IMMUNOHISTOCHEMICAL STUDY OF LEUCINE-ENKEPHALIN AND ITS SECRETORY EFFECTS IN THE ISOLATED PIG LACRIMAL GLAND

Background: In our previous studies immunohistochemical studies have d emonstrated the presence of leucine-enkephalin (Leu-Enk) in the intrin sic nerves of the pig lacrimal gland, which are discernible in the int erlobular and interacinar areas from where branches that innervate the acinar tissues are sent. Since the intrinsic nerves have been shown t o contain Leu-Enk, this study aimed to investigate the secretory effec ts of this neuropeptide in isolated segments of the pig lacrimal gland and to reconfirm its presence in the neuronal tissue of the lacrimal gland. Methods: Leu-Enk was identified using immunohistochemical techn iques, while total protein output was measured in the effluent samples by an automated on-line colorimetric method. The Ca2+ and Mg2+ concen trations in the effluent samples were determined using an atomic absor bance spectrophotometer. Results: Leu-Enk (10(-12)-10(-7) M) evoked ma rked increases in total protein output from superfused lacrimal gland segments. The secretory effect of Leu-Enk was not blocked by pretreatm ent of the tissue with atropine but was substantially reduced by a com bination of phentolamine and propranolol. The competitive antagonist, naloxone, has no effect on basal protein output, but when combined wit h Leu-Enk it caused a significant reduction in total protein output. C ombining theophylline (10(-3) M) with Leu-Enk resulted in a marked pot entiation of total protein output. In superfused lacrimal gland segmen ts Leu-Enk (10(-8) M) evoked a net efflux of magnesium (Mg2+ release) and a net influx of calcium (Ca2+ uptake). Conclusion: The results ind icated a physiological role for Leu-Enk in the regulation of protein s ecretion in the pig lacrimal gland.