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 car bachol-induced and nerve-evoked contractions were studied in vitro usi ng muscle strips from the distal esophagus. Antagonists displayed simi lar 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 b y the same receptor subtype. 4-DAMP, pirenzepine and methoctramine eff ects on carbachol responses gave pA(2) values characteristic of the M- 3 receptor in both the circular muscle (9.25 +/- 0.12, 6.79 +/- 0.09 a nd 6.04 +/- 0.11, respectively) and longitudinal muscle (9.46 +/- 0.14 , 7.25 +/- 0.07 and 6.10 +/- 0.06, respectively). Reverse transcriptio n-polymerase chain reaction analysis was done using primer sequences b ased on the cloned human muscarinic receptor subtypes. Messenger RNA f or the m(3) receptor was readily identified, whereas m(2) was not dete cted in esophageal muscle, but was present in cardiac muscle. Sequence homology between the amplified products from cat tissue and the corre sponding human m(2) and m(3) receptors genes were 93% and 89%, respect ively. In the cat esophagus, the M-3 receptor mediates functional resp onses and messenger RNA for the corresponding molecular form of this r eceptor is abundant in this tissue.