Effects of long-term ouabain treatment on blood pressure, sodium excretion, and renal dopamine D1 receptor levels in rats
To examine the effects of chronic ouabain treatment on blood pressure (BP), sodium excretion, and renal dopamine D1 receptor level, male Sprague-Dawley (SD) rats were treated with ouabain (27.8 μg kg−1 d−1) intraperitoneally for 5 weeks, and systolic blood pressure (SBP) were recorded weekly. After 5 weeks, sodium excretion and dopamine D1 receptor agonist fenoldopam-mediated natriuresis were measured, and the expression and phosphorylation levels of the renal cortical dopamine D1 receptor were confirmed by Western blot analysis. The effects of ouabain on fenoldopam-mediated inhibition of Na+-K+-ATPase activity were determined by colorimetric assays in human proximal tubular epithelial cells (HK-2 cells). After 5 weeks, the SBP in ouabain group was significantly higher than that in the control group (P < 0.01), but the sodium excretion and renal cortical D1 receptor expression levels were reduced, and D1 receptor phosphorylation levels were increased after ouabain treatment. Intravenous administration of fenoldopam caused an increased sodium excretion in control rats, but failed to induce natriuresis in ouabain-treated rats. In addition, fenoldopam induced a dose–respone (10−9 to 10−6 M) inhibition of Na+-K+-ATPase activity in HK-2 cells,but these effects were significantly diminished in HK-2 cells pretreated with nanomolar concentration of ouabain for 5 days (P < 0.01). We propose that the ouabain-induced reduction of the renal dopamine D1 receptor function serves as a mechanism responsible for sodium retention, and this contributes to the hypertension induced by chronic ouabain treatment.