Further characterization and evidence for a precursor in the formation of plasma antinatriferic factor

Natriuretic hormone activity in the urine of cirrhotic patients

The ability of urine extracts to inhibit sodium and potassium-activated ATPase, cross-react with antidigoxin antibodies and induce natriuresis in rats was investigated in 10 healthy subjects, 10 cirrhotic patients without ascites (compensated cirrhotics), 27 nonazotemic cirrhotic patients with ascites and 10 cirrhotic patients with ascites and functional renal failure to assess whether reduced activity of natriuretic hormone contributes to sodium retention in cirrhosis. No significant differences were seen between healthy subjects and compensated cirrhotic patients in any of these parameters (sodium and potassium-activated ATPase inhibition = 178.5 +/- 19.8 vs. 247.4 +/- 48.7 nmol equivalent of ouabain/day; digoxinlike activity = 43.9 +/- 6.1 vs. 48.0 +/- 5.6 ng equivalent of digoxin/day; natriuretic activity = 0.36 +/- 0.15 vs. 0.63 +/- 0.27 mumol/min). Cirrhotic patients with ascites with and without functional renal failure showed significantly higher values of sodium and potassium-activated ATPase inhibition (708.1 +/- 94.0 and 529.2 +/- 53.9 nmol equivalent of ouabain/day, respectively), digoxinlike activity (136.9 +/- 7.2 and 116.3 +/- 7.9 ng equivalent of digoxin/day) and natriuretic activity (1.78 +/- 0.48 and 1.93 +/- 0.37 mumol/min) than healthy subjects and compensated cirrhotic patients. We saw no significant differences between these two groups of cirrhotic patients with ascites with respect to these parameters. In the cirrhotic patients studied, sodium and potassium-activated ATPase inhibition and antidigoxin antibodies directly correlated with the degree of impairment of hepatic and renal function, plasma renin activity and plasma levels of aldosterone and norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Digitalislike circulating factor in hypertension: potential messenger between salt balance and intracellular sodium

Sodium chloride has no clearly established local direct action on blood vessels to produce constriction; on the contrary, it has an immediate local indirect action via osmolality, which produces vasodilation. Thus in order to explain salt-induced hypertension, a delayed remote indirect vasoconstrictor action must be postulated. This indirect vasoconstrictor action is apparently the result of volume expansion. Acute volume expansion imparts three physiologic properties to the plasma; these are the ability to inhibit Na,K-ATPase and the Na-K pump, to cause natriuresis, and to sensitize blood vessels to vasoconstrictor agents. Similarly, low-renin, volume-expanded hypertension endows the plasma with the capacity to inhibit the Na,K-ATPase pump, to sensitize blood vessels to vasoconstrictor agents, and to raise blood pressure. These properties apparently result from a circulating digitalislike substance(s), perhaps derived from the hypothalamus and/or adrenals. We here review the considerable effort expended in identifying the agent or agents, and conclude that both steroidal and peptidic structure must be considered. Regardless of its structure, we hypothesize that when sodium excretion does not keep pace with sodium intake, its release leads to increased contractile activity of cardiac and vascular smooth muscle and hence hypertension. Inhibition of the Na-K pump increases the intracellular sodium concentration, particularly when superimposed on genetic- or aldosterone-induced increased sodium permeability, resulting in depolarization and increased calcium influx (vascular smooth muscle) or altered Na(+)-Ca2+ exchange and decreased calcium efflux (heart muscle). The increased intracellular calcium concentration then accounts for the increased contractile activity. Depolarization may also increase the sensitivity of vascular smooth muscle to vasoconstrictor agents such as norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertensive dog plasma inhibits the Na+-K+ pump of cultured vascular smooth muscle

We investigated the effects of plasma from dogs with perinephritic hypertension on the Na+-K+ pump of cultured dog vascular smooth muscle cells. We also measured [3H]ouabain binding by myocardium and vascular tissue. Fresh, unprocessed plasma from healthy dogs during the first 6 weeks of benign one-kidney, one wrapped hypertension and from paired normotensive control dogs was layered over confluent primary cultured puppy aortic smooth muscle cells that had been sodium-loaded with monensin. In 26 paired assays of plasma from four pairs of dogs, cells incubated in the presence of plasma from hypertensive dogs had significantly reduced total (p less than 0.01) and ouabain-sensitive (p less than 0.001) 86Rb+ uptakes, but their intracellular sodium content did not differ from cells incubated in paired normotensive plasma. We no longer detected these uptake differences when passaged cells or cells cocultured with bovine endothelial cells were used for assay or when plasma was treated with protease inhibitors or boiled. However, boiled plasma increased the sodium content of the assay cells, suggesting an ionophorelike effect. Levels of pump inhibitory activity in plasma appeared to remain constant during Weeks 1 to 6 of hypertension. We found no evidence for altered numbers of pump sites in cardiovascular tissues from these hypertensive dogs. These findings support the hypothesis that plasma factors inhibit the membrane Na+-K+ pump in vascular smooth muscle cells in this form of hypertension. These plasma inhibitory factors apparently do not induce pump molecules.

Humoral sodium transport inhibitor in acute volume expansion and low renin hypertension

This review summarizes our bioassay methods for determining the level of humoral sodium pump inhibiting factor after acute volume expansion in experimental animals and humans, and in low renin experimental and human essential hypertension. In brief, ouabain-sensitive 86Rb uptake and membrane potential in blood vessels from normal animals are measured after incubation in plasma supernate from experimental subjects and animals and their respective controls. The data show that humoral sodium pump inhibitor is elevated after acute volume expansion in normal animals (dogs and rats) and in normal humans. The level of inhibitor is also elevated in patients with low renin essential hypertension and in experimental animals with low renin, volume-dependent types of hypertension, namely, one-kidney, one wrapped hypertension in dogs, and one-kidney, one clip and reduced renal mass-saline hypertension in rats. Humoral sodium pump inhibiting factor inhibits the Na+-K+ pump in the cardiovascular system. Such inhibition by other means (hypokalemia, cardiac glycosides) activates the system. Therefore, we also discuss the possible role of humoral sodium pump inhibitor in low renin volume-dependent hypertension.

Evidence for the existence of an endogenous inhibitor of Na, K-ATPase in plasma from cats and rats

Plasma samples from cats and rats were filtered through molecular filters to obtain a concentrate of the plasma molecules with a mass between 500 and 10,000. This concentrate was placed on a gel filtration column and eluted with a Tris buffer. Fractions corresponding to a molecular mass of 1500-2000 exhibited a marked Na,K-ATPase inhibitory 'activity', which was not influenced by heating to 95 degrees C for 10 min or by enzymatic degradation by pronase or trypsin.

Endogenous digitalis-like factors

Recent research has demonstrated the presence of endogenous compounds in blood and urine that crossreact with antibodies raised against digoxin. Given the widespread therapeutic use of digoxin and its being monitored clinically by immunoassay, such digoxin-like immunoreactive compounds pose significant diagnostic and interpretive problems. Serum levels of this factor(s) approaching therapeutic digoxin levels have been found in digoxin-free patients in renal failure, pregnant women, and newborns. The compound is incompletely characterized; however, existing data suggest that it is a small, neutral, nonpeptidic compound. In serum it is highly protein bound, and alterations in this binding appear to give rise to the false-positive assay results. The urinary form is probably conjugated. Digoxin-like immunoreactive substances may play a role in volume homeostasis and appear associated with essential and pregnancy-induced hypertension. If such roles are primary, measurement of digoxin-like immunoreactive substances may prove to be of value in and of itself.

Saralasin blocks the effect of angiotensin II and extracellular fluid saline expansion on the Na-K-ATPase inhibitor release in rats

A low molecular weight substance which behaves like ouabain as inhibitor of brain membrane Na-K-ATPase and 3H-ouabain binding was found in plasma after saline expansion of extracellular fluid or angiotensin II infusion into the third brain ventricle in the rat. Intracerebroventricular infusion of angiotensin II antagonist, saralasin, blocks the increase of the Na-K-ATPase inhibitor produced by infusion of angiotensin II into the third ventricle or extracellular fluid saline expansion.

The kidney in the pathogenesis of hypertension: the role of sodium

In this review, we first summarize the evidence which indicates that the inability of the kidney to excrete salt and water normally, particularly when combined with increased salt intake, is frequently associated with hypertension. We then concentrate on the link between sodium and water retention and hypertension. The increase in blood pressure probably results from the increase in volume rather than from the increase in salt. Recent evidence suggests that an increase in volume in the lesser circulation stimulates the release of a sodium pump inhibitor, probably the putative natriuretic hormone, from the hypothalamus. This agent appears to affect cardiac and vascular smooth muscle by suppressing Na+,K+-ATPase, and hence Na+-K+ pump activity in both muscle cells and adrenergic nerve terminals. The sodium pump inhibitor is a heat stable small molecule but its chemical structure is still unknown. It is clearly different from atrial natriuretic factor. We conclude the review with speculations on the possible role of renotropin and various growth and growth inhibitory factors in the vascular structural changes.

Observations on the "cascade" of Na-K-ATPase inhibitory and digoxin-like immunoreactive material in human urine: possible relevance to essential hypertension

Previous investigations have demonstrated an increased amount of a sodium pump inhibitor (N.H.) in plasma from humans with essential hypertension and from animals with various forms of experimental hypertension. The present study has employed Sephadex column and C18 reverse phase separation of urines from patients with essential hypertension and normal controls to distinguish "high", "intermediate" and "low" molecular weight forms of N.H., measured through properties of Na-K-ATPase inhibition and digoxin-like immunoreactivity. The major difference between hypertensive and normotensive urines was a highly significant increase in the "intermediate" molecular weight form of N.H., as measured by Na-K-ATPase inhibition. In contrast, digoxin-like immunoreactivity was significantly decreased in urine from hypertensive patients. The results are compatible with an hypothesis that the defect in some forms of essential hypertension may be partial inhibition of enzymatic conversion of intermediate to final form of N.H., with the increased sodium pump inhibition primarily related to the precursor.

Measurement of endogenous Na+,K+-ATPase inhibitors in human plasma and urine using high-performance liquid chromatography

This study was undertaken to assess endogenous Na+,K+-ATPase inhibitors in both plasma and urine in the same subjects. Samples were chromatographed on reverse-phase HPLC using an acetonitrile gradient and the eluent screened using Na+,K+-ATPase inhibition and cross-reaction with anti-digoxin antibodies. The donors were divided into inhibiting and non-inhibiting subjects using a previously described method, plasma action on ouabain binding and on Na+,K+-ATPase activity. Three Na+,K+-ATPase inhibitors (1P, 2P and 3P) were detectable in plasma; the antibodies cross-reaction of the peaks 2P and 3P were larger than that of peak 1P. The peaks 2P and 3P were significantly higher in inhibiting subjects as compared to non-inhibiting subjects. The 24-h urine is resolved into two peaks inhibiting Na+,K+-ATPase activity (1U and 2U). Peak 2U cross-reacted with anti-digoxin antibodies to a greater extent than peak 1U and is significantly larger in inhibiting subjects in terms of Na+,K+-ATPase inhibition. These data support the heterogeneity of human Na+,K+-ATPase inhibitor in both plasma and urine.

Natriuretic and hypertensive activities reside in a fragment of ACTH

The hypertensive and natriuretic effects of chronic administration of adrenocorticotrophic hormone (ACTH) cannot be duplicated by the administration of glucocorticoids and/or mineralocorticoids. We investigated the effects of a fragment of this hormone (ACTH4-10) and an analog of the fragment (D-Phe7) ACTH4-10 and found them to have pressor and cardioaccelerator actions in rats as determined by bolus intravenous (i.v.) injections of 30 to 1000 nmol/kg. The pressor and cardioaccelerator effects of (D-Phe7) ACTH4-10 were attenuated by alpha-receptor (phentolamine) and beta-receptor (metoprolol) antagonists. The cardiovascular actions of ACTH4-10 were produced in adrenalectomized or ganglionic-blocked (with mecamylamine) rats. At a lower dose (7 nmol/kg i.v.), ACTH4-10 was natriuretic and had a pattern of activity similar to that of a larger ACTH fragment, alpha-melanocyte-stimulating hormone. Extraadrenal effects of the intact ACTH molecule or the in vivo production of an ACTH4-10-like fragment from ACTH may contribute to the hypertensive and natriuretic actions associated with this hormone.

Partial purification and properties of the inhibitors of Na, K-ATPase and ouabain-binding in bovine central nervous system

Endogenous inhibitors of Na,K-ATPase and ouabain-binding were partially purified from bovine central nervous system, and some of their properties were studied. They were eluted as low-molecular-weight fractions by gel filtration. They could be adsorbed by both Amberlite IR 120 and Amberlite IRA 400 at acidic and basic pH, respectively, indicating that they could act as both anions and cations at different pH. These inhibitors of ouabain-binding appeared to affect specific binding of ouabin, and Scatchard plot analysis showed that the inhibition was competitive, suggesting that they could bind to the same site as ouabain, presumably to Na,K-ATPase itself. The inhibitory activities were heat stable, but charring inactivated them completely.

Proteolytic activation of a bioactive cardiac peptide by in vitro trypsin cleavage

Mammalian cardiac atria possess several unidentified biologically active peptides. Fractionation of rat atrial extracts by gel filtration chromatography revealed two major fractions [apparent molecular weights of 20,000-30,000 (peak I) and less than 10,000 (peak II)], both of which were potent natriuretic agents (eliciting a 25-fold increase in sodium excretion) and smooth muscle relaxants. Vigorous treatment with trypsin (100 units/ml at 37 degrees C for 15 min) of both fractions abolished all biological activity. Further purification of the lower molecular weight fraction (peak II) by ion-exchange chromatography indicated two subfractions that possessed potent natriuretic activity and that preferentially relaxed either intestinal (designated peak IIA) or vascular (peak IIB) smooth muscle assay tissues. The similarity of the biological effect of the high (peak I) and low (peak II) molecular weight peptides led us to test the possibility of precursor-product relationship. Mild proteolytic treatment of the high molecular weight peptide with trypsin (1 unit/ml at room temperature) markedly enhanced the smooth muscle relaxant activity. Subsequent analysis of the trypsin (1 unit/ml)-treated high molecular weight peptide (peak I) by gel filtration and ion-exchange chromatography revealed that the peptide now resembled the low molecular weight peptides (peaks IIA and IIB) present in the original atrial extract. These data suggest that the cardiac atria contain a relatively inactive (smooth muscle relaxant) high molecular weight peptide and suggest that biologically active low molecular weight peptides can subsequently be generated by proteolytic cleavage.

Red blood cell sodium in the DOCA hypertensive pig

The influence of deoxycorticosterone acetate (DOCA) on the sodium content of the red blood cell was determined in the pig. DOCA (100 mg/kg), impregnated in Silastic, was implanted subcutaneously (s.c.) in six male pigs; seven additional pigs received Silastic implants without the DOCA. Those receiving DOCA had an increase in mean arterial pressure (MAP) that was significant in 48 hours and reached a plateau that was 24 mm Hg greater than that of the controls after 15 days. These animals also developed hypokalemia and polydipsia over approximately the same time course. Red blood cell sodium content increased in DOCA-treated pigs 24 hours after implant (5.57 +/- 0.17 vs 5.23 +/- 0.05, mEq/liter cells). The sodium content continued to rise, reaching a plateau 28% above that of control value by the 5th postimplant day (6.37 +/- 0.40 mEq/liter cells). In vitro tests of possible mechanisms that might have caused the in vivo increase in red blood cell sodium content gave the following results: 1) Incubations of red blood cells in a physiological salt solution (PSS) containing deoxycorticosterone failed to cause an increase in cell sodium content. 2) No ouabain-like factor was demonstrated in plasma from the DOCA hypertensive pigs. 3) An elevation in bicarbonate concentration in the PSS caused an increase in red blood cell sodium content. 4) A decrease in potassium concentration in the PSS also caused an increase in red blood cell sodium content.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormalities of membrane transport in hypertension

In this review, postulated passive and active fluxes of sodium, potassium, and calcium across the sarcolemma of the normal vascular smooth muscle cell are first summarized. Some practical problems encountered in their measurement are also mentioned. The review then considers how these fluxes appear to be altered in various forms of hypertension in animals and humans. Emphasis is given to abnormal fluxes of sodium and potassium due to altered sodium pump activity and permeability. Increasing evidence indicates that sodium retention due to increased sodium intake or decreased sodium excretion causes hypertension by releasing a humoral pressor substance from brain. This substance, which may be the putative natriuretic hormone, inhibits Na+, K+-ATPase and sodium pump activities in blood vessels and heart, thereby increasing contractile activity. In the genetic models of hypertension, the primary defect appears to be increased permeability of the vascular smooth muscle cell wall to sodium; pump activity increases to compensate for the increased inward leak of sodium. This may also be the case in patients with heritable essential hypertension. The possible consequences of super-imposing the sodium pump inhibitor on the primary defect are also considered. This may occur when animals with genetic hypertension or patients with heritable essential hypertension retain sodium subsequent to increased sodium intake and/or decreased ability to excrete sodium. Such superimposition should raise intracellular sodium concentration to high levels since now the pump would not fully compensate for the increased inward leak of sodium.

Evidence for a vascular sensitizing factor in plasma of saline-loaded dogs

This study investigates whether plasma extracts previously demonstrated to have natriuretic and antinatriferic activity have effects on vascular reactivity of rat cremaster arterioles. Plasma from hydropenic and saline-loaded dogs was subjected to Diafiltration, and eluted on a strong cation exchange column (SCX). The effects of intraarterial injections of various column fractions on constrictor responses to repeated injections of 33.3 ng of norepinephrine (NE) were used to indicate changes in vascular responsiveness in third order cremaster arterioles. SCX fraction I (void volume) from saline-loaded dogs (FI-S) caused an increase in constrictor response to NE of 101%. Increased vascular responsiveness peaked at 40 minutes and remained significantly elevated (p less than 0.05) for 130 minutes. Fraction I from plasma of hydropenic dogs (FI-H) and fraction III from plasma of saline-loaded dogs (FIII-S) did not increase vascular responsiveness to NE. FI-S shifted the dose response curves for NE, arginine vasopressin, and angiotensin II parallel and to the left relative to control by a factor of 3.05-, 2.95-, and 5.63-fold, respectively, at the 50% constrictor dose. Systemic injections of FI-S, but not FI-H, caused a 10 mm Hg rise in blood pressure at 50 minutes, and blood pressure was significantly elevated for 30 to 90 minutes after injection (p less than 0.01). These data demonstrate a vascular-sensitizing factor in FI-S. The factor appears in the same chromatographic fraction previously demonstrated to contain natriuretic, antinatriferic, and digoxin-like activity. The correlation of these activities with salt loading suggests they are due to the same substance, which may be the putative natriuretic hormone.

Increased circulating levels of an endogenous digoxin-like factor in hypertensive monkeys

An endogenous, immunoreactive digoxin-like factor (endoxin) was measured in the plasma of nonhuman primates with hypertension. Both normotensive and hypertensive rhesus monkeys had levels of endoxin that significantly correlated with their systolic or diastolic blood pressure. Vervet monkeys with experimentally produced chronic Goldblatt hypertension had significantly elevated endoxin, but not plasma renin. These data suggest that increased plasma endoxin may be a contributing factor in the development of hypertension.

Progress in isolation and purification of an inhibitor of sodium transport obtained from dog urine

Among the potential modulators of transtubular sodium transport is the putative natriuretic hormone. Widespread efforts are underway to isolate this substance in pure form. The present studies describe a series of experiments directed to this goal. Urine samples in the amount of 150 liters were taken from normal, mineralocorticoid hormone "escape" dogs and were chromatographed through Sephadex G-25. The active fraction of the eluate (that is, the fraction containing the inhibitor of sodium transport) was then subjected to high pressure liquid chromatography (HPLC) in four consecutive steps using three different resins. Approximately 5% of the each column eluate was diverted by use of a stream-splitting apparatus, and the fluorescent pattern was measured and recorded, in most instances following the addition of fluorescamine. Based on the respective fluorescent patterns obtained from the eluates of the successive chromatographic steps, the residual portion (95%) of each eluate was divided into fractions, and each fraction was bioassayed. In each instance only the biologically active fraction was subjected to further purification. In the first step involving HPLC and a cation exchange resin, six fractions were obtained. Only one was active. When this fraction was subjected to reverse-phase chromatography, seven new fractions emerged. Again only one was active. When it was chromatographed using a second cation exchange resin, two fluorometrically detectable peaks, termed N ad H, were identifiable. N exhibited spontaneous fluorescence; H exhibited fluorescamine-dependent fluorescence. In the final step, N and H were separated and bio-assayed individually. N was inactive; H proved to be a potent inhibitor of sodium transport. Accumulation of H in sufficient quantity will determine whether it is a single compound and will permit analysis of its chemical nature.