Characterization of the muscarinic receptor subtype mediating contractions of the guinea‐pig uterus

Abstract

The muscarinic receptor present on the guinea-pig uterus has been characterized by use of both functional and radioligand binding studies. The contractile responses to carbachol were antagonized by atropine, pirenzepine, methoctramine and hexamethonium with pA₂ values consistent with the presence of an M₂ receptor (i.e. 9.5, 7.0, 8.0 and 3.7 respectively). In binding studies, pKi values were also observed for pirenzepine, methoctramine and AF-DX 116 which were consistent with the presence of a homogeneous M₂ receptor population (i.e. 6.6, 7.9 and 7.1 respectively). The uterus therefore possesses M₂ receptors through which a functional contractile response is mediated and in this respect it is clearly different from other smooth muscles. M₂ receptors in the uterus mediate excitatory responses, an effect in contrast to their action in other tissues.