Autoradiographic analysis of muscarinic receptors in rat nasal glands

An in vitro method was developed for the biochemical and autoradiographic demonstration of low muscarinic receptor densities in peripheral tissue. Histological criteria point clearly to the necessity for fixation to preserve tissue quality. [ 3H]l-Quinuclidinylbenzilate bound specifically to a homogeneous class of binding sites in 0.5% glutardialdehyde-fixed cryostat sections (10 μm) of rat nasal glands with high affinity (K d = 0.47 ± 0.06 nM) and with a receptor density (B max) of 41 ± 1 fmol/mg protein. This binding was linearly dependent on the thickness of the sections. Kinetic experiments resulted in a K d value of 0.19 nM. Binding was stereoselectively inhibited by benzetimide enantiomers. Autoradiograms, generated after incubation with 0.6 nM [ 3H]l-quinuclidinylbenzilate and dippling in nuclear K2 emulsion, showed specific labelling of the glandular acini and excretory ducts. These in vitro observations provide conclusive evidence for the presence of acetylcholine receptors in the nasal glands of the rat.