INTERACTIONS OF Cd++ AND CYSTEINE ON Ca++ CONTENT AND CONTRACTILITY OF ISOLATED AORTAS AND CARDIAC MUSCLES

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In isolated rabbit aortas, ventricles and left atria, the addition of Cd++ (0.5 mM) caused a decrease in the content of Ca++ and an increase in the Cd++ content. Contents of Mg++ and Zn++ were not significantly altered. Cysteine (1 mM) applied simultaneously with Cd++ prevented the Ca++-depleting action and also the accumulation of Cd++. Treatment of the tissues with Ca++-free media, EGTA (5 mM) and EDTA (5 mM) also reduced the content of Ca++. Treatment with Cd++ and EGTA did not significantly alter the extracellular space measured with 14C-inulin. The contractile response of aortic strips to noradrenaline and K+ was abolished by 0.5 mM Cd++. This inhibition was completely prevented by cysteine (1 mM) when noradrenaline was used as a stimulant, and partially prevented when K+ was used. Aortic contractions induced by noradrenaline and K+ were also attenuated by removal of Ca++ from bathing media and EGTA, the attenuation of the response to the ion being greater than that with the amine. Contractions of left atria driven electrically were abolished by 0.5 mM Cd++. Cysteine prevented only partially the inhibitory effect of Cd++. It may be concluded that Cd++ binds SH groups of membrane constituents to prevent influxes of Ca++, thus decreasing the content of Ca++ and reducing the reactivity of aortas and the contractility of atria.
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