Production of ouabain-like factors of hypothalamo-pituitary origin, determined by a sensitive ELISA for ouabain, is increased in DOCA-salt hypertensive rats

The central mechanism underlying hypertension: a review of the roles of sodium ions, epithelial sodium channels, the renin-angiotensin-aldosterone system, oxidative stress and endogenous digitalis in the brain

The central nervous system has a key role in regulating the circulatory system by modulating the sympathetic and parasympathetic nervous systems, pituitary hormone release, and the baroreceptor reflex. Digoxin- and ouabain-like immunoreactive materials were found >20 years ago in the hypothalamic nuclei. These factors appeared to localize to the paraventricular and supraoptic nuclei and the nerve fibers at the circumventricular organs and supposed to affect electrolyte balance and blood pressure. The turnover rate of these materials increases with increasing sodium intake. As intracerebroventricular injection of ouabain increases blood pressure via sympathetic activation, an endogenous digitalis-like factor (EDLF) was thought to regulate cardiovascular system-related functions in the brain, particularly after sodium loading. Experiments conducted mainly in rats revealed that the mechanism of action of ouabain in the brain involves sodium ions, epithelial sodium channels (ENaCs) and the renin-angiotensin-aldosterone system (RAAS), all of which are affected by sodium loading. Rats fed a high-sodium diet develop elevated sodium levels in their cerebrospinal fluid, which activates ENaCs. Activated ENaCs and/or increased intracellular sodium in neurons activate the RAAS; this releases EDLF in the brain, activating the sympathetic nervous system. The RAAS promotes oxidative stress in the brain, further activating the RAAS and augmenting sympathetic outflow. Angiotensin II and aldosterone of peripheral origin act in the brain to activate this cascade, increasing sympathetic outflow and leading to hypertension. Thus, the brain Na(+)-ENaC-RAAS-EDLF axis activates sympathetic outflow and has a crucial role in essential and secondary hypertension. This report provides an overview of the central mechanism underlying hypertension and discusses the use of antihypertensive agents.

An ouabain-like factor is secreted from immortalized hypothalamic cells in an aldosterone-dependent manner

Ouabain-like factor (OLF) modulates blood pressure via sodium pump inhibition in the central nervous system and in the peripheral circulation. Ouabain-like factor (OLF) is thought to be produced in the adrenal gland and hypothalamus, and it may relate locally to the renin-angiotensin-aldosterone system. However, the evidence for the latter was obtained from in vivo experiments using animals. In the present study, we investigated ouabain production in the immortalized hypothalamic cell line N1. First, cell culture supernatant was collected from the immortalized hypothalamic cell line N1 at 0.5, 4, 8, and 24 h. A newly developed enzyme-linked immunosorbent assay (ELISA) that used anti-ouabain antibody showed that immunoreactivity in the supernatant was increased significantly at 24 vs. 0.5 h (0.01±0.004 vs. 0.16±0.033 pmol/mg protein, p<0.01). A combination of HPLC and ELISA was used to characterize N1 cell-derived OLI, showing that the highest peak of OLI had the same retention time as authentic ouabain. Thereafter, N1 cells were cultured with (1-10 μM) aldosterone, and supernatant was collected after 24 h of culture. In addition, N1 cells were cultured with 5 μM eplerenone, a mineralocorticoid receptor blocker, plus aldosterone. OLI was significantly increased in the supernatant of the cells cultured with 10 μM aldosterone (0.40±0.078 pmol/mg protein), and this increase was abolished by the addition of the aldosterone antagonist eplerenone (0.12±0.030 pmol/mg protein). These data suggest that the immortalized hypothalamic N1 cells secrete OLF and that aldosterone stimulates its secretion via mineralocorticoid receptors.

Novel Digitalis-Like Factor, Marinobufotoxin, Isolated From Cultured Y-1 Cells, and Its Hypertensive Effect in Rats

Marinobufagenin and telecinobufagin have been identified as digitalis-like factors in mammals. In toads, marinobufagenin-related compounds, such as marinobufotoxin (MBT), have been isolated in some tissues but not in mammals, and its biological action has not been elucidated. Herein, we aimed to explore the possible production and/or secretion of MBT and the biological action in rats. First, the MBT in culture supernatant of the adrenocortical-originated cell line Y-1 was analyzed by high-performance liquid chromatography and sensitive ELISA for marinobufagenin-like immunoreactivity. Moreover, the structural information was obtained by mass spectrometry. To determine the biological action, MBT (9.6 and 0.96 μg/kg per day) was intraperitoneally infused via an osmotic minipump for 1 week. Blood pressure and renal excretion of marinobufagenin-like immunoreactivity were measured. Marinobufagenin-like immunoreactivity was found in Y-1 cell culture media, and the concentration increased until 24 hours. The structural analysis suggested that marinobufagenin-like immunoreactivities were marinobufagenin and MBT, and tandem mass spectrum analysis revealed them with the specific daughter ions. The highest sensitive ELISA-positive peak of marinobufagenin-like immunoreactivity in the media was MBT. Continuous administration of MBT in rats for 1 week significantly increased systolic blood pressure and renal excretion of marinobufagenin-like immunoreactivity compared with control rats (135±3.0 versus 126±2.0 mm Hg and 1.41±0.286 versus 0.34±0.064 ng/day, respectively). These data suggest that MBT, arginine-suberoyl ester of marinobufagenin, can be a novel digitalis-like factor with hypertensive action and is secreted from the adrenocortical cells.

Endogenous ouabain secretion in man is not regulated by ACTH

It has been suggested that endogenous ouabain-like substance (OLS) is of adrenal origin and the secretion of OLS may be ACTH dependent. To determine if OLS is influenced by the pituitary-adrenal axis, we studied the effect of adrenal stimulation (0.25 mg Synacthen) and suppression (1 mg Dexamethasone) on two separate groups of nine subjects. Serum OLS was measured by a radioimmunoassay (RIA) developed in our lab, and cortisol and ACTH were measured by commercial assay kits. Dexamethasone significantly (P< 0.001) suppressed serum cortisol and ACTH concentrations, without effecting endogenous OLS concentration (0.64+/-0.17 vs 0.85+/-0.18nmol/l). Synacthen increased the concentration of cortisol in serum (p < 0.001) over the test period; OLS concentration, again, remained unchanged (0.45+/-0.04 vs 0.43+/-0.05 nmol/l). In further studies, serum concentrations of cortisol and OLS were compared between left (LAV) and right (RAV) adrenal veins with that from the inferior vena cava (IVC). Concentration of cortisol in the LAV and RAV was five-fold greater than that in IVC. However, there was no difference in OLS concentration at the corresponding sites. In addition, serum OLS concentrations in patients having undergone bilateral adrenalectomy or diagnosed with Addison's disease (0.62+/-0.19 nmol/l) were similar to concentrations in healthy subjects (0.67+/-0.21 nmol/l). Examination of bovine adrenal, liver, kidney, heart and human placenta demonstrated that OLS content of bovine adrenal was comparable with other tissues analysed. HPLC studies of human serum and bovine adrenal gland produced identical elution profiles, resolving a single peak which coincided with the retention time observed for standard ouabain. We conclude that the adrenal is unlikely to be the source of endogenous OLS, the secretion of which appears to be independent of ACTH.

Effect of high salt intake on plasma and tissue concentration of endogenous ouabain-like substance in the rat

The effect of high salt intake on serum concentration and tissue distribution of ouabain-like substance (OLS) was examined in rats. Sprague-Dawley rats (n=8) were placed on a high salt diet by the inclusion of 1.8% sodium chloride in drinking water for 7 days and a 'control' group (n=8) was maintained on normal drinking water during the study period. Serum and tissue OLS was measured by radioimmunoassay after solid phase extraction. High salt intake significantly increased serum OLS concentration (1.43 +/- 0.06 vs 1.14 +/- 0.05 nmol/L; mean +/- SEM, P=0.002). In both groups, the adrenal showed significantly (p < 0.001) higher OLS content compared to liver, kidney, heart and brain. HPLC of rat serum extract resolved a major peak with a retention time identical to that of standard ouabain, further confirming the nature of OLS. We conclude that high salt intake increases endogenous production of OLS, which appears to originate from the adrenal gland in the rat.

Vasodepressor effects of exercise are accompanied by reduced circulating ouabainlike immunoreactivity and normalization of nitric oxide synthesis

Our object was to evaluate the effects of regular mild exercise on blood pressure and on circulating level of ouabainlike factors (OLF) and of nitrate anion, an endproduct of nitric oxide (NO) in humans. We measured plasma ouabainlike immunoreactivity (OLI) and nitrate ions (NO3.) before and after mild exercise for 3 months' duration in 16 patients with essential hypertension, diabetes mellitus, obesity, or hyperlipidemia. Plasma OLI was measured using an amplified ELISA system with anti-ouabain antibody and biotinyl-tyramide. Serum NO3. was measured with high-performance liquid chromatography (HPLC) with an anion-exchange column. With the reverse phase HPLC system with an octa decylsilyl silicagel column, the elution volume of plasma OLI of a healthy volunteer matched that of authentic ouabain in a gradient elution system of acetonitrile/H2O. Plasma OLI levels decreased significantly by about 34% after mild exercise, and NO3. levels tended to be within the reference interval in normal volunteers. Body weight, diastolic and systolic blood pressure, serum triglyceride and acetylcholine esterase (a marker of the fatty liver) were significantly decreased (p < 0.01) after 3 months of regular mild exercise. The plasma OLI level was significantly correlated with plasma NO3., there was a trend toward a correlation with diastolic blood pressure (p = 0.06) before and after regular exercise. Regular mild exercise led to a decrease in plasma levels of OLI, and acetylcholine esterase activity and blood pressure in adult patients. Results suggest that changes in OLF production contribute to the blood pressure regulation seen in patients who exercise regularly.