Pharmacological characterization of muscarinic receptors in the uterus of oestrogen-primed and pregnant rats

1 Radioligand binding and contractility studies were undertaken to determine the subtype/s of muscarinic receptors present in uteri of oestrogen-treated and late pregnant rats. 2 Competition binding studies with uterine membrane preparations and [H-3]-QNB (quinuclidinyl benzilate) provided negative log dissociation constants (pK(i)) for each antagonist as follows; oestrogen-treated - atropine (7.98) greater than or equal to himbacine (7.83) > methoctramine (7.52) greater than or equal to hexahydrosiladiphenidol (HHSiD; 7.32) greater than or equal to 5,11-dihydro-11-[[[2-[2-[(dipropylamino)methyl]-1piperidinyl]ethyl]amino]-carbonyl]-6H-pyrido-[2,3-b] [1,4]-benzodiazepin-6-one (AF-DX 384; 7.10) > 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]5,11-dihydro-6H-pyridol]2,3,-b][1,4]bemzodiazepin-6-one (AF-DX 116, 6.77) > pirenzepine (6.17); late pregnant - atropine (8.05) greater than or equal to methoctramine (7.95) greater than or equal to himbacine (7.71) greater than or equal to HHSiD (7.52) greater than or equal to AF-DX 384 (7.34) > AF-DX 116 (6.72) > pirenzepine (6.18). 3 The potency of carbachol in causing uterine contraction was similar in preparations from pregnant and non-pregnant animals (pD(2)=5.57 and 5.46, respectively). Each muscarinic antagonist caused parallel, rightward shifts of carbachol concentration-response curves. The pA(2) estimates were: oestrogen-treated - atropine (9.42) > himbacine (8.73) greater than or equal to HHSiD (8.68) greater than or equal to methoctramine (8.49) greater than or equal to AF-DX 384 (7.91) greater than or equal to AF-DX 116 (7.36) greater than or equal to pirenzepine (7.26); late pregnant - atropine (9.48) > himbacine (8.37) greater than or equal to HHSiD (8.22) greater than or equal to methoctramine (8.01) greater than or equal to AF-DX 116 (7.73) greater than or equal to AF-DX 384 (7.44) greater than or equal to pirenzepine (6.92). 4 The relative pK(i) estimates for antagonists obtained in membrane preparations from oestrogen-treated rats suggest the presence of muscarinic M-2 subtypes. In functional studies pA(2) values indicated the additional presence of muscarinic M-3 receptor or, possibly an atypical receptor subtype. The similarity between pK(i) and pA(2) estimates obtained in uteri from oestrogen-treated and pregnant animals, respectively, indicates that pregnancy does not affect myometrial muscarinic receptors in the rat.