title: Differential antagonism by Bay k 8644, a dihydropyridine calcium agonist, of the negative inotropic effects of nifedipine, verapamil, diltiazem and manganese ions in canine ventricular muscle.
abstract: Antagonism between either the dihydropyridine calcium agonist, Bay k 8644, or high external Ca2+ and the calcium antagonists, nifedipine, verapamil and diltiazem, and Mn2+ was investigated in canine isolated ventricular trabeculae. Bay k 8644 (10(-7)-10(-5)M) produced a slowly developing increase in developed tension which reached a maximum at 10(-6)M. A small decrease in the positive inotropic effect of Bay k 8644 at 10(-5)M was probably due to the negative inotropic effect of the solvent, 0.5% ethanol. Bay k 8644 (10(-7)-10(-5)M) produced a rightward parallel shift of the concentration-response curves for the negative inotropic effects of nifedipine (10(-8)-10(-5)M) and verapamil (10(-7)-3 X 10(-5)M). The slopes of the Schild plots were -0.92 for nifedipine (pA2 value = 6.58) and -0.48 for verapamil. Bay k 8644 (10(-6) and 10(-5)M) produced only a slight rightward shift of the concentration-response curves for the negative inotropic effect of diltiazem (10(-7)-3 X 10(-5)M) and did not affect the negative inotropic effect of Mn2+ (10(-4)-10(-2)M). Addition of 2.5 X 10(-3)M Ca2+ (5.05 X 10(-3)M Ca2+) to the medium produced a greater maximum positive inotropic effect than Bay k 8644. The concentration-response curves for the negative inotropic effects of nifedipine, verapamil and diltiazem obtained under these conditions were not essentially different from those under control conditions (2.55 X 10(-3)M Ca2+). 6 These results indicate that Bay k 8644, while producing a positive inotropic effect, antagonizes the negative inotropic effect of nifedipine by competing with the latter for the same site closely associated with the calcium channel. In contrast, Bay k 8644 antagonizes the negative inotropic effects of verapamil and diltiazem by interfering allosterically with the binding of these calcium antagonists to their sites of action. Bay k 8644 does not antagonize the negative inotropic effect of Mn2+. No pharmacological antagonism was observed between the three organic calcium antagonists and high external Ca2+_
authors.forename: K
authors.initials: K
authors.lastname: Ishii
authors.forename: N
authors.initials: N
authors.lastname: Taira
authors.forename: T
authors.initials: T
authors.lastname: Yanagisawa
mesh_terms: D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002120:Calcium Channel Agonists / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D004110:Diltiazem / Q000037:antagonists & inhibitors; D004285:Dogs; D005260:Female; D006321:Heart / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D008345:Manganese / Q000037:antagonists & inhibitors*; D009200:Myocardial Contraction / Q000187:drug effects*; D009543:Nifedipine / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors / Q000494:pharmacology; D014700:Verapamil / Q000037:antagonists & inhibitors
chemical_list: D002120:Calcium Channel Agonists; D002121:Calcium Channel Blockers; D008345:Manganese; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D014700:Verapamil; D004110:Diltiazem; D009543:Nifedipine; D002118:Calcium