Receptor signaling mechanisms underlying muscarinic agonist-evoked contraction in guinea-pig ileal longitudinal smooth muscle

1 In guinea-pig ileal longitudinal muscle, muscarinic partial agonists, 4-(N-[3-chlorophenyl]-carbomoyloxy)-2-butynyl-trimethylammonium (McN-A343) and pilocarpine, each produced parallel increases in tension and cytosolic Ca2+ concentration ([Ca2+]c) with a higher EC50 than that of the full agonist carbachol. The maximum response of [Ca2+]c or tension was not much different among the three agonists. The Ca2+ channel blocker nicardipine markedly inhibited the effects of all three agonists 2 The contractile response to any agonist was antagonized in a competitive manner by M-2 receptor selective antagonists (N,N'-bis[6-[[(2-methoyphenyl)methyl]amino]hexyl]-1,8-octanediamine tetrahydrochloride and 11-[[2-[(diethlamino)methyl]-1-piperidinyl]acetyl]-5,11- dihydro-6H-pyrido[2,3-b][ 1,4] benzodiazepine-6-one), and the apparent order of M-2 antagonist sensitivity was McN-A343>pilocarpine>carbachol. M-3 receptor selective antagonists, 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide and darifenacin, both severely depressed the maximum response for McN-A343, while darifenacin had a similar action in the case of pilocarpine. Both M3 antagonists behaved in a competitive manner in the case of the carbachol response. 3 McN-A343 failed to release Ca2+ from the intracellular stores, and the Ca2+-releasing action of pilocarpine was very weak compared with that of carbachol. All three agonists were capable of increasing Ca2+ sensitivity of the contractile proteins. 4 McN-A343 rarely produced membrane depolarization, but always accelerated electrical spike discharge. Pilocarpine effect was more often accompanied by membrane depolarization, as was usually seen using carbachol. 5 The results suggest that muscarinic agonist-evoked contractions result primarily from the integration of Ca2+ entry associated with the increased spike discharge and myofilaments Ca2+ sensitization, and that Ca2+ store release may contribute to the contraction indirectly via potentiation of the electrical membrane responses. They may also support the idea that an interaction of M-2 and M-3 receptors plays a crucial role in mediating the contraction response.