Digoxin Prevents Ouabain and High Salt Intake-Induced Hypertension in Rats With Sinoaortic Denervation

Digoxin prevents ouabain-induced hypertension in rats. In the present study, we tested whether this effect of digoxin depends on its sensitizing effect on baroreflex function or is due to an antagonistic action on exogenous ouabain or endogenous ouabainlike activity ("ouabain") in the brain. In Wistar rats, resting mean arterial pressure (MAP) was significantly increased by long-term subcutaneous (SC) ouabain (75 μg/d) plus high salt (8%) intake for 12 days (but not after only 5 days). In rats with chronic sinoaortic denervation (SAD), MAP was increased within 5 days of ouabain treatment to the same extent as MAP after 12 days of treatment in intact rats. The effect of ouabain and high salt was prevented when digoxin was given SC concomitantly via osmotic minipump (200 μg · kg · d). Resting MAP was not changed in rats treated with digoxin alone. In a second set of rats with chronic SAD or sham surgery, high salt intake was given for 14 days, with or without SC digoxin (200 μg · kg · d) or intracerebroventricular (ICV) antibody Fab fragments (200 μg/d), which bind "ouabain" with high affinity. On day 14, MAP, central venous pressure, heart rate, and renal sympathetic nerve activity were recorded in conscious rats at rest and in response to air-jet stress. IV phenylephrine and nitroprusside, and acute volume expansion with 5% dextrose IV. In rats with SAD versus sham surgery, high salt significantly increased resting MAP as well as excitatory responses of MAP, heart rate, and renal sympathetic nerve activity to air stress. These effects of high salt in rats with SAD were prevented by digoxin or Fab fragments. Arterial baroreflex function was blunted but cardiopulmonary baroreflex function was not affected in rats with SAD. Digoxin and Fab fragments had no effects on either function. In an in vitro assay for the inhibitory effects on Na,K-ATPase activity, 20 ng of ouabain caused 29% inhibition, but 20 ng of ouabain plus 13 or 53 ng of digoxin caused only 16% or 4% inhibition, respectively. These data indicate that the arterial baroreflex opposes sympathoexcitatory responses to ouabain and "ouabain" in the brain, thereby delaying ouabain- and preventing high salt-induced hypertension in Wistar rats. In addition to possible effects on the arterial baroreflex, digoxin appears to act centrally to prevent the sympathoexcitatory and pressor effects of increased brain "ouabain" or ouabain.