The Mode of Action of Contractile Effects Induced by External Calcium and its Related Bivalent Cations in the KCL-Depolarized Rat Uterus.

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Abstract
The cumulative log dose-response curve (DRC) isometrically produced by CaCl2 on depolarized uteri of rats in Ca2+-free KCl Ringer''s solution (pH 7.8) showed a plateau-shaped type, when responses were plotted as a percent of the maximal tension of 50 .mu.M acetylcholine, whereas those produced by BaCl2 or SrCl2 were a simple sigmoid type with the maximal response near the height of the plateau induced by Ca2+. The plateau phase of Ca2+ was inhibited competitively by Mg2+ (0.5-50 mM) and non-competitively by Mn2+ (30 .mu.M-1 mM), whereas the maximal contraction of Ca2+ was not inhibited by either ion. Dantrolene (0.1 mM) inhibited both the plateau and the maximal contraction of Ca2+ and retained the plateau-shaped type. EGTA [ethyleneglycol-bis(.beta.-aminoethyl ether)N,N,N'',N''-tetraacetic acid] (2 mM) potentiated the plateau to the maximal response level and changed the DRC induced by Ca2+ to the simple sigmoid type. The plateau of DRC induced by Ca2+ seems to be mainly due to actions of external Ca2+ on the calcium binding of internal surface of cell membrane, suggesting that the internal surface is the site of action of the bivalent cations tested.