Na+/Ca2+ exchange mediation in the ouabain-induced contraction in human placental vessels

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. Ouabain (10(-4) M) elicited a time-dependent enhancement of the 45Ca2+ uptake, which remained equal in presence of nifedipine or Bay K 8644. 3. The Na+/Ca2+ exchange blocker amiloride reduced both the contractions and the 45Ca2+ uptake induced by ouabain, whereas the Na ionophore monensin produced a parallel shift to the left of the concentration-response curve to ouabain. 4. These results suggest that ouabain-induced contractions in these vessels are dependent on the extracellular Ca2+, which mainly enters into the cell through the Na+/Ca2+ exchange system.