DISTINCT MUSCARINIC RECEPTORS, G-PROTEINS AND PHOSPHOLIPASES IN ESOPHAGEAL AND LOWER ESOPHAGEAL SPHINCTER CIRCULAR MUSCLE

Acetylcholine (ACh)-induced contraction of esophageal circular smooth muscle cells was inhibited by the M2 muscarinic antagonist methoctrami ne. In lower esophageal sphincter (LES) cells contraction was inhibite d by the M3 antagonist p-fluoro-hexahydro-sila-difenidol (pF-HSD). Per tussis toxin (PTX) reduced ACh-induced contraction of esophageal but n ot of LES cells, which suggested that different receptor-linked G prot eins are involved. Antibodies against G,3 antagonized contraction of e sophageal cells and G(q)-G11 antibodies antagonized contraction of LES cells. The phosphatidylinositol-specific phospholipase C (PLC) inhibi tors, U-73122 and neomycin, reduced ACh-induced contraction of LES but not of esophageal cells. Conversely, propranolol and p-chloromercurib enzoic acid (pCMB), which inhibit a phosphatidlycholine-specific phosp holipase D (PLD)-dependent pathway, reduced contraction of esophageal but not of LES muscle cells. At 1 and 5 sec after the administration o f ACh (10(-5) M), inositol 1,4,5-trisphosphate (IP3) increased only in LES muscle, which suggested that contraction results from PLC-induced IP3 production in the LES but not in the esophagus. The IP3 receptor antagonist heparin, and depletion of intracellular Ca++ stores by thap sigargin or A23187, inhibited ACh-induced contraction of LES but not o f esophageal muscle. It was concluded that ACh-induced esophageal cont raction depends preferentially on M2 receptors, a PTX-sensitive G13 pr otein, phosphatidylcholine-specific PLD and production of diacylglycer ol (DAG) and is independent Of IP3 formation and the release of intrac ellular Ca++. Conversely, LES contraction is mediated through M3 recep tors, a PTX-insensitive G(q)-G11 protein, activation of PLC, IP3 forma tion and the release of intracellular Ca++.