Inhibitory effect of ouabain on the palytoxin-induced contraction of human umbilical artery.

Palytoxin (PTX), C129H223O3N54, isolated from marine coelenterates of Palythoa tuberculosa, caused contraction of the human umbilical artery in a dose-dependent manner (10(-11)-10(-8) M). Pretreatment with ouabain (10(-5) M) abolished the PTX (10(-8) M)-induced contraction but had no effect on the serotonin- (10(-6) M) and potassium- (40 mM) induced contractions. When the muscle was exposed to a potassium-free medium, application of PTX was able to cause a contraction similar to the contraction in normal medium. In the presence of verapamil (3 X 10(-6) M) or in the calcium-free medium. PTX-induced contraction was inhibited. In the depolarized muscle with 126 mM potassium, PTX did not induce a contraction whereas serotonin did. Our results suggest that PTX causes calcium influx through the plasma membrane of the umbilical artery, causing contraction. The site of action of PTX is presumably related to the Na,K-ATPase on the plasma membrane, although the precise relation between the site of action and increase in calcium influx is not known now.