The effects of hypothyroidism and fasting on electrogenic amino acid transfer: possible evidence for multiple neutral amino acid carrier systems in rat jejunum.

The jejunal mechanisms for the electrogenic transfer of four neutral amino acids (alanine, leucine, methionine, valine) and for sarcosine were characterised by an electrical method in vitro. The values for apparent Km obtained electrically agree well with those assessed by conventional chemical techniques. Hypothyroidism and/or fasting rats for 3 days induced differential changes in the apparent Km and p.d.max for the various amino acids. These alterations were interpreted as indicating the presence of at least three mechanisms for neutral amino acid transfer and one for sarcosine. In euthyroid rats, only alanine showed changes in apparent Km (decrease) and p.d.max (decrease) after fasting for 3 days. With hypothyroidism the kinetic parameters of electrogenic transfer for alanine, valine and sarcosine were significantly altered while those for leucine and methionine were unaffected.