Control of alanine metabolism in rat liver by transport processes or cellular metabolism

Effect of alanine transport on ionic fluxes across the cell membrane in rat liver

Changes in cell volume and alanine, Na+ and K+ concentrations and fluxes were studied in rat isolated hepatocytes or perfused liver. Addition of 10 mM alanine elicited a noticeable entry of Na+ which accumulated into the cells, along with a leakage of cell K+. Concurrently, the cell volume was 13% increased. These processes were also present with perfused livers in which alanine uptake, Na+ entry and cell swelling were proportional to the induction of the A system. K+ efflux was parallel to cell swelling and occurred very early after exposure of liver cells to alanine (and was transitory). The rôle of extracellular anions was investigated: alanine transport was not affected by replacement of Cl- and HCO-3 by a permeant anion whereas it was inhibited with the non-permeant anion isethionate. The present results indicate that the disturbances of the ionic gradients consecutive to alanine transport in isolated hepatocytes also operate in the liver organ itself. The possible implications of these phenomenons are discussed.

Organic ion transport during seven decades. The amino acids

The amino acids are ions of various charge combinations, and one can argue that historically they were the first ions for which the ongoing problem of membrane transport was presented; also that among transported ions these may undergo a highly detailed molecular recognition. Furthermore, the distribution of charge on the amino acid molecule determines by what route or routes it is conducted across the biological membrane, with what directional and structural specificity, and therefore what regulation is imposed, and where. Cases where a presumably charged chemical group behaves as if it were somehow absent from the amino acid have been observed to fall into several categories: Straightforward cases where the pH has been low enough or high enough to remove the charge by protonation or deprotonation, even in free solution. Cases where that protonation or deprotonation is facilitated at the binding site, and perhaps by the total transport process. The cystine molecule can apparently thus be rendered either a tripolar anion or a tripolar cation for transport. Cases where an otherwise co-transported Na+ is omitted to redress charge, or where a Na+ serves as a surrogate for a missing charged group on the amino acid molecule. A case where the protonation occurs reversibly at the receptor site rather than on the amino acid molecule.

Bidirectional transport of glutamine across the cell membrane in rat liver

Hepatocytes isolated from fed rats were used to investigate glutamine transport. Glutamine transport appears as a composite process involving at least two saturable components. The Na+-dependent component probably represents the entry through the N system. The Na+-independent component was also inhibited by histidine and exhibited trans-stimulation, suggestive of a facilitated diffusion process. Kinetic parameters for both systems suggest that facilitated diffusion only plays a minor role in glutamine influx. In contrast, the Km for glutamine efflux was consistent with a physiological role of the facilitated-diffusion component in glutamine release. In Na+ medium, relatively constant distribution ratios (about 8) between intra- and extra-cellular concentrations were observed, with external glutamine ranging from 0.5 to 5 mM. The present observations suggest that glutamine influx might largely be mediated by the N system, whereas facilitated diffusion allows hepatocytes to release glutamine when intracellular concentrations are elevated. The physiological consequences of this bidirectional transfer of glutamine across the liver cell membrane is discussed.

Hypothesis: control of hepatic utilization of alanine by membrane transport or by cellular metabolism? Review

The rate of alanine transport into the liver limits its utilization even under the high alanine load resulting from a 90% casein diet, given that the rat has been adapted to that diet. A coordinated acceleration of alanine catabolism allows transport to remain rate-limiting, which in turn allows the adaptive regulation of transport to remain effective at high alanine loads. Accelerated degradation of alanine may change the hepatic amino acid content in a way that derepresses the activity of the alanine carrier system(s).