The guinea pig ileum lacks the direct, high-potency, M(2)-muscarinic, contractile mechanism characteristic of the mouse ileum.
We explored whether the M(2) muscarinic receptor in the guinea pig ileum elicits a highly potent, direct-contractile response, like that from the M(3) muscarinic receptor knockout mouse. First, we characterized the irreversible receptor-blocking activity of 4-DAMP mustard in ileum from muscarinic receptor knockout mice to verify its M(3) selectivity. Then, we used 4-DAMP mustard to inactivate M(3) responses in the guinea pig ileum to attempt to reveal direct, M(2) receptor-mediated contractions. The muscarinic agonist, oxotremorine-M, elicited potent contractions in ileum from wild-type, M(2) receptor knockout, and M(3) receptor knockout mice characterized by negative log EC(50) (pEC (50)) values +/- SEM of 6.75 +/- 0.03, 6.26 +/- 0.05, and 6.99 +/- 0.08, respectively. The corresponding E (max) values in wild-type and M(2) receptor knockout mice were approximately the same, but that in the M(3) receptor knockout mouse was only 36% of wild type. Following 4-DAMP mustard treatment, the concentration-response curve of oxotremorine-M in wild-type ileum resembled that of the M(3) knockout mouse in terms of its pEC (50), E (max), and inhibition by selective muscarinic antagonists. Thus, 4-DAMP mustard treatment appears to inactivate M(3) responses selectively and renders the muscarinic contractile behavior of the wild-type ileum similar to that of the M(3) knockout mouse. Following 4-DAMP mustard treatment, the contractile response of the guinea pig ileum to oxotremorine-M exhibited low potency and a competitive-antagonism profile consistent with an M(3) response. The guinea pig ileum, therefore, lacks a direct, highly potent, M(2)-contractile component but may have a direct, lower potency M(2) component.
Griffin
Michael T
MT
Department of Chemistry, Chapman University, Orange, CA, USA.
Matsui
Minoru
M
Ostrom
Rennolds S
RS
Ehlert
Frederick J
FJ
D000818:Animals; D004158:Diphenylacetic Acids / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018721:Muscarinic Agonists / Q000494:pharmacology; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D010095:Oxotremorine / Q000031:analogs & derivatives / Q000494:pharmacology; D010880:Piperidines / Q000494:pharmacology; D043585:Receptor, Muscarinic M2 / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D043587:Receptor, Muscarinic M3 / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D013045:Species Specificity
D004158:Diphenylacetic Acids; D018721:Muscarinic Agonists; D018727:Muscarinic Antagonists; D010880:Piperidines; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3; C074183:N-(2-chloroethyl)-4-piperidinyl diphenylacetate; D010095:Oxotremorine; C042743:oxotremorine M