Structure-Activity Relationships for the Hypertensinogenic Activity of Ouabain: Role of the Sugar and Lactone Ring.
Elevated levels of an endogenous ouabain (EO) occur in many patients with essential hypertension (e.g., Manunta et al, Hypertension 1999;34:450-456) and the prolonged infusion of ouabain induces hypertension (Manunta et al J. Hypertension 1994;12:549-560). This study used a series of closely related structural analogs of ouabain to test the key hypothesis that sodium pump inhibition underlies ouabain-dependent hypertension. The functional roles of the sugar and lactone ring were tested for their effects on inhibition of the Na,K-ATPase, and on long term blood pressure. Normal male SD rats received infusions of vehicle (VEH), rhamnose (RHA), ouabain (OUA), ouabagenin (OGN), dihydroouabain (DHO), isoouabain (ISO), and ouabain in which the lactone ring was opened (ORO)at 30 μg/Kg/24 hr for 4-5 weeks via s.c. osmotic pumps. Cuff pressures were taken weekly. At the end of the study, blood and tissues were harvested, extracted by C-18 and analyzed by ouabain-RIA using appropriate standards. HPLC showed that plasma and tissues contained one main peak of immunoreactivity corresponding to the infused agent. Systolic pressures (SBP)in VEH, RHA, OUA, OGN, DHO, ISO and ORO were 132±2.5, 133±1.5, 159±2.6*, 154±4*, 167±4*†, 171±2.2*†, 169±2.4*†,respectively (*P<0.01 vs VEH and RHA, †P<0.05 vs OUA). The hypertensinogenic activity was significantly greater than ouabain in three of the analogs (DHO, ORO, ISO)in which the lactone was either saturated, conformationally restrained by linkage with C14, or opened. These compounds were weak or inactive as inhibitors of the Na,K-ATPase. Similarly, absence of RHA from ouabain affected SBP minimally. We conclude that the hypertensinogenic activity of ouabain is mediated exclusively by the steroid nucleus. The lactone ring is not required for ouabain to induce hypertension but is essential for inhibition of the Na,K-ATPase. A novel mechanism mediates ouabain-dependent hypertension.