Although Na-K-ATPase plays an important role in vascular smooth muscle function, it is unknown which isoforms of the enzyme are present in the pulmonary vasculature and whether they possess different affinities for ouabain. Unlike rodents, Na-K-ATPase in sheep and humans displays greater affinity for ouabain. Thus, the present study examined the presence of various isoforms of the enzyme by Western blot analysis and their sensitivity to inhibition by ouabain (biochemical estimation of enzyme activity/K-relaxations) in the ovine pulmonary artery. Further, we studied the effect of ouabain on the basal tone and agonist-induced contractions in this vessel. Our results show the presence of both alpha1 and alpha2 isoforms of Na-K-ATPase in this vessel. The biphasic shape of the ouabain inhibition curve indicates that the alpha1 and alpha2 isoforms of the enzyme may possess low and high affinity, respectively, for the cardiac glycoside. Concentrations of ouabain <1 microM had no significant effect on the basal tone of the vessel. At 1 microM concentration, however, there was a significant increase in the basal tension (55% of 5-HT 1 microM contraction). Ouabain (0.1 microM) selectively increased the vasoconstrictor potency of 5-HT (pD2 6.81 +/- 0.10 versus control pD2 5.95 +/- 0.07), but not that of phenylephrine (pD2 5.80 +/- 0.07 versus control pD2 6.05 +/- 0.05). Neither endothelium removal nor treatment with PKG inhibitor KT 5823 (2 microM) had any effect on the sodium pump function. These results indicate that the low, but not the high, ouabain-sensitive isoform of Na-K-ATPase regulates basal tone and agonist-induced contractility in the ovine pulmonary artery.