Subcellular distribution and pharmacological characterization of muscarinic receptors in tracheal smooth muscle.

Subcellular fractions isolated from tracheal smooth muscle have been identified using biochemical markers and measuring the [3H]QNB muscarinic receptor binding activity in these fractions. This muscarinic receptor (mAchR) activity was slightly enriched 1.6 times in the crude mitochondrial fraction (M), 2.6 times in the crude microsomal fraction (P), and greatly enriched in the highly purified plasma membranes fractions, being 5.3 times in a heavy plasma membrane fraction designed as P2 and 9.1 times in a light plasma membrane fraction named P1 fraction. The muscarinic receptor subtypes present in the subcellular fractions were identified using competition experiments. The binding of five selective antagonists, pirenzepine, AF-DX 116, hexahydrodifenidol, methoctramine and 4-DAMP were examined. In this sense, the M1 antagonist pirenzepine showed pKi's values between 6.44-7.45 and the M2 antagonist AF-DX 116 showed pKi's values ranging from 6.75 to 7.45 being the lowest pKi's values here described. The antagonist hexahydrodifenidol showed higher affinities than pirenzepine-derivated compounds with pKi's values from 7.25 to 7.65. The antagonist 4-DAMP exhibited pKi's values from 8.18-8.41. Finally, methoctramine showed similar affinities as 4-DAMP, with pKi's ranging from 8.09 to 8.22 suggesting the existence of M2 receptors in these fractions. These data suggest that M2 mAchR are present in all particulate fractions here studied. It is important to emphasize that the M2 muscarinic receptor presents in the light plasma membrane fraction (P1) shows poor selectivity towards the muscarinic antagonists being different from the M2 mAchRs associated with other subcellular fractions isolated from bovine tracheal smooth muscle.