Marinobufagenin, an endogenous α-1 sodium pump ligand, in hypertensive Dahl salt-sensitive rats

Dahl salt-sensitive rats (DS), which have a mutation in the α-1 subunit of Na⁺/K⁺ATPase, exhibit impaired pressure natriuresis and on a high-salt diet, retain Na⁺ and exhibit increased blood pressure. Recently, we have shown that mammalian tissues contain a bufadienolide Na⁺/K⁺-ATPase inhibitory factor, marinobufagenin (MBG), that exhibits greater affinity for the α-1 than α-3 sodium pump isoform. The present study investigated the possible role of MBG in hypertension in DS on a high NaCl intake. Eight DS and 8 Dahl salt-resistant rats (DR) were placed on an 8% NaCl diet. Within 2 weeks, systolic blood pressure increased in DS (162±9 mm Hg at week 2 versus 110±2 mm Hg in baseline, P<0.01), and increased less in DR (124±3 mm Hg at week 2 versus 112±2 mm Hg in baseline). Renal excretion of MBG increased 4-fold (38.9±7.6 pmol versus 9.1±1.3 pmol in baseline, P<0.01) in DS, but by only 25% in DR (13.2±0.9 pmol versus 10.3±0.7 pmol in baseline). Excretion of endogenous ouabain did not change in either strain. MBG-immunoreactive material was purified from the urine of hypertensive DS by means of 2 steps of reverse-phase high performance liquid chromatography (HPLC) and compared with plant ouabain and amphibian MBG for its ability to inhibit the Na⁺/K⁺-ATPase from rat kidney (which expresses only α-1 Na⁺/K⁺-ATPase isoform). Unlike ouabain (IC₅₀=248 μmol/L), serially diluted, HPLC-purified MBG immunoreactivity from DS and authentic MBG potently inhibited rat kidney Na⁺/K⁺-ATPase (IC₅₀=70 and 78 nmol/L. respectively). Our results suggest that an α-1 Na⁺/K⁺-ATPase ligand. MBG, is elaborated to promote natriuresis in hypertensive DS. MBG acts as a selective inhibitor of the ouabain-resistant α-1 Na⁺/K⁺-ATPase subunit, ie. the major sodium pump isoform of the kidneys, as would be expected of a putative natriuretic hormone.