Characterization of functional melanotropin receptors in lacrimal glands of the rat

The specific melanotropin (MSH) binding sites of rat lacrimal glands were characterized with respect to anatomic distribution, peptide specificity and selectivity, and coupling to a biological response. Tissue distribution of MSH binding sites was determined by autoradiography following in situ binding of a radiolabeled, biologically active preparation of a superpotent alpha-MSH analog, 125I]-Nle4,D-Phe7]-alpha-MSH (125I]-NDP-MSH). Intense, specific (i.e., alpha-MSH-displaceable) 125I]-NDP-MSH binding was observed throughout lacrimal acinar tissue, but not in ducts or stroma. In freshly isolated lacrimal acinar cells, specific binding of 125I]-NDP-MSH was maximal within 30 min and rapidly reversible, with a dissociation half-time of about 15 min. A number of melanotropins alpha-MSH, N,O-diacetyl-Ser1]-alpha-MSH, des-acetyl-Ser1]-alpha-MSH, beta-MSH, ACTH(1-24) and ACTH(1-39)] were recognized by these binding sites, as assessed by their inhibition of 125I]-NDP-MSH binding; NDP-MSH was the most potent (IC50 = 1.3 x 10(-9) M). In contrast, other peptides, including ACTH(4-10) and the nonmelanotropic peptides VIP, substance P, somatostatin, and ACTH(18-39) (CLIP), had no effects on tracer binding. In isolated lacrimal acinar cells, alpha-MSH and NDP-MSH stimulated intracellular cyclic AMP accumulation. We conclude that lacrimal acinar cells express functional receptors recognizing melanotropins, suggesting that the lacrimal gland may be a target for physiological regulation by endogenous melanotropins.