Binding of the muscarine receptor antagonist heptane-1,7-bis(dimethyl-3′-phthalimidopropyl) ammonium bromide at cholinoceptor sites. European Journal of Pharmacology - Molecular Pharmacology Section, 246 (1993) 1-8

The binding of the bisquaternary muscarine receptor antagonist heptane-1,7-bis(dimethyl-3′-phthalimidopropyl)-ammonium bromide (C₇/3-phth) was investigated at a number of cholinergic binding sites using (-)-[³H]nicotine, [³H]pirenzepine and (-)-[³H]quinuclidinyl benzilate ([³H]QNB) in both central and peripheral tissues. C₇/3-phth displayed an affinity for muscarine M₂ receptors in rat atria (70.1 nM) which was 1.6-fold greater than for putative M₄ receptors in rabbit lung, and 4- to 5-fold greater than for M₁ receptors in rat cerebral cortex. Its affinity for nicotine receptors in the cortex was low, being 808-fold lower than its affinity for the M₂ receptor. Although the displacement of (-)-[³H]nicotine and [³H]pirenzepine binding in rat cortex by C₇/3-phth was best described in terms of one-site modelling, low Hill coefficients were observed with C₇/3-phth in displacement studies using [³H]QNB in this tissue. The possibility of allosteric interactions or multiple receptor subtype interactions is discussed.