Effect of methionine-enkephalin analog (FK 33-824) on plasma motilin.

In an attempt to elucidate the effect of opioid peptide on the secretion of various gastrointestinal hormones, the methionine-enkephalin analog, FK 33-824 (FK) was injected intramuscularly in healthy male adults. The plasma levels of motilin, gastrin and cholecystokinin were assessed by specific radioimmunoassay. After the injection of FK, the plasma level of motilin markedly decreased from the baseline value of 456 +/- 70.2 pg/ml to 264 +/- 44.7 pg/ml at 120 min. On the other hand, the plasma levels of cholecystokinin and gastrin remained unchanged. These data indicate that endogenous methionine-enkephalin may have an direct inhibitory effect on the secretion of motilin.