The influence of intestinal absorption of an amino acid on the subsequent absorption of the same amino acid.

Cysteine absorption was determined in an intestinal segment "in situ" in the rat under general anaesthesia, perfusing the segment twice for 30 minutes, at one hour interval, with the same amount of cysteine dissolved in a 0.85% NaCl solution. At the second perfusion, absorption of the amino acid was about half that at the first perfusion. As the first perfusion reduces absorption even in a segment not previously perfused, and intraperitoneal injection of cysteine does not influence absorption, it is postulated that an inhibitory factor, released from the intestinal wall, intervenes by humoral route in the regulating mechanism of the absorption of amino acids.