Relaxant activity in rat aorta and trachea, conversion to a muscarinic receptor antagonist and structure-activity relationships of new K(ATP) activating 6-varied benzopyrans.

To characterize ATP-sensitive channels (K(ATP) channels) benzopyrans with different substituents at position 6 were synthesized as new K(ATP)-activators. Their relaxant potencies were determined in rat aorta and trachea. In aorta, pEC50-values (-log, M) ranged from 7.37 to 5.43; in trachea, pEC50-values were 0.3 to 0.8 log units lower. Functional data were compared with binding data obtained in calf tracheal cells using the cyanoguanidine [3H]P1075 (N-cyano-N'-1,1-dimethyl[2,3(n)-3H]propyl)-N11-(3-pyridinyl)guanidine) as radioligand. A high correlation (r = 0.96) between pEC50- and pKD-values indicated that tracheal relaxation produced by benzopyrans is mediated via K(ATP) channels without signal amplification. The permanently charged trimethylammonium derivative designed as a probe for the membrane site of action completely lost its affinity for K(ATP) channels, but converted to an antagonist for muscarinic acetylcholine receptors (pK(B) = 6.12+/-0.10), as confirmed in radioligand binding studies (pK(D) = 5.77+/-0.04). Structure-activity analyses revealed that the 6-substituent influences biological activity by a direct receptor interaction of its own and not indirectly by withdrawing electrons from the benzopyran nucleus. The variance of the biological activity is primarily determined by electrostatic properties, but desolvation energies additionally contribute.