Muscarinic receptors: evidence for a nonuniform distribution in tracheal smooth muscle and exocrine glands.

Muscarinic receptor distribution in smooth muscle, exocrine glands, and epithelium of the ferret trachea was determined using [3H]propylbenzilylcholine mustard ([3H]PrBCM) binding and autoradiography. Specific, atropine-sensitive [3H]PrBCM binding was quantified autoradiographically in the trachealis muscle (approximately 21 binding sites/microns2), surface epithelium (approximately 6 binding sites/microns2), and submucosal glands (approximately 5 binding sites/microns2). Serous and mucous cells in the glands did not differ in receptor density. Binding sites on gland and epithelial cells were associated with basolateral membranes. In the trachealis muscle, a gradient in receptor density was observed, with outer layers of muscle containing 3 to 10 times more receptors per unit area than inner layers. Receptor distribution in both glands and muscle paralleled the distribution of cholinergic axons. However, at the light microscope level, there was no evidence for the presence of receptor "hot spots" related to the position of individual axons. The parallelism in the distribution of axons and receptors suggests the possibility of neural control of the genesis and/or maintenance of receptor distribution in these tissues.