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Opposite effects of Bay k 8644 and nicardipine on the inhibitory effect of Ca2+ on rat myometrium

1987; Elsevier BV; Volume: 140; Issue: 3 Linguagem: Inglês

10.1016/0014-2999(87)90286-x

1879-0712

Katsurou Matsuo, Masaatsu-Koujirou Uchida,

Chemical Reaction Mechanisms

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+ (10−8-3×10−6 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.