Muscarinic receptor localization and function in rabbit carotid body

Abstract

Acetylcholine and muscarinic agonists inhibit chemosensory activity in the rabbit carotid sinus nerve (CSN). Because the mechanism of this inhibition is poorly understood, we have investigated the kinetics and distribution of muscarinic receptors in the rabbit carotid body with the specific muscarinic antagonist [ 3H]quinuclidinylbenzilate ([ 3H]QNB). Equilibrium binding experiments identified displaceable binding sites (1 μM atropine) with a K d = 71.46pM and a B max = 9.23pmol/g tissue. These binding parameters and the pharmacology o the displaceable [ 3H]QNB binding sites are similar to specific muscarinic receptors identified in numerous other nervous, muscular and glandular tissues. Comparisons of specific binding in normal and chronic CSN-denervated carotid bodies suggest that muscarinic receptors are absent on afferent terminals in the carotid body; however, nearly 50% of the specific [ 3H]QNB binding is lost following chronic sympathectomy, suggesting the presence of presynaptic muscarinic receptors on the sympathetic innervation supplying the carotid body vasculature. Autoradiographic studies have localized the remainder of [ 3H]QNB binding sites to lobules of type I and type II parenchymal cells. In separate experiments, the muscarinic agonists, oxotremorine (100 μM) and bethanechol (100 μM) were shown to inhibit both the release of catecholamines and the increased CSN activity evoked by nicotine (50 μM) stimulation of the in vitro carotid body. Our data suggest that muscarinic inhibition in the rabbit carotid body is mediated by receptors located on type I cells which are able to modulate the excitatory actions of acetylcholine at nicotinic sites.