MUSCARINIC RECEPTOR SUBTYPES IN HUMAN NASAL-MUCOSA - CHARACTERIZATION, AUTORADIOGRAPHIC LOCALIZATION, AND FUNCTION-INVITRO

Muscarinic receptors play important roles in the regulation of glandul ar secretion and vasomotor tone in human nasal mucosa. M1, M2, and M3 muscarinic receptor subtypes were pharmacologically characterized in h uman inferior turbinates by receptor-binding assays using [H-3](-)quin uclidinyl benzilate (QNB, identifies total muscarinic receptors) and [ H-3]-pirenzepine (PZ). Receptors were localized by autoradiography, an d their function examined in vitro by assaying mucus secretion from cu ltured nasal mucosal explants. In competition assays, PZ was employed as a selective muscarinic antagonist for M1 receptors, gallamine and A F-DX 116 for M2 receptors, and 4-DAMP for M3 receptors. These ligands are selective at low nanomolar concentrations, but can interact with o ther muscarinic receptors at higher concentrations. It is not known if they can interact with putative M4 and M5 muscarinic receptor subtype s. Using [H-3](-)QNB, total muscarinic receptor binding was 688.4 +/- 49.6 fmol/mg protein (B(max)), with a K(d) of 1.47 +/- 0.13 nM. [H-3]- PZ bound to 45 % of the total QNB binding sites. In competition experi ments, 4-DAMP displaced [H-3](-)QNB with the lowest IC50, followed by PZ and AF-DX 116. Autoradiograms demonstrated that [H-3](-)QNB binding was completely displaced by 4-DAMP, partially displaced by PZ, but no t displaced by gallamine or AF-DX 116, and suggested that M1 and M3 su btypes coexist in submucosal glands. The localization of M1 receptors on submucosal glands was confirmed by direct labeling with [H-3]-PZ. [ H-3]-PZ also labeled vessels, but with a low silver grain density. Aut oradiographic [H-3]-QNB binding was displaced by 4-DAMP and atropine, but not by PZ, gallamine, or AF-DX 116. In studies of mucus secretion in vitro, 4-DAMP significantly inhibited methacholine-induced secretio n. Although less effective, PZ also had significant inhibitory effects . Neither gallamine nor AF-DX 116 had any inhibitory effect. M1 recept ors (PZ binding sites) may regulate glandular secretion while M3 recep tors (4-DAMP binding sites) may regulate glandular secretion and vasom otor tone in human nasal mucosa.