M3 muscarinic receptor mediates regulation of protein secretion in rabbit lacrimal gland.

PURPOSE: To identify which muscarinic receptor subtypes mediate protein secretion in isolated rabbit lacrimal glands. To compare protein secretory profiles in vitro with those in rabbit tears.
METHODS: Rabbit lacrimal gland slices were incubated with carbachol in the presence and absence of different muscarinic antagonists. Protein secretion into the incubation medium was characterized with a Bio-Rad protein assay dye reagent in conjunction with Laemmli's method of SDS-PAGE. The medium protein profile was compared to that in tear samples.
RESULTS: Dose dependent increases in protein secretion were elicited by carbachol between 10(-7) and 10(-4) M. During the first 20 min period, a maximal increase of 64% above the basal level was seen at the highest concentration. With 10(-4) M carbachol, the response was transient because after 100 min it decreased to its basal level. The increases in protein secretion caused by 10(-4) M carbachol were completely suppressed in the presence of 10(-5) M atropine. On the other hand, the relatively selective M1 antagonist, pirenzepine, at concentrations from 10(-6) M to 10(-4) M, had no effect on either the basal levels or the stimulatory effects of carbachol. Similarly, the relatively selective M2 antagonist, gallamine, at concentrations from 10(-6) M to 10(-4) M, had no effect on either of these levels. In contrast, the relatively selective M3 antagonist, 4-DAMP, at concentrations from 10(-6) M to 10(-4) M, progressively suppressed the stimulated level of protein secretion elicited by 10(-4) M carbachol without affecting the basal level. The protein profiles found in the tears and in the incubation medium were similar to one another.
CONCLUSION: In vitro rabbit lacrimal gland protein secretion is comparable to that in vivo. Cholinergic-mediated control of rabbit lacrimal gland protein secretion occurs through stimulation of the M3 muscarinic receptor subtype.