How NaCl raises blood pressure: a new paradigm for the pathogenesis of salt-dependent hypertension.

Excess dietary salt is a major cause of hypertension. Nevertheless, the specific mechanisms by which salt increases arterial constriction and peripheral vascular resistance, and thereby raises blood pressure (BP), are poorly understood. Here we summarize recent evidence that defines specific molecular links between Na(+) and the elevated vascular resistance that directly produces high BP. In this new paradigm, high dietary salt raises cerebrospinal fluid [Na(+)]. This leads, via the Na(+)-sensing circumventricular organs of the brain, to increased sympathetic nerve activity (SNA), a major trigger of vasoconstriction. Plasma levels of endogenous ouabain (EO), the Na(+) pump ligand, also become elevated. Remarkably, high cerebrospinal fluid [Na(+)]-evoked, locally secreted (hypothalamic) EO participates in a pathway that mediates the sustained increase in SNA. This hypothalamic signaling chain includes aldosterone, epithelial Na(+) channels, EO, ouabain-sensitive α(2) Na(+) pumps, and angiotensin II (ANG II). The EO increases (e.g.) hypothalamic ANG-II type-1 receptor and NADPH oxidase and decreases neuronal nitric oxide synthase protein expression. The aldosterone-epithelial Na(+) channel-EO-α(2) Na(+) pump-ANG-II pathway modulates the activity of brain cardiovascular control centers that regulate the BP set point and induce sustained changes in SNA. In the periphery, the EO secreted by the adrenal cortex directly enhances vasoconstriction via an EO-α(2) Na(+) pump-Na(+)/Ca(2+) exchanger-Ca(2+) signaling pathway. Circulating EO also activates an EO-α(2) Na(+) pump-Src kinase signaling cascade. This increases the expression of the Na(+)/Ca(2+) exchanger-transient receptor potential cation channel Ca(2+) signaling pathway in arterial smooth muscle but decreases the expression of endothelial vasodilator mechanisms. Additionally, EO is a growth factor and may directly participate in the arterial structural remodeling and lumen narrowing that is frequently observed in established hypertension. These several central and peripheral mechanisms are coordinated, in part by EO, to effect and maintain the salt-induced elevation of BP.

[Changes of endogenous ouabain in rat serum and certain tissues after exposure to simulated weightlessness].

OBJECTIVE: To investigate the changes of endogenous ouabain (EO) in rat serum and some tissues after exposure to simulated weightlessness and to investigate its possible pathophysiology. METHOD: Male Wistar rats were randomly divided into control group (Con) and 1 week tail-suspension group (TS). Enzyme-linked immunosorbent assay (ELISA) was used to detect the content of EO in serum, hypothalamus, pituitary, adrenal gland, kidney, heart and liver. RESULT: Compared with Con, EO increased significantly in serum, hypothalamus, adrenal gland and kidney after tail suspension (P<0.05). CONCLUSION: Simulated weightlessness induced changes of EO in serum and some tissues, which may have some effects on the regulation of hydro-electrolyte metabolism and cardiovascular functions.

Brain sodium channels mediate increases in brain "ouabain" and blood pressure in Dahl S rats.

Central infusions of benzamil prevent/reverse salt-induced hypertension in genetic models of salt-sensitive hypertension. Benzamil acts by blockade of ion--presumably sodium--channels. In the present study, we assessed in Dahl salt-sensitive (S) rats on high salt intake whether these channels mediate increases in brain "ouabain" and, thereby, hypertension. Intracerebroventricular (icv) infusions of a low (1.2 microg/kg per hour) or high (4.0 microg/kg per hour) dose of benzamil were given to Dahl S rats on high salt diet (1370 micromol Na+/g food) for 2 or 4 weeks. "Ouabain" content was measured using a specific enzyme-linked immunosorbent assay (ELISA). Systolic blood pressure (BP) in Dahl S rats on high salt for 4 weeks increased markedly (188+/-10 versus 128+/-4 mm Hg, n=8, P<0.05). Benzamil fully blocked this increase (131+/-7 mm Hg after the high dose of benzamil). Hypothalamic and pituitary "ouabain" increased significantly (22+/-7 versus 12+/-3 and 151+/-38 versus 69+/-6 ng/g tissue, respectively, P<0.05) in Dahl S rats on high salt versus regular salt diet for 2 weeks. Benzamil blocked these increases of brain "ouabain" to high salt intake. Similarly, high salt intake for 4 weeks increased hypothalamic (18+/-2 versus 13+/-1 ng/g tissue, P<0.05) and pituitary (183+/-30 versus 78+/-8 ng/g tissue, P<0.05) "ouabain." Benzamil also inhibited these increases of brain "ouabain." Both hypothalamic and pituitary "ouabain" showed significant positive correlations with BP. In contrast, high salt intake did not affect "ouabain" levels in the adrenal gland or plasma in Dahl S rats on high salt for either 2 or 4 weeks. These findings indicate that in Dahl S rats high salt intake only increases brain and not peripheral "ouabain" and that benzamil-blockable brain sodium channels mediate the increases in brain "ouabain" and the subsequent hypertension.

Ouabain, a new steroid hormone of adrenal gland and hypothalamus.

Ouabain has been isolated and identified as a constituent of human blood, bovine adrenal glands and hypothalamus. This water soluble inhibitor of the sodium pump (Na+/K+-ATPase) circulates in elevated concentrations in blood plasma of 50% of Caucasians with elevated blood pressure. It is released from adrenal cortical cells in tissue culture by angiotensin II. A ouabain antagonist, PST2238, lowers blood pressure in hypertensive rats. Hence, ouabain is most probably a new steroid hormone formed in adrenal glands and hypothalamus. Consistent therewith is the demonstration of a specific binding globulin for cardiac glycosides in blood plasma.

Effect of dietary cardiac glycosides on blood pressure regulation in rats.

To investigate the possible physiological significance of dietary cardiac glycosides in blood pressure regulation, the blood pressure of normal Sprague Dawley rats raised on a regular diet, which naturally contains large amounts of Na+-pump inhibitors, was compared with that of rats on a purified synthetic diet, which contains no Na+-pump specific inhibitors, and with that of rats on a synthetic diet supplemented with 10 microg x mL(-1) ouabain or 10 microg x mL- convallatoxin in the drinking water. After 6 weeks on the synthetic diet, the systolic blood pressure in the synthetic diet group was significantly elevated (145 +/- 5 vs. 128 +/- 4 mmHg, P < 0.05). At 10 weeks it reached a plateau (154 +/- 3 vs. 122 +/- 3 mmHg, P < 0.05). Plasma renin activity and Na+ level were significantly higher in animals fed synthetic diets than in the regular diet group (P < 0.01). Administration of either losartan or lisinopril or a switch to a low salt synthetic diet (0.03% sodium) normalized the synthetic diet-induced high blood pressure. Supplementation of the synthetic diet with the cardiac glycosides delayed the onset of the increase in blood pressure for 4 weeks. Plasma aldosterone levels were approximately doubled in the cardiac glycoside-treated groups. Higher plasma Na+ levels and hematocrit values present in the synthetic diet group were normalized by the glycoside supplements. These results suggest that supplemental dietary cardiac glycosides exert bidirectional effects on blood pressure regulation through actions that modulate extracellular fluid and electrolyte balance.

Elevation of ouabainlike compound levels with hypertonic sodium chloride load in rat plasma and tissues.

A major biologically active endogenous digitalis-like factor in the mammalian body may be an isomer of ouabain (ouabainlike compound, OLC). However, the exact role of OLC in sodium homeostasis is still unclear, and acute isotonic volume expansion does not enhance the secretion of OLC. We tested the hypothesis that OLC may be more important in the response to acute hypertonic NaCl load rather than isotonic volume expansion. We injected intraperitoneally 2 mL of 20% NaCl solution into male Wistar rats (n=34) and measured OLC levels in plasma, hypothalamus, pituitary, and adrenal at baseline (n=10) and 1, 2, and 4 hours (n=8 for each). In response to hypertonic NaCl loading, plasma Na-K ratio was elevated at 2 and 4 hours (P<.01). OLC levels in pituitary increased (P<.01) at 1 hour. Thereafter, plasma OLC levels increased at 2 and 4 hours (P<.05; basal, 75+/-11 pmol/L [+/-SEM]; 1 hour, 55+/-11; 2 hours, 130+/-24; 4 hours, 156+/-20). Concomitantly, OLC levels in adrenal increased at 2 and 4 hours (P<.01; basal, 1.7+/-0.2 pmol/g; 1 hour, 4.5+/-0.9; 2 hours, 5.0+/-0.7; 4 hours, 6.8+/-2.2). A significant correlation was observed between OLC levels in plasma and adrenal (P<.05). Plasma Na-K ratio positively correlated with OLC levels in plasma (r=.51, P<.01) and adrenal (r=.48, P<.01). Similar injection of physiological saline solution or hypertonic sucrose solution in physiological saline did not increase OLC levels in plasma and tissues. These findings indicate the elevation of OLC levels in plasma, pituitary, and adrenal in response to acute hypertonic NaCl load in rats and suggest that OLC may be involved in the response to the hypernatremic state.

Franz Volhard Lecture 1996. Sodium transport inhibitors and hypertension.

SODIUM EXCRETION IN HYPERTENSION: The concept that blood contains a sodium transport inhibitor with natriuretic and pressor properties emerged in the 1960's and 1970's from three separate lines of enquiry. The control of sodium excretion, in normal man and animals undergoing volume expansion, in uraemic man and animals, and thirdly the effect of cations on arteries from normal and hypertensive animals. Each of these studies led to the notion that the plasma contains a digitalis-like substance which increases vascular tone by raising intra-cellular calcium. Na-K,ATPase inhibitors were then found in increased quantities in the plasma in essential hypertension and experimental hypertension. As a result it was proposed that in essential hypertension a hereditary renal impairment to excrete the usual large amounts of sodium consumed by most populations increased the circulating concentration of this substance and thereby the arterial pressure. ENDOGENOUS OUABAIN: Substances spectrometrically identified to be plant ouabain have now been found in human plasma and bovine hypothalamus. Derivatization experiments have shown that the 'plant' ouabains in human plasma and bovine hypothalamus are the same substance but that they are different from true plant ouabain. The endogenous ouabain analogue may have direct pressor effects centrally and peripherally.

Chronic ouabain infusion does not cause hypertension in sheep.

Ouabain is claimed to be a hormone of adrenal origin, capable of raising arterial pressure in rats. We infused ouabain in conscious sheep under carefully controlled circumstances to determine its effects on blood pressure, urine electrolytes, and vasoactive hormones. Eight healthy ewes were studied while taking a constant intake of dietary sodium and potassium. Ouabain infusion at 0.25 mg daily over 22 days reduced heart rate and arterial pressure and had no effect on pressor responsiveness to incremental intravenous infusions of angiotensin II. Ouabain induced minor, but statistically significant, decrements in urine volume, urinary sodium excretion, plasma renin and angiotensin II concentrations, and a rise in plasma aldosterone and cortisol. Plasma ouabain levels averaged 1.37 +/- 0.28 nmol/l during ouabain infusion. In conclusion, high-dose chronic ouabain infusion in sheep did not elevate arterial pressure or alter pressor responsiveness to angiotensin II, was antidiuretic and antinatriuretic, and induced minor perturbations in circulating renin, angiotensin II, aldosterone, and cortisol.

Effect of hypothalamic lesions on interaction of centrally administered ANF and the circulating sodium-pump inhibitor.

We investigated the possible central interaction of atrial natriuretic factor (ANF) and an endogenous Na+, K(+)-ATPase (Na-pump) inhibitor in normal rats. Release of an endogenous Na-pump inhibitor associated with deoxycorticosterone acetate-salt hypertension may be regulated in the anteroventral third ventricle (AV3V) area of the CNS. We reported earlier that bolus injection of synthetic 26-amino acid ANF (Arg101-Tyr126, 6 micrograms/250 g rat) into the lateral brain ventricle (ICV) promotes the appearance in the plasma of a Na-pump inhibitor in rats. To determine whether the AV3V area of the brain is involved in the ICV effect of ANF, we introduced electrolytic lesions in this area. This treatment abolished the appearance of the Na-pump inhibitor after intraventricular injection of ANF. To further localize the area and the pathways involved in the interaction of ANF and the Na-pump inhibitor, we produced bilateral medial coronal knife cuts designed to transect the medially coursing pathway through the periventricular tissue of the AV3V region between the level of the medial preoptic area and the anterior hypothalamic nuclei. These knife cuts also abolished the appearance of the Na-pump inhibitor after ICV injection of ANF. Our data to date indicate that centrally administered ANF promotes the appearance of a Na-pump inhibitor in the plasma. A central site of interaction between ANF and the Na-pump inhibitor appears to be the AV3V area and a medial pathway coursing caudally from the AV3V region.

Existence of a polar digitalis-like factor in mammalian hypothalamus.

We were able to partially purify a polar digitalis-like factor from rat and bovine hypothalami based on the capacity to inhibit [3H]ouabain binding to intact human erythrocytes. This factor was characterized in reference to the digitalis-like factor that we have isolated and reported on. Hypothalamic factor shared digitalis-like activities and physicochemical properties with the one derived from human urine and mammalian plasma. These findings strongly suggest that a polar digitalis-like factor identical to the circulatory factor does exist in mammalian hypothalamus.

Recent advances on endogenous Na+,K+-ATPase inhibitors: clinical investigation and purification.

Evidence exists which demonstrates the relationship between a Natriuretic Factor or Na+,K+-ATPase inhibitor and volemic expansion, both in man and animal. Patients having extracellular volume expansion have been studied for the effect of their plasma on erythrocytes 3H-ouabain binding. High levels of ouabain-like activity was found in plasma from acromegalic patients and patients with chronic renal failure. High levels were also observed in some hypertensive patients. A partial purification of such a compound was performed from urine of hypertensives. The partially purified compound inhibited to a greater extent the Na+,K+-ATPase semi-purified from dog kidney than that from sheep brain. The present data are consistent with the possible regulation of the activity or the secretion of plasma ouabain-like activity by extracellular volume.

Investigation of the endogenous Na+-pump inhibitor in essential hypertension and blood volume expansion.

The digitalis-like activities of plasma extracts from 108 patients and normal subjects were measured by their ability to compete with ouabain for binding to the digitalis sites of the Na+-pump. High levels were found in 18 of 54 untreated patients with moderate hypertension, 10 of 14 patients with end-stage renal failure and six patients with active acromegaly. These levels returned to control values after dialysis in the patients with renal insufficiency and high levels of the inhibitor, and after successful surgery and cobalt therapy in seven acromegalic patients. An increase in circulating Na+, K+-ATPase inhibitor was also found in rats after chronic sodium loading. These results indicate that levels of the circulating compound with digitalis-like properties do not result from high blood pressure but, rather, are related to blood volume and Na+ balance.