• M2 and M3 receptors;
  • intestinal smooth muscle;
  • cationic current;
  • Ca2+-store release;
  • Ca2+ sensitization;
  • membrane potential;
  • carbachol;
  • pilocarpine;
  • McN-A343
  • 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

  • The contractile response to any agonist was antagonized in a competitive manner by M2 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 M2 antagonist sensitivity was McN-A343>pilocarpine>carbachol. M3 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.

  • 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.

  • 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.

  • 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 M2 and M3 receptors plays a crucial role in mediating the contraction response.

British Journal of Pharmacology (2003) 139, 337–350. doi:10.1038/sj.bjp.0705267