Abstract
1. 1. Ouabain induced concentration-dependent contractions in segments of human placental arteries and veins, which were practically abolished in a Ca 2+-free medium and not modified by the calcium antagonist nifedipine or the calcium agonist Bay K 8644. 2. 2. Ouabain (10 −4 M) elicited a time-dependent enhancement of the 45Ca 2+ uptake, which remained equal in presence of nifedipine or Bay K 8644. 3. 3. The Na +/Ca 2+ exchange blocker amiloride reduced both the contractions and the 45Ca 2+ uptake induced by ouabain, whereas the Na ionophore monensin produced a parallel shift to the left of the concentration-response curve to ouabain. 4. 4. These results suggest that ouabain-induced contractions in these vessels are dependent on the extracellular Ca 2+, which mainly enters into the cell through the Na +/Ca 2+ exchange system.
Published Version
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