The proabsorptive effect of D-ala²-metenkephalinamide is mediated by naloxone-sensitive opiate receptors in the isolated perfused ileum

Methionine-enkephalin is an endogenous opiate pentapeptide, originally isolated in the brain, that exists within enteric plexuses and enterocytes. The purpose of this study was to delineate the effects of the opiate agonist methionine-enkephalin on intestinal water and electrolyte transport, with the stable analog D-ala²-metenkephalinamide (m-ENK). Ileal segments from New Zealand white rabbits (n = 39) were harvested and vascularly and luminally perfused ex vivo. Net fluxes of H₂O, Na⁺, and Cl^- were calculated for three 20-minute periods: basal, drug infusion, and recovery. Six groups were studied: (1) control, (2-4) m-ENK at three doses, (5) naloxone, and (6) naloxone plus m-ENK. Oxygen consumption and arterial perfusion pressure were assessed as measures of metabolic activity and viability. The control and naloxone groups had no changes in the fluxes of water and electrolytes. Significant proabsorptive effects were demonstrated for the fluxes of H₂O, Na⁺, and Cl^- at increasing doses of m-ENK (p < 0.05). Naloxone completely prevented m-ENK-induced absorption. These results with exogenous m-ENK suggest that endogenous methionine-enkephalin, serving as an enteric neurotransmitter and acting through naloxone-sensitive opiate receptors, may function as a physiologic modulator of intestinal water and electrolyte absorption.