Muscarinic M-2 receptors are not primarily involved in the contraction of guinea-pig gallbladder smooth muscle

The presence of M-1-M-4 receptors in guinea-pig gallbladder smooth muscle c ells has been reported recently. The majority of these receptors are said t o be of M-2 subtype. However, there are controversial reports about the fun ctional muscarinic receptors that mediate contraction in this tissue. Simil ar to gallbladder, it was claimed that M-4 receptors mediate guinea-pig ute rine contractions, but these receptors have appeared to be of M-2 subtypes later. Therefore, the antagonistic affinities of three M-2-selective muscar inic antagonists were determined in contraction and radioligand binding exp eriments in guinea-pig gallbladder in the present study. The antagonistic a ffinity Values (pK(i)) of gallamine, tripitramine and imperialine were as f ollows, respectively: 6.28+/-0.15, 8.65+/-0.10 and 6.55+/-0.07 against 0.25 0 nM [H-3]QNB binding. All three antagonists displaced the concentration-re sponse curves to carbachol to the right in parallel without affecting the m aximum responses. The pA(2) 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 M-2 receptors are reported to be 6.3, 9. 6, 7.7, respectively. The pA(2) values obtained in this study clearly indic ate that the primary muscarinic receptors involved in carbachol-induced gui nea-pig gallbladder contraction are not of M-2 subtype. The poor correlatio n between the antagonistic affinity values of these antagonists obtained at radioligand binding (pK(i)) and contraction (pA(2)) experiments also suppo rt the conclusion that the majority of muscarinic receptors which have been reported to be of M-2 do not mediate the contractile responses. (C) 1999 A cademic Press.