Role of opiate receptors in regulation of enkephalin stimulation of active sodium and chloride absorption.

The mechanism of opioid peptide stimulation of active Na and Cl absorption in rabbit ileum is not known. Since vasoactive intestinal peptide (VIP) modulation of electrolyte transport is mediated by specific membrane receptors, these experiments sought to determine whether membrane receptors for opioid peptides are present on rabbit ileal enterocytes. Although we found specific binding both of radiolabeled opioid peptides to homogenates of cerebrum and ileal myenteric plexi and of 125I-VIP to ileal enterocytes, specific binding of radiolabeled opioid peptides to either ileal enterocytes or their homogenates was not identified. In parallel studies, tetrodotoxin, an inhibitor of neurotransmission, did not alter VIP-stimulation of Cl secretion but inhibited D-Ala2-methionine-enkephalinamide-induced electrolyte absorption. These studies suggest that opioid peptide stimulation of active Na and Cl absorption is mediated by an unidentified intermediary agonist.