Effect of enkephalin and naloxone on vagal and hormonal stimulation of pancreatic secretion in man.

Pancreatic secretory responses to modified sham-feeding (MSF) and secretin (1 U/kg-h) infusion were examined in eight healthy subjects with and without administration of the stable enkephalin analogue (Ala-Enk) and/or naloxone. MSF produced a clear increase in pancreatic protein output, reaching about 45% of caerulein (0.5 microgram/kg-h) maximal output. This response was significantly reduced by Ala-enk (20 micrograms/kg-h) but was not affected by naloxone (40 micrograms/kg-h). Ala-Enk given during a well-sustained plateau of pancreatic response to secretin also significantly suppressed both bicarbonate and protein; the effect was not influenced by the addition of naloxone. Naloxone alone without Ala-Enk did not affect basal or stimulated pancreatic secretion. We conclude that enkephalin inhibits pancreatic secretion induced by vagal or secretin stimulation, and this inhibition does not seem to be mediated by opiate receptors of the mu type, since naloxone was ineffective in the prevention of this effect.