title: Dualistic actions of cromakalim and new potent 2H-1,4-benzoxazine derivatives on the native skeletal muscle K ATP channel.
abstract: 1 New 2H-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. 2 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 x 10(-8) M) >-C(2)H(5) (DE(50)=1.11 x 10(-7) M)>-CH(3) (DE(50)=2.81 x 10(-7) M)>cromak-slim (DE(50)= 1.42 x 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. 3 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 x 10(-8) M)> or =-CH(3) (IC(50)=2.6 x 10(-8) M)>-C(2)H(5) (IC(50)=4.4 x 10(-8) M)>-C(3)H(7) (IC(50)=1.68 x 10(-7) M) derivatives. 4 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 2H-1,4-benzoxazine nucleus is pivotal in determining the potency of benzoxazine derivatives as agonists or antagonists.
authors.affiliation: Department of Pharmacobiology, Faculty of Pharmacy, via Orabona no. 4, University of Bari, I-70126 Bari, Italy.
authors.forename: Domenico
authors.initials: D
authors.lastname: Tricarico
authors.forename: Mariagrazia
authors.initials: M
authors.lastname: Barbieri
authors.forename: Laghezza
authors.initials: L
authors.lastname: Antonio
authors.forename: Paolo
authors.initials: P
authors.lastname: Tortorella
authors.forename: Fulvio
authors.initials: F
authors.lastname: Loiodice
authors.forename: Diana Conte
authors.initials: DC
authors.lastname: Camerino
mesh_terms: D000255:Adenosine Triphosphate / Q000502:physiology*; D000818:Animals; D001665:Binding Sites; D019806:Cromakalim / Q000494:pharmacology*; D008297:Male; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000378:metabolism; D010078:Oxazines / Q000737:chemistry / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000819:agonists*; D051381:Rats; D017208:Rats, Wistar; D013329:Structure-Activity Relationship
chemical_list: D010078:Oxazines; D015221:Potassium Channels; D019806:Cromakalim; D000255:Adenosine Triphosphate