Contraction Induced by Ouabain and Potassium-free Solution in Human Umbilical Arteries

Summary

Ouabain-induced contraction is either of the myogenic or the neurogenic type depending on the variety of artery. In the present study, mechanisms of the ouabain and potassium-free solution induced contraction were investigated in the human umbilical arteries. The umbilical arteries were obtained from full-term, uncomplicated deliveries of normal infants. The arteries were cut helically into strips, which were mounted in the modified Krebs-bicarbonate solution. Ouabain (3 × 10⁻⁷ to 10⁻⁴ M) induced concentration-dependent biphasic contractions which consisted of an early contraction — a transient rapid contraction — and a late contraction — a slow continuous contraction. Potassium-free solution also induced biphasic contractions. A calcium antagonist, nifedipine (5 × 10⁻⁹ and 1 × 10⁻⁷ M), inhibited the early contraction but did not inhibit the late contraction. In addition, Ca²⁺-free solution containing 0.2 mM EGTA completely inhibited the early contraction but did not completely inhibit the late contraction. Neither the early nor late contractions altered phenoxybenzamine (2 × 10⁻⁶ M). In conclusion, ouabain and potassium-free solution induced early contraction in umbilical arteries, which could have been caused by facilitation of Ca²⁺ entry into the vascular smooth muscle through voltage-sensitive Ca²⁺ channels.