Pharmacological and molecular characterization of muscarinic receptors in cat esophageal smooth muscle.

The muscarinic receptor subtypes that mediate cholinergic responses in cat esophageal smooth muscle were examined. Antagonist effects on carbachol-induced and nerve-evoked contractions were studied in vitro using muscle strips from the distal esophagus. Antagonists displayed similar relative selectivities in suppressing carbachol and nerve-mediated responses as follows: 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) > zamifenacin > para-fluoro-hexahydrosiladiphenidol > pirenzepine > AF-DX 116 > methoctramine, indicating that these responses are mediated by the same receptor subtype. 4-DAMP, pirenzepine and methoctramine effects on carbachol responses gave pA2 values characteristic of the M3 receptor in both the circular muscle (9.25 +/- 0.12, 6.79 +/- 0.09 and 6.04 +/- 0.11, respectively) and longitudinal muscle (9.46 +/- 0.14, 7.25 +/- 0.07 and 6.10 +/- 0.06, respectively). Reverse transcription-polymerase chain reaction analysis was done using primer sequences based on the cloned human muscarinic receptor subtypes. Messenger RNA for the m3 receptor was readily identified, whereas m2 was not detected in esophageal muscle, but was present in cardiac muscle. Sequence homology between the amplified products from cat tissue and the corresponding human m2 and m3 receptors genes were 93% and 89%, respectively. In the cat esophagus, the M3 receptor mediates functional responses and messenger RNA for the corresponding molecular form of this receptor is abundant in this tissue.