Low concentrations of ouabain activate vascular smooth muscle cell proliferation.

In vascular smooth muscle cells the sodium pump complex can act as an intracellular signal transducing complex activated by low ouabain concentrations, which inhibit sufficient pumps to activate a transduction cascade via transactivation of EGFR, but insufficient pumps to alter intracellular ions. Higher concentrations interfere with proliferation. This biphasic ouabain response occurs in human, canine, and rat VSMC at concentrations that reflect the differing ouabain affinities of the alpha1 isoforms of the three species. This supports the proposal that this effect occurs via ouabain binding to the alpha1 subunit of the Na pump. These data suggest a new transducing function of ouabain-Na pump interaction, distinct from the cellular ionic effects resulting from pump inhibition. This transducing function occurs at ouabain concentrations that do not perturb cytoplasmic ion content and requires specific localization of pumps to caveolae.