Atropine is a competitive antagonist of muscarinic acetylcholine receptor with IC50 values of 2.22±0.60nM, 4.32±1.63nM, 4.16±1.04nM, 2.38±1.07nM and 3.39±1.16nM for M1, M2, M3, M4 and M5, respectively [1].

Atropine is an antimuscarinic agent and has shown the pharmacological effects because of binding to muscarinic acetylcholine receptors. Atropine, the orthosteric antagonist, has been reported to compete with [3H]-NMS for all muscarinic subtypes with a potency consistent with the high affinities at the mAChR subtypes. The Ki values are 1.27±0.36nM, 3.24±1.16nM, 2.21±0.53nM, 0.77±0.43nM and 2.84±0.84nM, respectively [1, 2, 3].

References:
[1] Lebois EP1, Bridges TM, Lewis LM, Dawson ES, Kane AS, Xiang Z, Jadhav SB, Yin H, Kennedy JP, Meiler J, Niswender CM, Jones CK, Conn PJ, Weaver CD, Lindsley CW. Discovery and characterization of novel subtype-selective allosteric agonists for the investigation of M(1) receptor function in the central nervous system.ACS Chem Neurosci. 2010;1(2):104-121.
[2] BUSCH H, ALLEN H, ANDERSON DC.Effects of atropine and carbachol on labeling of protein fractions of mouse pancreas in vitro.J Pharmacol Exp Ther. 1959 Nov; 127:200-4.
[3] Feron O1, Smith TW, Michel T, Kelly RA.Dynamic targeting of the agonist-stimulated m2 muscarinic acetylcholine receptor to caveolae in cardiac myocytes. J Biol Chem. 1997 Jul 11; 272(28):17744-8.