Energization of alanine transport in isolated rat hepatocytes. Electrogenic Na+-alanine co-transport leading to increased K+ permeability.

The mechanism of alanine transport across the cell membrane was studied at 37°C in isolated hepatocytes from 48-h fasted rats. Amino-oxyacetate, an alanine aminotransferase inhibitor, was used to separate the transport of alanine from its metabolism. The influx of ['4C]alanine was dependent on the extracellular Na' concentration. Conversely, addition of alanine increased the "Na' influx, and the coupling ratio between Na' and alanine was 1.36:l. Dissipation of the Na' electrochemical gradient by gramicidin D or substitution of Na' with choline both decreased the steady state intracellular/extracellular distribution ratio of alanine from about 20 to about 1. An increase in the intracellular negativity, induced by valinomycin, led to an increase in the alanine influx. There was no correlation between the effect of these ionophores on alanine transport and their effect on cellular ATP. The data indicate that the uptake of alanine in rat hepatocytes is energized by the Na' electrochemical gradient. The rate constant of the passive *'K+ efflux from 42K+ preloaded hepatocytes was increased by alanine, maximally with a factor of 3 t o 4 at extracellular alanine concentrations of 10 to 30 m ~ . "he increase in 42K+ efflux was parallel with the Na+-dependent alanine influx and was not influenced by inhibition of the intracellular metabolism of alanine. Avariety of other amino acids also increased the 42K+ efflux. The passive (ie. ouabain noninhibitable) component of the 42K+ influx was increased with a factor of 4 to 5 by 10 m~ alanine, and a calculation showed that 10 m~ alanine increased the K' permeability with a factor of 5 from a basal value of 1.2 X cm-s-'. The transport-linked increase in the K' permeability, which tends to increase the intracellular negativity, represents a hitherto unrecognized mechanism for support of the Na' electrochemical gradient during the electrogenic co-transport of Na+ and amino acids.