In vitro characterization of tripitramine, a polymethylene tetraamine displaying high selectivity and affinity for muscarinic M2 receptors

1 The antimuscarinic effects of tripitramine were investigated in vitro in isolated driven left (force) and spontaneously beating right (force and rate) atria as well as in the ileum of guinea‐pig and rat and in the trachea and lung strip of guinea‐pig and compared with the effects of methoctramine. 2 Tripitramine was a potent competitive antagonist of muscarinic M2 receptors in right and left atria. The pA2 values ranged from 9.14 to 9.85. However, in the guinea‐pig and rat left atria but not in guinea‐pig right atria, tripitramine at lower concentrations (3–10 nM) produced a less than proportional displacement to the right of agonist‐induced responses owing to the presence of a possible saturable removal process. 3 Tripitramine was about three orders of magnitude less potent in ileal and tracheal than in atrial preparations (pA2 values ranging from 6.34 to 6.81) which makes it more potent and more selective than methoctramine. 4 Another intriguing finding was the observation that the pA2 value of 7.91 observed for tripitramine in guinea‐pig lung does not correlate with that found at both muscarinic M2 and M3 receptor subtypes, which clearly indicates that the contraction of guinea‐pig lung strip is not mediated by these muscarinic receptor subtypes. 5 A combination of tripitramine with atropine resulted in addition of the dose‐ratios for left atria as required for two antagonists interacting competitively with the same receptor site, whereas the same combination gave a supra‐additive antagonism on guinea‐pig ileum which suggests that tripitramine interacts with a second interdependent site. 6 Tripitramine was more specific than methoctramine since, in addition to muscarinic receptors, it inhibited only frog rectus abdominis muscular (pIC50 value of 6.14) and rat duodenum neuronal (pIC50 value of 4.87) nicotinic receptors among receptor systems investigated, namely α1‐, α1‐, and β1‐ adrenoceptors, H1‐ and H2‐histamine receptors, and muscular and neuronal nicotinic receptors. 1995 British Pharmacological Society