Dualistic actions of cromakalim and new potent 2 H ‐1,4‐benzoxazine derivatives on the native skeletal muscle K ATP channel

New 2 H ‐1,4‐benzoxazine derivatives were synthesized and tested for their agonist properties on the ATP‐sensitive K + channels (K ATP ) of native rat skeletal muscle fibres by using the patch‐clamp technique. The novel modifications involved the introduction at position 2 of the benzoxazine ring of alkyl substituents such as methyl (–CH 3 ), ethyl (–C 2 H 5 ) or propyl (–C 3 H 7 ) groups, while maintaining pharmacophore groups critical for conferring agonist properties. The effects of these molecules were compared with those of cromakalim in the presence or absence of internal ATP (10 −4 M ). In the presence of internal ATP, all the compounds increased the macropatch K ATP currents. The order of potency of the molecules as agonists was −C 3 H 7 (DE 50 =1.63 × 10 −8 M ) >−C 2 H 5 (DE 50 =1.11 × 10 −7 M )>–CH 3 (DE 50 =2.81 × 10 −7 M )>cromak‐slim (DE 50 = 1.42 × 10 −5 M ). Bell‐shaped dose–response curves were observed for these compounds and cromakalim indicating a downturn in response when a certain dose was exceeded. In contrast, in the absence of internal ATP, all molecules including cromakalim inhibited the K ATP currents. The order of increasing potency as antagonists was cromakalim (IC 50 =1.15 × 10 −8 M )–CH 3 (IC 50 =2.6 × 10 −8 M )>–C 2 H 5 (IC 50 =4.4 × 10 −8 M )>–C 3 H 7 (IC 50 =1.68 × 10 −7 M ) derivatives. These results suggest that the newly synthesized molecules and cromakalim act on muscle K ATP channel by binding on two receptor sites that have opposite actions. Alternatively, a more simple explanation is to consider the existence of a single site for potassium channel openers regulated by ATP which favours the transduction of the channel opening. The alkyl chains at position 2 of the 2 H ‐1,4‐benzoxazine nucleus is pivotal in determining the potency of benzoxazine derivatives as agonists or antagonists.British Journal of Pharmacology (2003) 139 , 255–262. doi: 10.1038/sj.bjp.0705233