Myocardial (Na+,K+)-ATPase activity in Dahl salt-sensitive and resistant rats

Coordinated shifts in Na/K-ATPase isoforms and their endogenous ligands during cardiac hypertrophy and failure in NaCl-sensitive hypertension

OBJECTIVES

NaCl loading of Dahl salt-sensitive rats (DS) stimulates marinobufagenin (MBG), an alpha1 Na/K-ATPase (NKA) isoform ligand. Cardiac function depends on NKA, which is regulated in part by endogenous digitalis-like ligands. Our goal was to study whether changes occur in MBG and endogenous ouabain (EO) production during cardiac remodelling in hypertensive DS, and whether these are associated with changes in myocardial NKA isoforms and sensitivity to MBG and ouabain.

METHODS

Changes in MBG and EO levels, changes in myocardial NKA isoform composition, and sensitivity to endogenous ligands during development of cardiac hypertrophy and the transition to heart failure were studied in DS rats with an 8% NaCl intake.

RESULTS

The animals developed compensated left ventricular hypertrophy after 4 weeks, which progressed to heart failure at 9-12 weeks. The hypertrophic stage was associated with increased plasma MBG levels (mean +/- SEM of 1.22 +/- 0.22 versus 0.31 +/- 0.03 nmol/l; P < 0.01), increased sensitivity of NKA to MBG, and an increased abundance of alpha1 NKA. Plasma levels of EO did not change, and the sensitivity of NKA to ouabain decreased. The transition to heart failure was accompanied by a decrease in alpha1 NKA, a reduction in plasma MBG, and decreased sensitivity of NKA to MBG. In addition, an increased abundance of ouabain-sensitive alpha3 NKA, a three-fold rise in plasma EO (1.01 +/- 0.13 versus 0.27 +/- 0.06 nmol/l), and a seven-fold increase in the ouabain sensitivity of NKA compared with controls were observed.

CONCLUSIONS

During cardiac hypertrophy and the transition to heart failure, a shift in endogenous NKA ligands production is linked to a shift in myocardial NKA isoforms.

Pharmacologic agents for the in vivo detection of vascular sodium transport defects in hypertension

Anatagonists to angiotensin, catecholamines, aldosterone, and vasopressin have long been used to help determine agonist roles in hypertension. We here call attention to a possible extension of this approach to detect, evaluate, and treat vascular sodium transport defects in hypertension. Two basic types of transport defects have been identified in the blood vessels of hypertensive animals, increased sodium permeability and decreased sodium pump activity. Intravenous injection of 6-iodo-amiloride, a sodium channel blocker and vasodilator, produces an immediate and sustained decrease in blood pressure in two genetic models of hypertension characterized by increased permeability of the vascular smooth muscle cell membrane to sodium (Okamoto spontaneously hypertensive rat, Dahl salt sensitive rat), whereas it produces only a transient fall in arterial pressure in two renal models of hypertension having normal sodium permeability in vascular smooth muscle cells (reduced renal mass-saline rat, one-kidney, one clip rat). Canrenone, a metabolic product of spironolactone which can compete with oubain for binding to Na+,K+-ATPase at the digitalis receptor site, decreases blood pressure in a low renin, volume expanded model of hypertension which has been shown to have depressed sodium pump activity in arteries and increased sodium pump inhibitor in plasma (reduced renal mass-saline rat) but has no effect on blood pressure in a genetic model of hypertension which has been shown to have increased sodium pump activity secondary to increased sodium permeability (spontaneously hypertensive rat). Thus, a sodium channel blocker and a competitor to ouabain binding can detect and determine the functional significance of sodium transport defects in the blood vessels of intact hypertensive animals. Studies in red and white blood cells suggest that similar defects may exist in the blood vessels of hypertensive humans. Thus, this approach, probing for vascular transport defects in the intact animal, may ultimately also be useful in the clinical setting.