A probable relationship between an endogenous digitalis-like substance and concentric cardiac hypertrophy in primary aldosteronism

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

Calpain is a mediator of myocardial injury in experimental uremia: is it activated by endogenous ouabain?

BACKGROUND

Incidence of cardiovascular disease is more than 20-fold higher in patients with chronic renal failure than in aged-matched individuals with normal renal function. Little is understood about the causes or the mechanism of uremia-induced cardiovascular injury, but the involvement of calpain as a possible mediator has recently been under investigation. Mean calpain activity was found to be 3.4-fold higher in the hearts of uremic rats than in control or spontaneously hypertensive (SHR) rats. In addition, calpain activity was found to be stimulated in myoblasts (Girardi) treated with media enriched with uremic serum compared with cells treated with serum from healthy volunteers. In this study, we assessed the impact of calpain activation in uremia and explored the possibility that calpain might be activated in uremia by endogenous ouabain. Ouabain is known to be elevated in uremia and is strongly associated with left ventricular hypertrophy in essential hypertension.

METHODS

Calpain activity was measured in situ in human-derived myoblasts treated with low doses of ouabain similar to those concentrations found in uremic patients.

RESULTS

Low concentrations of ouabain (10 nmol/L) caused a highly significant increase in calpain activity, which could be completely inhibited by the simultaneous chelation of intracellular and extracellular Ca2+, and by the chelation of extracellular Ca2+ alone.

CONCLUSIONS

Calpain activity can be stimulated by nanomolar concentrations of ouabain due to an influx of extracellular Ca2+. As circulating ouabain is known to be elevated in uremia and strongly associated with LVH remodeling, we hypothesize that endogenous ouabain might be one of the factors that facilitates the remodeling of the left ventricle in patients with renal failure.