport of alanine across the liver cell membrane was studied in hepatocytes isolated from fed and 48-h-fasted rats. Aminooxyacetate was used to render alanine nonmetabolizable. The steady-state intracellular-to-extracellular distribution ratio of alanine was maximal at extracellular concentrations below 0.5 mM and was increased from about 10 to about 20 by fasting. This increase was the net effect of a two- to threefold increase in the alanine influx and about a 50% increase in the rate constant of alanine efflux. The results suggest that alanine efflux occurred partly via the transport system mediating Na+-dependent alanine influx and partly by another pathway. The transmembrane Na+ electrochemical gradient remained unchanged by fasting and could apparently account for a maximal distribution ratio of alanine well above 20. In both nutritional states, the simplest kinetic model adequately describing the alanine influx implicated one saturable component besides a passive component. The apparent Vmax of the former was doubled by fasting while the apparent Km was insignificantly decreased. At low extracellular alanine concentrations, the rate of alanine consumption (aminooxyacetate absent) was only 40% of the alanine influx in the fed state but was increased at least fivefold by fasting and thereby balanced with the increased alanine influx. These results suggest a rate limitation at the transport level in the fasted state.