Further biochemical characterization of an Na+ pump inhibitor purified from human urine

No endogenous ouabain is detectable in human plasma by ultra-sensitive UPLC-MS/MS

BACKGROUND

The presence of a binding site for cardiac glycosides, such as digitoxin and digoxin, in the sodium-potassium-ATPase, stimulated attempts to isolate endogenous cardiotonic steroids. Using immunoassays, clinical studies found the cardenolide ouabain to be secreted endogenously in response to exercise and untreated hypertension and to be correlated with severity of clinical conditions such as kidney failure and dilated cardiomyopathy. The assays used were not standardized and the mean concentrations of endogenous ouabain reported for healthy controls ranged from 60 to 530 pmol/l. None of these immunoassays is available any more. Therefore, the aim of this study was to develop a highly specific and reliable method for measurement of ouabain in human plasma based on isotope dilution liquid chromatography tandem-mass spectrometry (ID-LC-MS/MS).

METHOD

An ultra-sensitive and specific ultra performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) method was developed which applied solid phase extraction of plasma for sample preparation.

RESULTS

The method was comprehensively validated and had a lower limit of quantification of 1.7 pmol/l. However, despite this very low detection limit ouabain was not observed in plasma samples from patients with and without heart failure.

CONCLUSION

Our results suggest that immunoassays previously used to quantify assumed endogenous ouabain detected compounds which are not structurally identical with ouabain. Cross reactivity of structurally related compounds of endogenous origin may cause these discrepancies between immunological and mass spectrometric analyses. Conclusive characterization of assumed endogenous counterparts of digoxin in a biomarker discovery approach seems to require distinct analytical techniques.

A new endogenous natriuretic factor: LLU-alpha

For over three decades, renal physiology has sought a putative natriuretic hormone (third factor) that might control the body's pool of extracellular fluid, an important determinant in hypertension, congestive heart failure, and cirrhosis. In our search for this hormone, we have isolated several pure natriuretic factors from human uremic urine that would appear, alone or in combination, to mark a cluster of phenomena previously presumed to be that of a single "natriuretic hormone." This paper reports the purification, chemical structure, and total synthesis of the first of these compounds, LLU-alpha, which proved to be 2,7,8-trimethyl-2-(beta-carboxyethyl)-6-hydroxychroman, presumably a metabolite of gamma-tocopherol. Both natural LLU-alpha and synthetic material are identical (except for optical activity) with respect to structure and biological activity. It appears that the natriuretic activity of LLU-alpha is mediated by inhibition of the 70 pS K+ channel in the apical membrane of the thick ascending limb of the kidney.

Alteration of erythrocyte membrane Na, K-ATPase in children with borderline or essential hypertension

The aim of this study was to evaluate the substrate (ATP) kinetics of erythrocyte membrane Na, K-ATPase in children with borderline or essential hypertension. Although the activity of Na, K-ATPase in the presence of in vivo concentrations of ATP was not significantly altered, kinetic studies showed an obvious inhibition of enzyme activity in the erythrocyte membrane of children with borderline or essential hypertension. Hanes plot analysis revealed a decrease of V(max) from 7.19 in erythrocytes from control subjects to 4.93 and 3.33 in those from children with borderline or essential hypertension, respectively. A mean value of the K(m) decreased from 0.10 in the control to 0.08 and 0.02 in children with borderline or essential hypertension, respectively. The energy status of erythrocytes, estimated by ATP, ADP and AMP levels, ATP/ADP ratio, and adenylate energy charge (AEC) was not significantly changed in the cells from hypertensive children. The use of a free radical-generating system (FeSO4/ascorbate) in vitro significantly reduced enzyme activity in the control erythrocytes while in those from hypertensive children it was abolished completely. The level of lipid peroxides was considerably higher (+ 37 per cent) in the plasma, while that of reduced glutathione was significantly lower both in the erythrocytes and the plasma of children with essential hypertension than in healthy children. These results indicate significant alterations of the antioxidant status which could be the cause of the inhibited Na, K-ATPase activity in erythrocyte membranes from hypertensive children.

Endogenous natriuretic factors 3: isolation and characterization of human natriuretic factors LLU-alpha, LLU-beta 1, and LLU-gamma

A low molecular weight endogenous substance believed to be responsible for extracellular fluid homeostasis in mammals has been sought for many years. Our goal is to isolate and structurally characterize this putative "natriuretic hormone." We have developed an assay using the conscious rat to measure prolonged natriuresis (Benaksas et al (1993) Life Sciences, 52, 1045-1054), the activity originally described for this putative substance. Using this assay we have identified a number of natriuretic compounds isolated from human uremic urine. The collected urine is processed by ultrafiltration (< or = 3 kDa), gel filtration chromatography (G-25) and extraction with isopropanol and diethyl ether. The organic soluble material is then subjected to sequential high-performance liquid chromatography. We report here the initial characterization of two pure isolates (LLU-alpha and LLU-gamma) obtained by this method, and the structural elucidation of a third pure compound, LLU-beta 1, a natriuretic and previously unreported metabolite of the drug diltiazem.

Isolation of an endogenous Na-pump specific inhibitor from normal pig urine: characterization and comparison with the inhibitor purified from bovine adrenal glands

An endogenous Na-pump specific inhibitor has been purified to homogeneity from normal pig urine using Amberlite XAD-2 adsorption chromatography followed by five steps of reverse phase HPLC. Although most of the dose response curves for this purified Na-pump inhibitor, designated uroxin, in the various assay systems paralleled those of authentic ouabain and the specific Na-pump inhibitor previously purified from bovine adrenal glands (designated adrexin C), the cross-reactivity curve with anti-ouabain antibodies did not. The retention times of uroxin on various types of reverse phase HPLC columns were also different from those of plant-derived cardiotonic steroids and adrexin C. The cross-reaction curve of adrexin C was superimposable with that of ouabain, and adrexin C coeluted with ouabain from all of the HPLC columns tested. The results from physical and chemical characterization of both purified inhibitors suggest that uroxin is a novel Na-pump inhibitor which is structurally different from any of the known cardiotonic steroids or other substances previously reported to exhibit Na-pump inhibitory activity. The results also indicate that adrexin C is indistinguishable from ouabain. These results suggest that there are at least two different types of endogenous Na+,K(+)-ATPase inhibitors in the mammalian body.

In search of synaptosomal Na+, K+-ATPase regulators

The arrival of the nerve impulse to the nerve endings leads to a series of events involving the entry of sodium and the exit of potassium. Restoration of ionic equilibria of sodium and potassium through the membrane is carried out by the sodium/potassium pump, that is the enzyme Na+,K(+)-ATPase. This is a particle-bound enzyme that concentrates in the nerve ending or synaptosomal membranes. The activity of Na+,K(+)-ATPase is essential for the maintenance of numerous reactions, as demonstrated in the isolated synaptosomes. This lends interest to the knowledge of the possible regulatory mechanisms of Na+,K(+)-ATPase activity in the synaptic region. The aim of this review is to summarize the results obtained in the author's laboratory, that refer to the effect of neurotransmitters and endogenous substances on Na+,K(+)-ATPase activity. Mention is also made of results in the field obtained in other laboratories. Evidence showing that brain Na+,K(+)-ATPase activity may be modified by certain neurotransmitters and insulin have been presented. The type of change produced by noradrenaline, dopamine, and serotonin on synaptosomal membrane Na+,K(+)-ATPase was found to depend on the presence or absence of a soluble brain fraction. The soluble brain fraction itself was able to stimulate or inhibit the enzyme, an effect that was dependent in turn on the time elapsed between preparation and use of the fraction. The filtration of soluble brain fraction through Sephadex G-50 allowed the separation of two active subfractions: peaks I and II. Peak I increased Na+,K(+)- and Mg(2+)-ATPases, and peak II inhibited Na+,K(+)-ATPase. Other membrane enzymes such as acetylcholinesterase and 5'-nucleotidase were unchanged by peaks I or II. In normotensive anesthetized rats, water and sodium excretion were not modified by peak I but were increased by peak II, thus resembling ouabain effects. 3H-ouabain binding was unchanged by peak I but decreased by peak II in some areas of the CNS assayed by quantitative autoradiography and in synaptosomal membranes assayed by a filtration technique. The effects of peak I and II on Na+,K(+)-ATPase were reversed by catecholamines. The extent of Na+,K(+)-ATPase inhibition by peak II was dependent on K+ concentration, thus suggesting an interference with the K+ site of the enzyme. Peak II was able to induce the release of neurotransmitter stored in the synaptic vesicles in a way similar to ouabain. Taking into account that peak II inhibits only Na+,N(+)-ATPase, increases diuresis and natriuresis, blocks high affinity 3H-ouabain binding, and induces neurotransmitter release, it is suggested that it contains an ouabain-like substance.

Endogenous digitalis-like factors

The postulate of a natriuretic factor inhibiting the sodium pump in the kidney led to the detection of increased concentrations of endogenous digitalis-like factors in blood after salt loading, in essential hypertension, in pregnancy-induced hypertension and in chronic hypervolaemia. The recent isolation of ouabain or a close isomer thereof from human plasma and the demonstration of a compound similar if not identical to digoxin in adrenals and human urine shows that mammals like non-vertebrates and toads may synthesize cardiac glycosides in their adrenals and possibly in hypothalamus. The hypothalamus also forms other compounds of unknown structure which bind to the cardiac glycoside receptor site. The differential functions of endogenously formed ouabain and of a digoxin-like substance are unclear. The detailed knowledge of the physiological role of both endogenously formed cardiac glycosides in the regulation of blood pressure has still to be worked out.

Stimulation of atrial natriuretic peptide secretion and synthesis by Na-K-ATPase inhibitors

Ouabain has been reported to increase the secretion of ANP in vitro. In this study, we focused on whether this action is common in Na-K-ATPase inhibitors (ATPI) and whether ATPI simply increase the release of ANP or stimulate both its biosynthesis and release. The effects of ouabain and digoxin on secretion of ANP and accumulation of ANP mRNA were investigated in the rat cardiocyte superfusion system. Ouabain and digoxin increased the immunoreactive ANP (iANP) output into perfusate and accumulation of ANP mRNA significantly. These results suggest that ATPI may stimulate both ANP biosynthesis and release in vitro.

Increased activity of digoxin-like substance in low-renin hypertension in acromegaly

Arterial hypertension is common in acromegaly, but the pathogenesis of this complication remains unknown. To determine the role of an endogenous Na,K pump inhibitor/digoxin-like substance (DLS) in the pathogenesis of hypertension in acromegaly 76 subjects: 28 with acromegaly, 20 with essential hypertension and 28 healthy controls were studied. Serum DLS was measured with the use of radioimmunoassay and bioassay by the inhibition of digoxin-sensitive erythrocyte 86-Rb uptake. In acromegaly, the activity of DLS was significantly increased and plasma renin activity decreased in the hypertensive group, as compared with that of the normotensive group and controls. Moreover, DLS was elevated in the low-renin group of essential hypertension, as compared with that of the normal/high-renin group or controls. The activity of DLS correlated positively with mean arterial pressure and negatively with plasma renin activity, but not with growth hormone levels.

IN CONCLUSION

an endogenous sodium pump inhibitor/digoxin-like substance may play a role in the pathogenesis of low-renin hypertension in acromegaly.

Positive inotropic effects of the endogenous Na+/K(+)-transporting ATPase inhibitor from the hypothalamus

Bovine hypothalamus contains a nonpeptidic substance that inhibits purified Na+/K(+)-transporting ATPase [ATP phosphohydrolase (Na+/K(+)-transporting), EC 3.6.1.37] reversibly with high affinity by a mechanism similar to, but not identical to, that of the cardiac glycosides. It possesses some of the characteristics ascribed to a putative endogenous "digitalis-like" compound that has been implicated in the control of renal sodium excretion and the pathogenesis of essential hypertension in man. To determine whether this hypothalamic Na+/K(+)-transporting ATPase inhibitor might have physiologic properties in cardiac tissues, its effects on Na+ pump inhibition, accumulation of cytosolic free calcium, and contractile response were studied in cultured, spontaneously contracting neonatal rat cardiocytes. The hypothalamic factor potently inhibited the Na+ pump in these cells, increased myoplasmic free calcium in a dose-dependent manner, and reversibly enhanced myocyte contractility by up to 40%, comparable in degree to maximal positive inotropic effects caused by the cardiac glycoside ouabain. Comparative studies further indicate that cardiotoxic effects of ouabain in the myocytes may be more complex than simple progressive elevation of intracellular free calcium concentration because at a free calcium concentration in excess of that produced by a toxic dose of ouabain, no toxicity with the hypothalamic Na+/K(+)-transporting ATPase inhibitor occurred.

Isolation and characterization of a sodium pump inhibitor from human plasma

An endogenous sodium pump inhibitor has been purified from human plasma. The purification scheme involved large scale dialysis, extraction of lyophilized dialysate by methanol followed by preparative and semipreparative scale reverse-phase high-performance chromatography. A single peak of biologically active material was obtained enriched by a factor of greater than 10 billion. This material showed high chromatographic polarity, was inactivated by charring, strong acid, or alkali, and was resistant to short-term boiling. The purified material had a molecular weight between 300 and 900 g/mol and was insensitive to type I esterase and a variety of proteolytic enzymes. The purified factor inhibited the ouabain-sensitive uptake of 86Rb by human erythrocytes, binding of [3H]ouabain, and activity of dog kidney Na,K-adenosine triphosphatase (ATPase) with high affinity (less than 0.3 nM) in a time- and dose-dependent manner. Maximally effective concentrations of the digitalislike factor showed no effect on either human red blood cell Mg- or Ca-ATPase, rabbit muscle sarcoplasmic reticulum Ca-ATPase, or guinea pig stomach H,K-ATPase. The purified material is a highly potent selective inhibitor of the ion transport, receptor, and hydrolytic functions of the sodium pump. The characteristic properties of this substance suggest it may be a mammalian endogenous digitalis and may be similar to the sodium transport inhibitor detected in the plasma of volume-sensitive forms of experimental and human hypertension.

Ouabain induces secretion of proatrial natriuretic factor by rat atrial cardiocytes

Neonatal rat atrial and ventricular cardiocytes in primary culture secrete proatrial natriuretic factor (proANF), the 126-amino acid precursor of atrial natriuretic factor (ANF). The cellular mechanisms of proANF secretion differ in the two cell types: atrial cardiocytes store proANF in abundant secretory granules before secretion, whereas ventricular cardiocytes secrete the precursor rapidly after synthesis. In this study, we used the Na+-K+-adenosinetriphosphatase (ATPase) inhibitor ouabain to investigate the functional significance of these differing secretory mechanisms. Ouabain increased immunoreactive ANF (iANF) secretion by atrial cardiocytes 1.5- to 2.4-fold. Metabolic labeling studies demonstrated that proANF was the form of iANF released in response to ouabain. Ventricular secretion of iANF was unaffected by the Na+-K+-ATPase inhibitor. These observations are consistent with a model in which atrial cardiocytes are able to release proANF via a regulated secretory pathway, whereas ventricular cardiocytes utilize a constitutive secretory pathway. The ability of ouabain to stimulate proANF secretion suggests that an increase in intracellular calcium concentration may promote fusion of secretory granules with atrial cell membranes thereby mediating exocytosis of stored proANF.

Endogenous digitalislike compounds. A tentative update of chemical and biological studies

Endogenous digitalislike compound (or compounds) has been described as involved in some diseases. Questions remain concerning its chemical nature, origin, and biological properties. The methods of measuring the compound are based on biological properties of digitalis, mainly Na+, K+-adenosine triphosphatase (ATPase) inhibition and related properties. Chemically, digitalislike compound has been described as a peptide, as fatty acids, and as a steroid. Its origin could be the brain, particularly the hypothalamus and pituitary gland. The adrenal glands were also proposed as its origin. The reported biological properties of digitalislike compound are mainly dependent on Na+, K+-ATPase inhibition. No definitive conclusions can be drawn from the available data.