Chronic Hypertension Induced by Ouabain but Not Digoxin in the Rat: Antihypertensive Effect of Digoxin and Digitoxin

Elevated circulating levels of an endogenous ouabain (EO) have been associated with essential hypertension. To investigate structure-activity relationships relevant to blood pressure, we infused either ouabain, ouabagenin, digoxin or digitoxin at 30 microg/kg/day in normal Sprague Dawley rats. After five weeks, the ouabain and ouabagenin infused rats were hypertensive, whereas blood pressures declined below their vehicle controls in rats infused with digoxin or digitoxin. In a second study, mean blood pressures were 118.5+/-1.7 mmHg in rats infused with ouabain (15 microg/kg/day) on day 35 vs. 98.3+/-1.8 and 100.3+/-1.1 mmHg in the digoxin (30 microg/kg/day) and vehicle infused groups (both p<0.005 vs. ouabain), respectively. Plasma and kidney levels of ouabain immunoreactivity were increased 4-8 fold in ouabain infused rats while blood pressure and plasma levels of ouabain returned to normal one week following discontinuation of the steroid infusion. In rats with ouabain-dependent hypertension, secondary infusions of digoxin or digitoxin (30 microg/kg/day) normalized blood pressure even though circulating ouabain remained elevated. In digoxin infused rats, neither blood pressure nor kidney digoxin immunoreactivity was raised whereas plasma digoxin was increased. Collectively, the results show that the hemodynamic effects of these sodium pump inhibitors differ dramatically during prolonged administration and that tissue rather than circulating levels of these agents appear to better explain their effects on blood pressure. These studies suggest that sodium pump inhibition is not the exclusive mediator of the hemodynamic effects of these cardiac glycosides and demonstrate the presence of structure-specific mechanisms that regulate their tissue levels and effects on long-term blood pressure.