Biphasic contraction induced by ouabain in human umbilical arteries.

The effects of nifedipine on the contraction induced by ouabain were investigated in helically cut human umbilical arterial strips. Ouabain (10(-7)-10(-4) M) induced a rapid transient (early) contraction and a persistent slow (late) contraction. Both effects were dose-dependent. The early contractions were inhibited by nifedipine (5 x 10(-9) and 10(-7) M) and by Ca2+-free solution containing 0.2 mM EGTA but the late contractions were not altered by nifedipine. The results suggest that ouabain could facilitate the transmembrane Ca2+ entry through voltage-sensitive Ca2+ channels.