Prompt effect of progesterone on the adrenergic response of smooth muscles.

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1) The contractile effects of epinephrine on the uterus and ductus deferens of the rabbit and the ductus deferens of the monkey were inhibited by the preincubation with progesterone (6.4×10-5 M) for 1 or 3 min in Locke-Ringer solution. Epinephrine relaxed the guinea pig uterus and taenia caecum. The relaxant effects were enhanced by preincubation with progesterone. Their effects were in a dose-dependent manner. 2) There was no apparent change in the number and affinity of alpha-adrenergic receptors in the uterus of rabbits and the ductus deferens of guinea pigs during the incubation with progesterone. Progesterone has no direct effect on alpha-adrenergic receptors. 3) All smooth muscles yielded reproducible contractile reactions to Ca2+ when maintained in depolarizing Tyrode's solution containing K+ (40 mmol/I). Their concentration-response curves were inhibited by preincubation with progesterone (6.4×10-5 M), and they were shifted to the right in a concentration-dependent manner. Established Ca2+induced contractions were rapidly relaxed by the addition of progesterone (6.4×10-5 M). 4) It suggests that progesterone directly affects the plasma membrane and inhibits the voltage-dependent Ca2+ channel and then inhibits smooth muscle contraction.
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