Subcellular distribution of muscarinic acetylcholine receptors in rat exorbital lacrimal gland

The muscarinic acetylcholine receptor (MAChR) is an important mediator of parasympathetic regulation of secretion by the rat exorbital lacrimal gland. In order to survey the subcellular distribution of MAChR in lacrimal acinar cells, we have measured the binding of the specific muscarinic cholinergic antagonist [³H]-quinuclidinyl benzilate ([³H]-QNB) to membrane samples isolated from rat exorbital lacrimal glands by differential and equilibrium density gradient centrifugation. Binding of [³H]-QNB in all membrane fractions was consistent with the presence of a single class of receptor which was muscarinic in nature on the basis of its K(d) for [³H]-QNB (0.30-0.35 nM) and its ability to interact with the muscarinic agonists carbachol and methachol and the antagonist atropine. MAChR were present at the highest specific activity in acinar cell basal-lateral plasma membrane-derived populations, where B(max) was as high as 1960 fmole/mg protein. However, the density distributions of MAChR and of other membrane markers indicated that the receptors were present also in membranes derived from cytoplasmic structures, where B(max) values ranged from 50.4 to 152.8 fmole/mg protein. Stimulation with 10 μM carbachol for 30 min led to a 20% (P < 0.05) increase in the relative MAChR content of a population of membranes derived from the acinar cell basal-lateral membrane; an apparent tendency for MAChR activity to decrease in other membrane populations suggests that stimulation might cause a redistribution of MAChR between cytoplasmic pools and the cell surface membranes.