Comparison of the effects of ouabain and brain natriuretic peptide in saline-loaded sheep.

1. It has been claimed that ouabain is an endogenous hormone that may be pivotal in the pathogenesis of some forms of hypertension and may exaggerate natriuresis in situations characterized by volume overload. We compared the haemodynamic, renal and endocrine effects of ouabain (at approximately 187 ng/kg per min for 2 h) with those of brain natriuretic peptide (BNP; at 5 pmol/kg per min for 2 h) in nine saline-loaded sheep in a balanced, randomized, single-blind, placebo-controlled crossover study. 2. Brain natriuretic peptide infusion reduced mean arterial pressure whereas ouabain infusion caused no change. Haematocrit rose steadily during BNP infusion but fell during ouabain infusion. Neither ouabain nor BNP affected urine volume, sodium, potassium or creatinine excretion. Mean heart rate declined during the ouabain and placebo infusions, but was not altered during BNP infusion. Endogenous ouabain concentrations were not detectable at baseline or during BNP or placebo infusions, but rose to concentrations of 11 +/- 1.3 nmol/L during the ouabain infusion. 3. These results suggest that ouabain is not an endogenous hormone present at physiologically relevant concentrations. Furthermore, ouabain does not cause natriuresis during saline-loading in sheep and is therefore unlikely to be responsible for the exaggerated natriuresis seen in some forms of hypertension.

Differing metabolism and bioactivity of atrial and brain natriuretic peptides in essential hypertension.

Plasma concentrations of both atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are elevated in severe hypertension, acute myocardial infarction, and heart failure. In the current study of individuals with essential hypertension, we have documented the hemodynamic, hormonal, and endocrine effects of infusions of these two peptides given alone or in combination in equimolar doses calculated to induce increments in plasma peptides to concentrations (30 to 60 pmol/L) observed in these disease states. The metabolic clearance rate of ANP (4.56 +/- 0.62 L/min) was greater than that for BNP (3.4 +/- 0.23 L/min, P <.001). Infusions of each cardiac hormone impaired the clearance of coinfused peptide. All peptide infusions enhanced natriuresis (17% to 70% above preinfusion levels versus placebo, 6%; P <.001), lowered blood pressure (10 to 18 mm Hg fall in mean arterial pressure below placebo levels; P <.001), increased hematocrit, suppressed the renin-angiotensin-aldosterone system, and enhanced plasma norepinephrine concentrations. The natriuretic and blood pressure-lowering effects of BNP were twofold to threefold those of ANP. In contrast, ANP-induced increments in plasma and urinary second messenger (cGMP) levels were greater than those for BNP. Both peptides suppressed the renin-angiotensin-aldosterone system (approximately one-third fall in renin activity and plasma aldosterone) and enhanced plasma norepinephrine concentrations (+30%) to a similar degree. Increments in plasma ANP and BNP that occur simultaneously in cardiovascular disease states appear capable of causing hemodynamic, endocrine, and renal effects that would tend to ameliorate conditions such as hypertension or heart failure.

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 ouabain on pressor responsiveness in normal man.

To assess the effects of ouabain on pressor and vasoactive hormone responsiveness, 10 healthy volunteers were pretreated with ouabain (0.5 mg i.v. 42 and 18 h before study) or placebo before pressor challenge with angiotensin II (ANG II; 2, 4, and 8 ng.kg-1.min-1 for 30 min/dose) and norepinephrine (NE; 5, 15, and 45 ng.kg-1.min-1 for 15 min/dose). There were no differences at baseline between the two study days regarding mean arterial pressure (MAP) or heart rate. Baseline pulse pressure, however, was significantly greater after ouabain (47 +/- 3 vs. 41 +/- 1 mmHg; P < 0.05). The mean maximum increments in MAP during ANG II and NE infusions were 17.5 +/- 1.1 and 10.5 +/- 1.3 (SE) mmHg, respectively, after ouabain and 19.2 +/- 1.3 and 10.4 +/- 1.5 mmHg after placebo (not significant). The mean heart rate was lower during both infusion periods on the ouabain study day compared with control (P < 0.05). Baseline plasma levels of ANG II, aldosterone, plasma renin activity, atrial and brain natriuretic peptide, guanosine 3',5'-cyclic monophosphate, NE, and epinephrine and achieved levels during the two infusions were similar on the two study days. We conclude that short-term ouabain administration does not alter pressor responsiveness or plasma levels of vasoactive hormones in healthy volunteers.

Ouabain is not detectable in human plasma.

An enzyme-linked immunosorbent assay is described for the measurement of ouabain in human plasma. This assay is specific for ouabain, strophanthidin, and ouabagenin, with other steroids, including digoxin and vasopressor hormones, exhibiting negligible cross-reactivity. Assay sensitivity was 0.06 nmol/L if 1 mL plasma was extracted and less than 0.005 nmol/L when 20 mL plasma was analyzed. Extracted plasma samples showed ouabainlike immunoreactivity that diluted in parallel with the ouabain standard curve. Repeated extraction and assay of single plasma samples, however, did not produce consistent results in the assay. Increased specificity was obtained by high-performance liquid chromatography of sample extracts before assay. When high-performance liquid chromatographic profiles of plasma spiked with ouabain standard or following bolus intravenous injections of ouabain into normal human volunteers were compared with profiles of unspiked plasma, there was no support for the immunoreactive material in the latter samples being ouabain. We propose that if ouabain is present in the human circulation, its concentration is less than 0.005 nmol/L.

Acute effects of intravenous ouabain in healthy volunteers.

1. The influence of single-dose intravenous ouabain on haemodynamics, renal function and vasoactive hormones was investigated in healthy subjects. 2. After ouabain injection there was a significant reduction in minimum and average hourly heart rates. Diastolic blood pressure fell immediately after ouabain, without a significant change in systolic blood pressure. 3. There was no change in renal blood flow, glomerular filtration rate, hourly urine volume or sodium and potassium excretion. 4. Ouabain injection was associated with a fall in plasma angiotensin II levels and a rise in plasma adrenaline levels, but plasma noradrenaline levels were unchanged. Both plasma atrial and brain natriuretic peptide levels were increased after ouabain administration. 5. The present study demonstrates that ouabain has significant although minor, acute effects on plasma levels of vasoactive hormones. It fails, however, to confirm that ouabain is an acute pressor or natriuretic substance. If it has an important influence on electrolyte and haemodynamic homeostasis, its actions are likely to be slowly developed.

Renal artery stenosis as a cause of heavy albuminuria.

We report three cases of heavy albuminuria occurring in hypertensive patients with unilateral or bilateral renal artery stenosis. In two of the patients the albuminuria and hypertension were corrected by removing the ischaemic kidney. The mechanism of the albuminuria, particularly in atheromatous renal artery stenosis, is probably multifactorial.