Selective blockade of muscarinic M2 receptors in vivo by the new antagonist tripitramine

The antimuscarinic effects of tripitramine (1, 1, 24--tris [[5, 11-dihydro-6-oxo-6H-pyrido [2, 3-b][1, 4]-benzodiazepin-11-yl)(carbonyl] methyl]-8, 17-dimethyl-1, 8, 17, 24-tetraazatetracosane tetraoxalate), a member of a series of polymethylene tetraamines with in vitro cardioselectivity, were assessed in two in vivo preparations: anaesthetized and pithed rats. The well-known M₂ selective antagonist methoctramine was used in a comparative study. Tripitramine (0.0202 mol/kg i.v.) proved to be a potent antagonist at cardiac M₂ receptors that mediate the decrease in heart rate in the pithed rat; the same dose of this antagonist in the anaesthetized rat did not significantly affect the depressor action of methacholine mediated by vascular M₃ receptors. In the pithed rat, this dose did not affect the ganglionic M₁ receptor-mediated tachycardia and pressor response to muscarme or McN-A-343. These in vivo data are consistent with the in vitro findings and confirm that tripitramine is a more potent and selective muscarinic M₂ receptor antagonist than methoctramine.