Increase of [Ca²⁺]i and release of arachidonic acid via activation of M2 receptor coupled to Gi and Rho proteins in oesophageal muscle

We have previously shown that acetylcholine-induced contraction of oesophageal circular muscle depends on activation of phosphatidylcholine selective phospholipase C and D, which result in formation of diacylglycerol, and of phospholipase 2 which produces arachidonic acid. Diacylglycerol and arachidonic acid interact synergistically to activate protein kinase C. We have therefore investigated the relationship between cytosolic Ca²⁺ and activation of phospholipase A₂ in response to acetylcholine-induced stimulation, by measuring the intracellular free Ca²⁺ ([Ca²⁺]i), muscle tension, and [³H] arachidonic acid release. Acetylcholine-induced contraction was associated with increased [Ca²⁺]i and arachidonic acid release in a dose-dependent manner. In Ca²⁺-free medium, acetylcholine did not produce contraction, [Ca²⁺]i increase, and arachidonic acid release. In contrast, after depletion of Ca²⁺ stores by thapsigargin (3 μM), acetylcholine caused a normal contraction, [Ca²⁺]i increase and arachidonic acid release. The increase in [Ca²⁺]i and arachidonic acid release were attenuated by the M2 receptor antagonist methoctramine, but not by the M3 receptor antagonist p-fluoro-hexahydro siladifenidol. Increase in [Ca²⁺]i and arachidonic acid release by acetylcholine were inhibited by pertussis toxin and C3 toxin. These findings indicate that contraction and arachidonic acid release are mediated through muscarinic M2 coupled to Gi or rho protein activation and Ca²⁺ influx. Acetylcholine-induced contraction and the associated increase in [Ca²⁺]i and release of arachidonic acid were completely reduced by the combination treatment with a phospholipase A₂ inhibitor dimethyleicosadienoic acid and a phospholipase D inhibitor pCMB. They increased by the action of the inhibitor of diacylglycerol kinase R59949, whereas they decreased by a protein kinase C inhibitor chelerythrine. These data suggest that in oesophageal circular muscle acetylcholine-induced [Ca²⁺]i increase and arachidonic acid release are mediated through activation of M2 receptor coupled to Gi or rho protein, resulting in the activation of phospholipase A₂ and phospholipase D to activate protein kinase C. Copyright (C) 2000 Elsevier Science Inc.