Methoctramine is a selective antagonist of M2 muscarinic acetylcholine receptors (IC50 = 6.1 nM in CHO-K1 cell membranes).[1] It is selective for M2 over M1, M3, M4, and M5 receptors (IC50s = 92, 770, 260, and 217 nM, respectively). In vitro, methoctramine inhibits acetylcholine-induced reductions in isolated guinea pig tracheal tube contractions when used at a concentration of 1 μM.[2] In vivo, methoctramine inhibits bradycardia and bronchoconstriction induced by acetylcholinein guinea pigs with ED50 values of 38 and 81 nmol/kg, respectively. In a rat model of spinal cord injury, methoctramine suppresses bladder overactivity induced by the non-selective muscarinic acetylcholine receptor agonist oxotremorine M.[3]

Reference:
[1]. Buckley, N.J., Bonner, T.I., Buckley, C.M., et al. Antagonist binding properties of five cloned muscarinic receptors expressed in CHO-K1 cells. Mol. Pharm. 35(4), 469-476 (1989).
[2]. Watson, N., Barnes, P.J., and Maclagan, J. Actions of methoctramine, a muscarinic M2 receptor antagonist, on muscarinic and nicotinic cholinoceptors in guinea-pig airways in vivo and in vitro. Br. J. Pharmacol. 105(1), 107-112 (1992).
[3]. Matsumoto, Y., Miyazato, M., Yokoyama, H., et al. Role of M2 and M3 muscarinic acetylcholine receptor subtypes in activation of bladder afferent pathways in spinal cord injured rats. Urology 79(5), 1184.e1115-e1120 (2012).