Muscarinic M₂ receptors are not primarily involved in the contraction of guinea-pig gallbladder smooth muscle

The presence of M₁-M₄ receptors in guinea-pig gallbladder smooth muscle cells has been reported recently. The majority of these receptors are said to be of M₂ subtype. However, there are controversial reports about the functional muscarinic receptors that mediate contraction in this tissue. Similar to gallbladder, it was claimed that M₄ receptors mediate guinea-pig uterine contractions, but these receptors have appeared to be of M₂ subtypes later. Therefore, the antagonistic affinities of three M₂-selective muscarinic antagonists were determined in contraction and radioligand binding experiments in guinea-pig gallbladder in the present study. The antagonistic affinity values (pK(i)) of gallamine, tripitramine and imperialine were as follows, respectively: 6.28 ± 0.15, 8.65 ± 0.10 and 6.55 ± 0.07 against 0.250 nM [³H]QNB binding. All three antagonists displaced the concentration-response curves to carbachol to the right in parallel without affecting the maximum responses. The pA₂ values obtained from constrained Schild plots (-log K(B)) were 4.14 ± 0.18 for gallamine, 6.79 ± 0.09 for tripitramine, and 7.02 ± 0.09 for imperialine. The antagonistic affinity values of gallamine, tripitramine and imperialine for M₂ receptors are reported to be 6.3, 9.6, 7.7, respectively. The pA₂ values obtained in this study clearly indicate that the primary muscarinic receptors involved in carbachol-induced guinea-pig gallbladder contraction are not of M₂ subtype. The poor correlation between the antagonistic affinity values of these antagonists obtained at radioligand binding (pK(i)) and contraction (pA₂) experiments also support the conclusion that the majority of muscarinic receptors which have been reported to be of M₂ do not mediate the contractile responses.