PHARMACOLOGICAL ANALYSIS OF AGONIST-ANTAGONIST INTERACTIONS AT ACETYLCHOLINE MUSCARINIC RECEPTORS IN A NEW URINARY-BLADDER ASSAY

1 Agonist-antagonist interactions at acetylcholine (ACh) muscarinic receptors have been analysed by use of an improved urinary bladder assay, isolated and intact, from the mouse. With 5-methylfurmethide as agonist, validated cumulative concentration-effect curves were obtained in less than 7 min by re-dosing before the response plateaux began to fade. 2 The pK(B) value estimated for pirenzepine was 6.76. The pK(B) values estimated for atropine and N-methylatropine from data obtained at concentrations which produced dose-ratios greater than 20 and 60 were 8.90 and 9.58, respectively. 3 The deviation from simple competitive behaviour at low dose-ratios with atropine and N-methylatropine was consistent with the operation of saturable antagonist removal processes. The deviation observed with atropine was corrected by pre-incubation with methylbutyrate, an alternative substrate for 'atropine esterase'. 4 The simple competitive behaviour of N-methylatropine was restored following pre-incubation with the neuronal choline uptake blocker hemicholinium-3 (HC-3). However, the pK(B) estimated for N-methylatropine under these conditions was low. This latter result could be accounted for by the observed behaviour of HC-3 as a competitive antagonist of ACh muscarinic receptors (pK(B) = 4.01). 5 We conclude that the modified mouse urinary bladder assay is suitable for the quantitative analysis of muscarinic receptor interactions. In addition, we postulate the existence of a previously undescribed uptake mechanism for quaternary muscarinic receptor antagonists.