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 pKB value estimated for pirenzepine was 6.76. The pKB 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 pKB 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 (pKB = 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.
Durant
P A
PA
James Black Foundation, London.
Shankley
N P
NP
Welsh
N J
NJ
Black
J W
JW
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001286:Atropine Derivatives / Q000494:pharmacology; D006168:Guinea Pigs; D006426:Hemicholinium 3 / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D009116:Muscarine / Q000031:analogs & derivatives / Q000494:pharmacology; D018727:Muscarinic Antagonists; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D010277:Parasympathomimetics / Q000494:pharmacology; D010890:Pirenzepine / Q000494:pharmacology; D011976:Receptors, Muscarinic / Q000187:drug effects*; D014132:Trachea / Q000187:drug effects; D001743:Urinary Bladder / Q000187:drug effects
D001286:Atropine Derivatives; D018727:Muscarinic Antagonists; D010277:Parasympathomimetics; D011976:Receptors, Muscarinic; C008446:5-methylfurtrethonium; D006426:Hemicholinium 3; D010890:Pirenzepine; D001285:Atropine; D009116:Muscarine; C006649:methylatropine