[3H]TA-3090, a selective benzothiazepine-type calcium channel receptor antagonist: in vitro characterization.
Binding of the new benzothiazepine calcium channel blocker, (+)-(2S,3S)-3-acetoxy-8-chloro-5-(2-(dimethylamino)ethyl)-2,3-dihydro-2- (4- methoxyphenyl)-1,5-benzothiazepine-4-(5H)-one maleate, [3H]TA-3090), was characterized and its specificity for rat myocardial benzothiazepine receptors described. Scatchard plots and nonlinear regression analysis of specific [3H]TA-3090 binding best fit a one-site binding model (Kd = 8.8 +/- 2.7 nM, Bmax = 132 +/- 38 fmol/mg protein). Kinetically derived affinity constants were in close agreement (Kd = 7.86 nM) with those obtained from analysis of equilibrium binding data. In comparison, under identical conditions [3H]diltiazem exhibited a Kd of 38 nM and Bmax, 106 fmol/mg protein. Specific binding was saturable, reversible and stereoselective (d-cis-TA-3090 Ki = 14 nM; 1-cis-TA-3090 Ki = 2700 nM). Competitions for [3H]TA-3090 binding were conducted with nifedipine, propranolol, prazosin, quinuclidinyl benzilate, verapamil and yohimbine. Only the calcium channel blockers nifedipine and verapamil inhibited specific [3H]TA-3090 binding. Nifedipine could maximally inhibit only 52% of specifically bound [3H]TA-3090 at 10 microM. In contrast, however, 10 microM verapamil completely inhibited specific radioligand binding (Ki = 93 +/- 28 nM) but with six times less efficacy than TA-3090. Thus, these data demonstrate that [3H]TA-3090 is a potent radioligand selective for the benzothiazepine binding site and is consistent with the hypothesis that [3H]TA-3090 interacts with a myocardial benzothiazepine receptor site.
Zobrist
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RH
Department of Cardiovascular Pharmacology, Marion Merrell Dow Inc., Kansas City, Missouri.
Mecca
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TE
D000818:Animals; D001667:Binding, Competitive; D002121:Calcium Channel Blockers / Q000378:metabolism* / Q000494:pharmacology; D046508:Culture Techniques; D004110:Diltiazem / Q000031:analogs & derivatives*; D006321:Heart / Q000187:drug effects; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013841:Thiazepines / Q000378:metabolism*; D014316:Tritium
D002121:Calcium Channel Blockers; D013841:Thiazepines; D014316:Tritium; C056595:clentiazem; D004110:Diltiazem