Are m-cholinoceptors of guinea pig gallbladder smooth muscle of m<inf>4</inf> subtype?
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The antagonism of carbachol-induced contractions of guinea pig gallbladder smooth muscle strips via selective antagonists. methoctramine, HHSiD, pf-HHSiD and DABDMA has been investigated in order to find out the m-cholinoceptor subtype^) of gallbladder smooth muscle. All m-cholinoceptor antagonists examined, displaced the concentration-response curves to the right parallel in a concentration-dependent manner without affecting the maximum response. Schild analysis of data was consistent with competitive antagonism, -log Kb values of the antagonists were 7.37 for HHSiD. 7.53 for pf-HHSiD, 6.58 for DABDMA and 7.60 for methoctramine. These results, together with the high affinity of pirenzepine and low affinities of 4-DAMP and AF-DX 116. indicate that the m-cholinoceptors of the guinea pig gallbladder which mediate cholinergic contractions are not of mr, mi- and ma-subtypes but seem likely to be of iru-subtype. © 1994 S. Karger AG. Basel.