Proenkephalin A derivatives in lacrimal gland: occurrence and regulation of lacrimal function

The derivatives of proenkephalin A were measured in acid extracts of rat lacrimal glands by specific radioimmunoassay. Glands from adult male rats contained all four derivatives of the opiate precursor. The content in the gland of proenkephalin A-derived peptides Met5-enkephalin, Leu5-enkephalin, Met5-enkephalin Arg6-Phe7 and Met5-enkephalin Arg6-Gly7-Leu8 indicates tissue specific processing with an enhancement of the heptapeptide. The effect of enkephalins on the activity of adenylate cyclase in lacrimal membranes was measured and compared with the effect of the synthetic enkephalin analogue D-ala2-methionine enkephalinamide (DALA) that inhibits both lacrimal protein secretion and lacrimal adenylate cyclase. In the presence of the peptidase inhibitors thiorphan and bestatin, the inhibition of forskolin-stimulated adenylate cyclase activity by Met5-enk, Leu5-enk, Met5-enk Arg6-Phe7 and DALA were identical. Maximum inhibition was approximately 35% at a dose of 50 microM enkephalin. Addition of the octapeptide, Met5-enk Arg6-Gly7-Leu8 resulted in decreased adenylate cyclase activity; however, the effect was not statistically significant. Activation of delta opioid receptors by the endogenous enkephalins is indicated by the reversal of adenylate cyclase inhibition in the presence of the delta-receptor antagonist ICI 174864. The data support the physiological significance of in vitro inhibition of lacrimal secretion by DALA and indicate a possible role for endogenous enkephalins in lacrimal function.