Membrane potential of rat hepatoma cells in culture: Influence of factors affecting amino acid transport

Abstract

The effect has been studied of various media, hormones and of amino acids on the membrane potential of rat hepatoma cells in culture measured by microelectrode impalement. Cells in Eagle's minimal essential medium plus 5% serum had a value which varied daily from about 5-8 mV, inside negative. The membrane potential of rat hepatocytes was measured to be 8.7 +/- 0.2 mV, inside negative. The membrane potential of the hepatoma cells was decreased by insulin and increased by glucagon. Membrane potential was unaffected by change of medium to Hanks' or Earle's balanced salt solutions or deprivation of serum. It was, however, reduced in cells in phosphate-buffered saline and by reduction of pH. The former effect was shown to be due to the higher [Na+] of phosphate-buffered saline as opposed to the other media. Addition of alanine, glycine, serine, proline and methylaminoisobutyrate all reduced membrane potential by 2-3 mV. Smaller decreases were seen with methionine, leucine and phenylalanine, but none with glutamine, threonine, BCH (2-aminonorborane-2-carboxylic acid) and D-alanine. The results are compared with the effects of similar conditions on aminoisobutyrate uptake. Whilst there was a correlation under some conditions there was not under others. It is concluded that for the hepatoma cells factors additional to the membrane potential must exert some influence on the capacity for amino acid transport.

Threonine metabolism in isolated rat hepatocytes

“…3), indicating that transporters on the plasma membrane, the mitochondrial membranes, or both may be increased upon the binding of glucagon to its receptor on the plasma membrane, thus providing at least a partial explanation for the increased oxidation of threonine observed in the presence of glucagon. The specific acute mode of action whereby glucagon acts to increase threonine influx is not fully understood but may involve changes mediated through an increase in membrane potential (23). Whether mitochondrial amino acid transport is increased due to glucagon stimulation is unknown; however, Mabrouk et al (22) have shown that glucagon leads to a phosphorylation of cytoplasmic proteins and a concomitant increase in flux through the glycine cleavage system in hepatocytes.…”