Methoctramine, a polymethylene tetraamine, differentiates three subtypes of muscarinic receptor in direct binding studies.

A newly described cardiac selective antagonist methoctramine was examined in direct binding studies for affinity at three putative subtypes of the muscarinic receptor. The compound proved to be highly selective (158 fold) for cardiac as opposed to exocrine gland type M2 muscarinic receptors. In addition the compound was also 16 fold selective for M2 cardiac over M1 muscarinic receptors. These results indicate that methoctramine is a useful compound for the classification of muscarinic receptor subtypes.