In addition to inhibition of the Na-K
ATPase,
ouabain activates a signal transduction function, triggering growth and proliferation of cultured cells even at nanomolar concentrations. An isomer of
ouabain (EO) circulates in mammalians at subnanomolar concentrations, and increased levels are associated with
cardiac hypertrophy and
hypertension. We present here a study of cardiac and renal
hypertrophy induced by
ouabain infused into rats for prolonged periods and relate this effect to the recently described
ouabain-induced activation of the Src-EGFr-ERK signaling pathway.
Ouabain infusion into rats (15 microg/kg/day for 18 weeks) doubled plasma
ouabain levels from 0.3 to 0.7 nm and increased blood pressure by 20 mm Hg (p < 0.001), cardiac left ventricle (+11%, p < 0.05), and kidney weight (+9%, p < 0.01). These effects in vivo are associated with a significant enrichment of alpha1, beta1, gammaa Na-K
ATPase subunits together with Src and EGFr in isolated renal caveolae membranes and activation of ERK1/2. In caveolae, direct Na-K
ATPase/Src interactions can be demonstrated by co-immunoprecipitation. The interaction is amplified by
ouabain, at a high affinity binding site, detectable in caveolae but not in total rat renal membranes. The high affinity site for
ouabain is associated with Src-dependent
tyrosine phosphorylation of rat alpha1 Na-K
ATPase. The
antihypertensive compound,
PST 2238, antagonized all
ouabain-induced effects
at 10 microg/kg/day in vivo or 10(-10)-10(-8) m in vitro. These findings provide a molecular mechanism for the in vivo pro-hypertrophic and hypertensinogenic activity of
ouabain, or by analogy those of EO in humans. They also explain the pharmacological basis for
PST 2238 treatment.