Cholinoreceptors in the isolated costo-uterine muscle of the rat.

1 The aim of this study was to examine the cholinoreceptor population in the rat costo-uterine muscle. 2 The nicotinic cholinoreceptor agonists nicotine and DMPP, and the ganglionic muscarinic cholinoreceptor stimulant McNeil A-343, had no effects upon isolated preparations of this tissue. 3 Acetylcholine was more potent than carbachol and approximately equipotent with methacholine (the mean EC50 values were 7.0, 6.3 and 6.7 respectively) in producing contractions of the preparation; each was a full agonist. The potencies of carbachol and methacholine were similar in preparations taken from animals in oestrus and in dioestrus. 4 Atropine competitively antagonised the effects of carbachol and methacholine, the pA2 values were 9.37 and 9.41 respectively. The pA2 value for pirenzepine with carbachol as the agonist was 6.69. 5 Pilocarpine produced phasic contractions of the tissue (EC50 value = 4.17), and competitively antagonised the effects of carbachol with a pA2 value of 5.26. The anticholinesterase, physostigmine, produced only a small potentiation of the effects of acetylcholine. 6 It is concluded that the cholinoreceptors which mediate contraction of the rat costo-uterine muscle are muscarinic, homogeneous in nature and unaffected by fluctuating levels of ovarian hormones occurring during the oestrous cycle. The consequences of inhibition of cholinesterase activity in isolated preparations of the tissue are minimal.