title: Opposite effects of Bay k 8644 and nicardipine on the inhibitory effect of Ca2+ on rat myometrium.
abstract: The effect of Ca2+ on the oxytocin-induced, sustained contraction of rat uterine muscle in Ca-free medium after prolonged incubation with 3 mM EGTA (Ca-free contraction) was investigated. A micromolar concentration of Ca2+ caused phasic contraction followed by relaxation while a submicromolar concentration caused relaxation only. Cumulative addition of Ca2+ (10(-8)-3 X 10(-6) M) caused dose-dependent relaxation (Ca reversal). This relaxation was inhibited by nicardipine and enhanced by Bay k 8644, and the effects of these two drugs were potentiated in 45.6 mM K+ medium. It is concluded that the inhibitory effect of Ca2+ on Ca-free contraction is caused by the influx of a minute amount of Ca2+. Thus, Ca2+ has dual actions in the cell: activation at concentrations higher than 10(-6) M, and inhibition alone at concentrations lower than 10(-7) M.
mesh_terms: D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000494:pharmacology* / Q000502:physiology; D005260:Female; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects; D009126:Muscle Relaxation / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D009215:Myometrium / Q000187:drug effects; D009529:Nicardipine / Q000494:pharmacology*; D010052:Ovariectomy; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D009529:Nicardipine; D002118:Calcium