Regulation of hemolymph amino acid levels and active secretion of proline by Malpighian tubules of locusts

Levels of individual and total (40–50 mM) amino acids remained relatively constant in locust hemolymph despite 50% changes in hemolymph volume during hydration and dehydration: proline, glycine, and glutamine constituted 70% of the total. Likewise, amino acid levels in tubular fluid did not change substantially during osmotic stress: proline constituted 80% of the total. Most hemolymph amino acids (P) appeared at lower concentrations in the tubular fluid (U); however, U/P ratios for glutamate and proline were 5.8 and 3.2, respectively. These results suggest that neutrally charged proline is actively transported by locust tubules, and this was confirmed by further experiments in vitro. Secreted proline is actively reabsorbed in the rectum where it probably serves as the principal substrate for oxidative metabolism in this tissue.