Characterization of the muscarinic receptor in isolated uterus of sham operated and ovariectomized rats

1 The pharmacological characteristics of muscarinic receptors in rat isolated uterus were studied in ovariectomized (ov.) and sham operated (sh.) animals. 2 Competition radioligand binding studies, using uterine membranes and [H-3]-NMS, were undertaken with several muscarinic receptor antagonists. Most of the antagonists indicated a one-site fit with apparent affinity estimates (pK(i)) unchanged by ovariectomy. The selective M-2 antagonist, tripitramine revealed high (representing 33+/-8 and 38+/-2%) and low (67+/-8 and 62+/-2%) affinity binding sites in both sh. and ov. rat uterus, respectively. These sites likely represented muscarinic M-2 and M-3 receptors and the proportions were not significantly different in the two conditions. 3 Carbachol induced concentration-dependent contractions which were surmountably antagonized by several muscarinic receptor antagonists (pK(B), sh.; ov.): zamifenacin (9.19; 9.18), p-F-HHSID (8.50; 9.06), tripitramine (7.23; 7.54), himbacine (7.21; 7.41), methoctramine (6.79; 7.49), pirenzepine (6.48; 7.21), AF DX 116 (6.26; 6.61), MTx 3 (<7.00; <7.00) and PD 102807 (<7.00; <7.00). 4 The apparent affinity values obtained in functional studies using the uteri from both sh. and ov. animals correlated most closely with values reported at human recombinant muscarinic M-3 receptors. This suggests that the muscarinic M-3 receptor mediates contraction under both conditions. 5 Radioligand binding experiments indicate the presence of M-2 receptors, in addition to M-3 receptors, which probably explains the discrepancies between functional and binding affinities. These data further suggest that the pharmacological profile and proportions of the two populations of muscarinic receptors are unaffected by ovariectomy.