Atropine and glycopyrronium show similar binding patterns to M2 (cardiac) and M3 (submandibular gland) muscarinic receptor subtypes in the rat.

Atropine and glycopyrronium are frequently used for premedication to reduce oral and respiratory secretions and prevent bradycardia. Glycopyrronium is said to have similar antisialagogue effects, but is less likely to cause significant tachycardia than atropine. Different antimuscarinic receptor selectivity patterns could explain the differences. The aim of this investigation was to determine the possible selectivity of glycopyrronium for M2 and M3 muscarinic receptor subtypes. Muscarinic receptor subtypes in Wistar rat ventricle and submandibular gland homogenates were characterized with [3H]-N-methylscopolamine ([3H]-NMS) by ligand binding studies. Inhibition of [3H]-NMS binding by non-labelled compounds showed the following order: in rat ventricle: glycopyrronium > atropine >> otenzepad > hexahydrosiladiphenidol (HHSiD) > pirenzepine; in rat submandibular gland: glycopyrronium > atropine >> HHSiD >> pirenzepine > otenzepad. These were similar to the expected order of frequency of M2 and M3 subtypes, respectively. Glycopyrronium showed similarly high affinities for both M2 (Ki = 1.889 (SEM 0.049) nmol litre-1) and M3 (Ki = 1.686 (0.184) nmol litre-1) subtypes. Glycopyrronium bound to a homogeneous population of binding sites in both tissues and showed no selectivity for M2 or M3 muscarinic receptor subtypes.