Effect of ouabain on the responses to vasoconstrictor agents in isolated perfused rat tail arteries

In isolated rat tail arteries perfused with Krebs-Henseleit solution, ouabain, in concentrations of less than 10(-5) M, caused a concentration-dependent, noncompetitive inhibition of the vasoconstrictor response to phenylephrine. Concentrations of 10(-8) and 10(-6) M caused 30 and 61% inhibition, respectively. The inhibition was gradual in onset, with a maximal effect at 45 min. In concentrations greater than 10(-5) M, ouabain caused a parallel shift to the left of the phenylephrine dose-response curve, indicating potentiation. The potentiation was rapid in onset (within 1 min), and was approximately 1.4-fold. The cutoff point between inhibition and potentiation of vasoconstrictors occurred at a ouabain concentration of 10(-5) M. Ouabain-induced potentiation was nonspecific in regard to the vasoconstrictor used. Phenylephrine, clonidine, and serotonin were each potentiated. Potentiation probably resulted from a reduction in the smooth muscle membrane potential as a consequence of inhibition of Na-K ATPase. The mechanism of the inhibitory effect of ouabain on vasoconstrictors was not resolved but may operate via stimulation of Na-K ATPase. The results are consistent with a "partial agonist" action of ouabain on Na-K ATPase.