title: Allosteric interaction of semotiadil fumarate, a novel benzothiazine, with 1,4-dihydropyridines, phenylalkylamines, and 1,5-benzothiazepines at the Ca(2+)-channel antagonist binding sites in canine skeletal muscle membranes.
abstract: We assessed the binding characteristics of a benzothiazine Ca(2+)-channel antagonist, semotiadil, in canine skeletal muscle membranes. Semotiadil inhibited binding of (+)-[3H]PN 200-110 (maximum inhibition 80%), and almost completely inhibited binding of both (-)-[3H]desmethoxyverapamil and D-cis-[3H]diltiazem to their specific binding sites with an IC50 value of 0.2-2 microM and a Hill slope of 0.6-0.9. Saturation isotherm and dissociation kinetic studies suggest that semotiadil acts as a noncompetitive inhibitor at the 1,4-dihydropyridine, phenylalkylamine, and benzothiazepine (BTZ) recognition sites in the L-type Ca2+ channel: (a) Scatchard analysis showed that semotiadil decreased maximum binding (Bmax) of the three classes of Ca2+ channel antagonists, while causing a slight increase in the equilibrium dissociation constant (Kd) in the case of (+)-[3H]PN 200-110 binding or no significant change in Kd values for binding of (-)-[3H]desmethoxyverapamil and D-cis-[3H]diltiazem to their specific binding sites; and (b) dissociation kinetics of the (+)-[3H]PN 200-110 and D-cis-[3H]diltiazem bindings were accelerated by semotiadil. These results suggest that semotiadil has a strong negative allosteric interaction with three classes of Ca2+ channel antagonists, including 1,4-dihydropyridines, phenylalkylamines, and BTZ at their specific binding sites.
authors.affiliation: Department of Pharmacology, University of Shizuoka, School of Pharmaceutical Sciences, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Nakayama
authors.forename: Y
authors.initials: Y
authors.lastname: Nozawa
authors.forename: Y
authors.initials: Y
authors.lastname: Fukuta
mesh_terms: D000494:Allosteric Regulation; D000818:Animals; D002121:Calcium Channel Blockers / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D004110:Diltiazem / Q000378:metabolism*; D004285:Dogs; D005260:Female; D005711:Gallopamil / Q000378:metabolism*; D017275:Isradipine / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008566:Membranes / Q000378:metabolism; D009132:Muscles / Q000378:metabolism*; D013844:Thiazoles / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D002121:Calcium Channel Blockers; D015220:Calcium Channels; D013844:Thiazoles; D005711:Gallopamil; C064356:sesamodil; D004110:Diltiazem; D017275:Isradipine