Evidence for a digitalis-like substance in the hypothalamopituitary axis in rats: implications in the central cardiovascular regulation associated with an excess intake of sodium

Sodium pumps, ouabain and aldosterone in the brain: A neuromodulatory pathway underlying salt-sensitive hypertension and heart failure

Accumulating evidence obtained over the last three decades has revealed a neuroendocrine system in the brain that mediates long term increases in blood pressure. The system involves distinct ion transport pathways including the alpha-2 isoform of the Na,K pump and epithelial sodium channels, as well as critical hormone elements such as angiotensin II, aldosterone, mineralocorticoid receptors and endogenous ouabain. Activation of this system either by circulating or central sodium ions and/or angiotensin II leads to a cascading sequence of events that begins in the hypothalamus and involves the participation of several brain nuclei including the subfornical organ, supraoptic and paraventricular nuclei and the rostral ventral medulla. Key events include heightened aldosterone synthesis and mineralocorticoid receptor activation, upregulation of epithelial sodium channels, augmented synthesis and secretion of endogenous ouabain from hypothalamic magnocellular neurons, and sustained increases in sympathetic outflow. The latter step depends upon increased production of angiotensin II and the primary amplification of angiotensin II type I receptor signaling from the paraventricular nucleus to the rostral ventral lateral medulla. The transmission of sympathetic traffic is secondarily amplified in the periphery by increased short- and long-term potentiation in sympathetic ganglia and by sustained actions of endogenous ouabain in the vascular wall that augment expression of sodium calcium exchange, increase cytosolic Ca²⁺ and heighten myogenic tone and contractility. Upregulation of this multi-amplifier system participates in forms of hypertension where salt, angiotensin and/or aldosterone are elevated and contributes to adverse outcomes in heart failure.

Cardiotonic steroid ouabain stimulates steroidogenesis in Leydig cells via the α3 isoform of the sodium pump

Cardiotonic steroids such as ouabain are potent inhibitors of the sodium pump and have been widely used for centuries in the treatment of congestive heart failure. In recent decades, however, they have also been identified as hormone-like molecules that trigger signaling cascades of physiological relevance by using the various sodium pump α subunit isoforms as receptors. The murine Leydig cell line MLTC-1 expresses both the ubiquitous, relatively ouabain-insensitive α1 isoform of the sodium pump and the ouabain-sensitive α3 isoform that is normally found in neuronal cells. The physiological relevance of the simultaneous presence of the two isoforms in Leydig cells has not been previously addressed. MLTC-1 Leydig cells contain lipid droplets (LDs) and are capable of progesterone biosynthesis when stimulated by luteinizing hormone (LH). When exposed to low nanomolar concentrations of ouabain, they respond with stimulation of Erk1/2, CREB, and ATF-1 phosphorylation, LD enlargement, and perilipin2 mobilization to the LDs. As a result, progesterone biosynthesis is augmented. Abrogation of α3 isoform expression by siRNA prevents all of the above responses, indicating that it is the hormone/receptor-like interaction of ouabain exclusively with this isoform that triggers the signaling events that normally occur when LH binds to its receptor. Considering that ouabain is produced endogenously and is found in seminal fluid, one can speculate that effects of this substance on germ and somatic cells of the testis might play a role in male reproductive physiology.

Endogenous Na+, K+-ATPase inhibitors and CSF [Na+] contribute to migraine formation

There is strong evidence that neuronal hyper-excitability underlies migraine, and may or may not be preceded by cortical spreading depression. However, the mechanisms for cortical spreading depression and/or migraine are not established. Previous studies reported that cerebrospinal fluid (CSF) [Na+] is higher during migraine, and that higher extracellular [Na+] leads to hyper-excitability. We raise the hypothesis that altered choroid plexus Na+, K+-ATPase activity can cause both migraine phenomena: inhibition raises CSF [K+] and initiates cortical spreading depression, while activation raises CSF [Na+] and causes migraine. In this study, we examined levels of specific Na+, K+-ATPase inhibitors, endogenous ouabain-like compounds (EOLC), in CSF from migraineurs and controls. CSF EOLC levels were significantly lower during ictal migraine (0.4 nM +/- 0.09) than from either controls (1.8 nM +/- 0.4) or interictal migraineurs (3.1 nM +/- 1.9). Blood plasma EOLC levels were higher in migraineurs than controls, but did not differ between ictal and interictal states. In a Sprague-Dawley rat model of nitroglycerin-triggered central sensitization, we changed the concentrations of EOLC and CSF sodium, and measured aversive mechanical threshold (von Frey hairs), trigeminal nucleus caudalis activation (cFos), and CSF [Na+] (ultra-high field 23Na MRI). Animals were sensitized by three independent treatments: intraperitoneal nitroglycerin, immunodepleting EOLC from cerebral ventricles, or cerebroventricular infusion of higher CSF [Na+]. Conversely, nitroglycerin-triggered sensitization was prevented by either vascular or cerebroventricular delivery of the specific Na+, K+-ATPase inhibitor, ouabain. These results affirm our hypothesis that higher CSF [Na+] is linked to human migraine and to a rodent migraine model, and demonstrate that EOLC regulates them both. Our data suggest that altered choroid plexus Na+, K+-ATPase activity is a common source of these changes, and may be the initiating mechanism in migraine.

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

Upregulation of the Renin-Angiotensin-aldosterone-ouabain system in the brain is the core mechanism in the genesis of all types of hypertension

Basic research using animal models points to a causal role of the central nervous system in essential hypertension; however, since clinical research is technically difficult to perform, this connection has not been confirmed in humans. Recently, renal nerve ablation in humans proved to continuously decrease blood pressure in resistant hypertension. Furthermore, when electrical stimulation was continuously applied to the carotid baroreceptor nerve of human adults, their blood pressure lowered. These findings promoted the concept that the central nervous system may actually be involved in the pathogenesis of essential hypertension, which is closely associated with excess sodium intake. We have demonstrated that endogenous digitalis plays a key role in hypertension associated with excess sodium intake via sympathetic activation in rats. Increased sodium concentration inside the brain activates epithelial sodium channels and the renin-angiotensin-aldosterone system in the brain. Aldosterone releases ouabain from neurons in the paraventricular nucleus in the hypothalamus. Angiotensin II and aldosterone of peripheral origin reach the brain to augment sympathetic outflow. Collectively essential hypertension associated with excess sodium intake and obesity, renovascular hypertension, and primary aldosteronism and pseudoaldosteronism all seem to have a common cause originating from the central nervous system.

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.

Effects of central sodium on epithelial sodium channels in rat brain

We evaluated the effects of intracerebroventricular (icv) infusion of Na+-rich artificial cerebrospinal fluid (aCSF), with or without the mineralocorticoid receptor (MR) blocker spironolactone, on epithelial Na+ channel (ENaC) subunits and regulators, such as MR, serum/glucocorticoid-inducible kinase 1, neural precursor cells expressed developmentally downregulated 4-like gene, 11β-hydroxylase, and aldosterone synthase, in brain regions of Wistar rats. The effects of icv infusion of the amiloride analog benzamil on brain tissue and CSF Na+ concentration ([Na+]) were also assessed. In the choroid plexus and ependyma of the anteroventral third ventricle, ENaC subunits are present in apical and basal membranes. Na+-rich aCSF increased β-ENaC mRNA and immunoreactivity in the choroid plexus and increased α- and β-ENaC immunoreactivities in the ependyma. Na+-rich aCSF increased α- and β-ENaC-gold-labeled particles in the microvilli of the choroid plexus and in basolateral membranes of the ependyma. Spironolactone only prevented the increase in β-ENaC immunoreactivity in the choroid plexus and ependyma. In the supraoptic nucleus, paraventricular nucleus, and subfornical organ, Na+-rich aCSF did not affect mRNA expression levels of the studied genes. Benzamil significantly increased CSF [Na+] in the control, but not Na+-rich, aCSF group. In contrast, benzamil prevented the increase in hypothalamic tissue [Na+] by Na+-rich aCSF. These results suggest that CSF Na+ upregulates ENaC expression in the brain epithelia, but not in the neurons of hypothalamic nuclei. ENaC in the choroid plexus and ependyma appear to contribute to regulation of Na+ homeostasis in the brain.

The physiological significance of the cardiotonic steroid/ouabain-binding site of the Na,K-ATPase

The Na,K-ATPase is the membrane "pump" that generates the Na(+) and K(+) gradients across the plasma membrane that drives many physiological processes. This enzyme is highly sensitive to inhibition by cardiotonic steroids, most notably the digitalis/ouabain class of compounds, which have been used for centuries to treat congestive heart failure and arrhythmias. The amino acids that constitute the ouabain-binding site are highly conserved across the evolutionary spectrum. This could be fortuitous or could result from this site being conserved because it has an important biological function. New physiological approaches using genetically engineered mice are being used to define the biological significance of the "receptor function" of the Na,K-ATPase and its regulation by potential endogenous cardiotonic steroid-like compounds. These studies extend the reach of earlier studies involving the biochemical purification of endogenous regulatory ligands.

Endogenous ouabain in hypertensive disorder complicating pregnancy

Expression of endogenous ouabain in placenta and the concentrations of serum ET-1 and NO were examined in 30 patients with hypertensive disorder complicating pregnancy (HDCP) and 30 healthy pregnant women to investigate the effect of endogenous ouabain on HDCP. Compared with the healthy pregnant group, the expression of endogenous ouabain dramatically increased in the HDCP groups (P<0.01). There was a significantly positive correlation between the expression of endogenous ouabain with ET-1 (r=0.5567, P<0.01), while the correlation of endogenous ouabain and NO was significantly negative (r=-0.6895, P<0.01). As expected, the correlation between ET-1 and NO was negative (r=-0.7796, P<0.01). ET-1 concentrations of maternal and cord sera in HDCP groups were significantly higher in comparison with healthy pregnant group (P<0.01). On the contrast, NO concentrations were much lower in the maternal and cord sera of HDCP groups as compared with healthy pregnant group (P<0.01). Our data suggest that endogenous ouabain is directly involved in the nosogenesis of HDCP, with accompanying decreased NO and the elevated of ET-1.

Endogenous brain Na pumps, brain ouabain-like substance and the alpha2 isoform in salt-dependent hypertension

An endogenous ouabain-like substance (OLS) plays a critical role in the etiology of experimental models of human hypertension induced by a high salt diet. Early on, evidence for a role of this Na, K-ATPase inhibitor in blood pressure regulation was provided mainly by correlations of blood pressure with the levels of circulating Na, K-ATPase inhibitor. However, over the past decade, numerous studies have shown that endogenous Na pump inhibitors in the brain mediate salt-dependent hypertension in a variety of experimental models, including Dahl salt-sensitive (Dahl-S) and spontaneously hypertensive (SHR) rats on a high-salt diet. Other forms of hypertension that are known to be mediated by endogenous ouabain-like substances include steroid/salt- (e.g., DOCA-salt) and ACTH-induced hypertension. Even when exogenous ouabain is peripherally administered and/or the plasma ouabain/OLS level is increased in rats, the resulting hypertension is of CNS origin. After peripheral ouabain administration, ouabain levels increase in the plasma and the inhibitor subsequently accumulates in the brain. The ensuing hypertension is abolished by the intracerebroventricular (icv) administration of an anti-ouabain antibody (but not by the same antibody dose given iv), by discrete excitotoxic lesions in the brain or by ganglionic blockade, demonstrating that the response is neurally mediated. The pressor response to stimuli that increase the brain OLS (high salt diet, icv sodium) or to icv ouabain is abolished by icv losartan, demonstrating that the brain OLS activates the brain renin-angiotensin system (RAS) downstream. There are three isoforms of the catalytic alpha subunit of the Na, K-ATPase in the brain and cardiovascular system (alpha1, alpha2 and alpha3), but it is not known which brain isoform(s) mediate the hypertensive effects of circulating/CNS ouabain. Preliminary studies in gene-targeted mice suggest that the alpha2 isoform plays a critical role.

Endogenous and exogenous cardiac glycosides: their roles in hypertension, salt metabolism, and cell growth

Cardiotonic steroids (CTS), long used to treat heart failure, are endogenously produced in mammals. Among them are the hydrophilic cardenolide ouabain and the more hydrophobic cardenolide digoxin, as well as the bufadienolides marinobufagenin and telecinobufagin. The physiological effects of endogenous ouabain on blood pressure and cardiac activity are consistent with the "Na(+)-lag" hypothesis. This hypothesis assumes that, in cardiac and arterial myocytes, a CTS-induced local increase of Na(+) concentration due to inhibition of Na(+)/K(+)-ATPase leads to an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) via a backward-running Na(+)/Ca(2+) exchanger. The increase in [Ca(2+)](i) then activates muscle contraction. The Na(+)-lag hypothesis may best explain short-term and inotropic actions of CTS. Yet all data on the CTS-induced alteration of gene expression are consistent with another hypothesis, based on the Na(+)/K(+)-ATPase "signalosome," that describes the interaction of cardiac glycosides with the Na(+) pump as machinery activating various signaling pathways via intramembrane and cytosolic protein-protein interactions. These pathways, which may be activated simultaneously or selectively, elevate [Ca(2+)](i), activate Src and the ERK1/2 kinase pathways, and activate phosphoinositide 3-kinase and protein kinase B (Akt), NF-kappaB, and reactive oxygen species. A recent development indicates that new pharmaceuticals with antihypertensive and anticancer activities may be found among CTS and their derivatives: the antihypertensive rostafuroxin suppresses Na(+) resorption and the Src-epidermal growth factor receptor-ERK pathway in kidney tubule cells. It may be the parent compound of a new principle of antihypertensive therapy. Bufalin and oleandrin or the cardenolide analog UNBS-1450 block tumor cell proliferation and induce apoptosis at low concentrations in tumors with constitutive activation of NF-kappaB.

Endogenous and exogenous cardiac glycosides and their mechanisms of action

Cardiac glycosides have been used for decades to treat congestive heart failure. The recent identification of cardiotonic steroids such as ouabain, digoxin, marinobufagenin, and telocinobufagin in blood plasma, adrenal glands, and hypothalamus of mammals led to exciting new perspectives in the pathology of heart failure and arterial hypertension. Biosynthesis of ouabain and digoxin occurs in adrenal glands and is under the control of angiotensin II, endothelin, and epinephrine released from cells of the midbrain upon stimulation of brain areas sensing cerebrospinal Na(+) concentration and, apparently, the body's K(+) content. Rapid changes of endogenous ouabain upon physical exercise may favor the economy of the heart by a rise of intracellular Ca(2)(+) levels in cardiac and atrial muscle cells. According to the sodium pump lag hypothesis, this may be accomplished by partial inhibition of the sodium pump and Ca(2+) influx via the Na(+)/Ca(2+) exchanger working in reverse mode or via activation of the Na(+)/K(+)-ATPase signalosome complex, generating intracellular calcium oscillations, reactive oxygen species, and gene activation via nuclear factor-kappaB or extracellular signal-regulated kinases 1 and 2. Elevated concentrations of endogenous ouabain and marinobufagenin in the subnanomolar concentration range were found to stimulate proliferation and differentiation of cardiac and smooth muscle cells. They may have a primary role in the development of cardiac dysfunction and failure because (i) offspring of hypertensive patients evidently inherit elevated plasma concentrations of endogenous ouabain; (ii) such elevated concentrations correlate positively with cardiac dysfunction, hypertrophy, and arterial hypertension; (iii) about 40% of Europeans with uncomplicated essential hypertension show increased concentrations of endogenous ouabain associated with reduced heart rate and cardiac hypertrophy; (iv) in patients with advanced arterial hypertension, circulating levels of endogenous ouabain correlate with BP and total peripheral resistance; (v) among patients with idiopathic dilated cardiomyopathy, high circulating levels of endogenous ouabain and marinobufagenin identify those individuals who are predisposed to progressing more rapidly to heart failure, suggesting that endogenous ouabain (and marinobufagenin) may contribute to toxicity upon digoxin therapy. In contrast to endogenous ouabain, endogenous marinobufagenin may act as a natriuretic substance as well. It shows a higher affinity for the ouabain-insensitive alpha(1) isoform of Na(+)/K(+)-ATPase of rat kidney tubular cells and its levels are increased in volume expansion and pre-eclampsia. Digoxin, which is synthesized in adrenal glands, seems to counteract the hypertensinogenic action of ouabain in rats, as do antibodies against ouabain, for example, (Digibind) and rostafuroxin (PST 2238), a selective ouabain antagonist. It lowers BP in ouabain- and adducin-dependent hypertension in rats and is a promising new class of antihypertensive medication in humans.

Endogenous Cardiac Glycosides: Hormones Using the Sodium Pump as Signal Transducer

The search for an endogenous digitalis has led to the identification of the cardenolides ouabain and digoxin and the bufadienolide marinobufagenin in mammalian tissues and biological fluids. Ouabain's release from adrenal glands is under the control of epinephrine and angiotensin II; hence, its blood concentration changes rapidly on physical exercise. It also is controlled by brain areas sensing cerebrospinal Na+ concentration and apparently the body's K+ content because urinary K+ loss leads to an increase in its plasma concentration as well. Long-term treatment of rats with ouabain results in arterial hypertension, and 50% of Caucasians with low-renin hypertension have increased plasma concentrations of this cardenolide. Levels of digoxin, which is synthesized from acetate in adrenal glands, increase slightly in blood on prolonged exercise. It counteracts the hypertensinogenic action of ouabain in rats, as does the ouabain antagonist PST 2238. The plasma concentration of the bufadienolide marinobufagenin is increased after cardiac infarction. It may show natriuretic properties because it inhibits the alpha1 isoform of Na+/K+-adenosine triphosphatase (ATPase), the main sodium pump isoform of the kidney, much better than other sodium pump isoforms. These effects of endogenous cardiac glycosides are observed at concentrations that do not inhibit the sodium pump. Apparently, Na+/K+-ATPase is used by these steroids as a signal transducer to activate tissue proliferation, heart contractility, arterial hypertension, and natriuresis via various intracellular signaling pathways.

Nanomolar level of ouabain increases intracellular calcium to produce nitric oxide in rat aortic endothelial cells

Changes in [Ca(2+)](i) across the cell membrane and/or the sarcoplasmic reticulum regulate endothelial nitric oxide (NO) synthase activity. In the present study, we investigated the effect of ouabain, a specific inhibitor of Na(+)/K(+)-ATPase, on NO release and [Ca(2+)](i) movements in cultured rat aortic endothelial cells (RAEC) by monitoring NO production continuously using an NO-specific real-time sensor and by measuring the change in [Ca(2+)](i) using a fluorescence microscopic imaging technique with high-speed wavelength switching. The t((1/2)) (half-time of the decline of [Ca(2+)](i) to basal levels after stimulation with 10 micro mol/L bradykinin) was used as an index of [Ca(2+)](i) extrusion. A very low concentration of ouabain (10 nmol/L) did not increase the peak of NO production, but decreased the decay of NO release and, accordingly, increased integral NO production by the maximal dose-response concentration induced by bradykinin. The same dose of ouabain affected [Ca(2+)](i) movements across the cell membrane and/or sarcoplasmic reticulum induced by bradykinin with a time-course similar to that of NO release. Moreover, the t((1/2)) was significantly increased. Pretreatment of RAEC with Na(+)-free solution, an inhibitor of the Na(+)/Ca(2+) exchanger, and nickel chloride hexahydrate prevented the effects induced by bradykinin and ouabain. These observations using real-time recording indicate that a small amount of ouabain contributes to the bradykinin-stimulated increase of NO production through inhibition of plasma membrane Na(+)/K(+)-ATPase activity and an increase in intracellular Na(+) concentrations. The membrane was then depolarized, leading to a decline in the bradykinin-stimulated increase in [Ca(2+)](i) by forward mode Na(+)/Ca(2+) exchange to prolong the Ca(2+) signal time. From these results, we suggest that nanomolar levels of ouabain modulate [Ca(2+)](i) movements and NO production in RAEC.

Pressor response to CSF sodium in mice: mediation by a ouabain-like substance and renin-angiotensin system in the brain

Intracerebroventricular (i.c.v.) infusion of sodium in rats increases cerebrospinal fluid (CSF) [Na], mimicking the effects of a high salt diet in salt-sensitive strains and causing sympathetic hyperactivity and a pressor response that are mediated via both an endogenous brain ouabainlike substance (OLS) and the brain renin-angiotensin system (RAS). However, the concept that CSF sodium activates both the brain OLS and brain RAS to increase blood pressure has not been tested in any other species besides the rat. In the current study, it was established that continuous i.c.v. infusion of NaCl causes sustained increases in blood pressure and heart rate in both outbred (Swiss Webster, SW) and inbred (C57Bl/6) mouse strains. Subsequently, the mechanisms of the pressor effects were explored. In both SW and C57Bl/6, the i.c.v. administration of Fab fragments of an antibody with high affinity for ouabain and the OLS (Fab) abolished the pressor and tachycardic responses to i.c.v. sodium, as did the angiotensin II AT1 receptor antagonist losartan given i.c.v. In contrast, doses of NaCl, Fab and losartan that were effective i.c.v. were ineffective when given i.v. I.c.v. ouabain also caused the pressor and tachycardic responses, which were abolished by losartan (i.c.v.). In the reciprocal study, i.c.v. Fab had no effect on similar responses to i.c.v. angiotensin II. These studies demonstrate that the sustained blood pressure and heart rate responses caused by increases in CSF [Na] are mediated via both a brain OLS and the brain RAS. The RAS activation occurs downstream of the OLS effect.

Endogenous cardiac glycosides, a new class of steroid hormones

The search for endogenous digitalis has led to the isolation of ouabain as well as several additional cardiotonic steroids of the cardenolide and bufadienolide type from blood, adrenals, and hypothalamus. The concentration of endogenous ouabain is elevated in blood upon increased Na(+) uptake, hypoxia, and physical exercise. Changes in blood levels of ouabain upon physical exercise occur rapidly. Adrenal cortical cells in tissue culture release ouabain upon addition of angiotensin II and epinephrine, and it is thought that ouabain is released from adrenal cortex in vivo. Ouabain levels in blood are elevated in 50% of Caucasians with low-renin hypertension. Infusion over several weeks of low concentrations of ouabain, but not of digoxin, induces hypertension in rats. A digoxin-like compound, which has been isolated from human urine and adrenals, as well various other endogenous cardiac glycosides may counterbalance their actions within a regulatory framework of water and salt metabolism. Marinobufagenin, for instance, whose concentration is increased after cardiac infarction, may show natriuretic properties because it inhibits the alpha1 isoform of Na(+)/K(+)-ATPase, the main sodium pump isoform of the kidney, much better than other sodium pump isoforms. In analogy to other steroid hormones, cardiotonic steroid hormones in blood are bound to a specific cardiac glycoside binding globulin. The discovery of ouabain as a new adrenal hormone affecting Na(+) metabolism and the development of the new ouabain antagonist PST 2238 allows for new possibilities for the therapy of hypertension and congestive heart failure. This will lead in turn to a better understanding of the disease on a physiological and endocrinological level and of the action of ouabain on the cellular level as a signal that is transduced to the plasma membrane as well as to the cell nucleus.

Is ATP a substrate for 15-lipoxygenase?

Lipoxygenases catalyze peroxidation of polyunsaturated fatty acids containing the 1-cis, 4-cis pentadiene structure. Linoleic (18:2), linolenic (18:3), and arachidonic (20:4) acids are the predominant substrates for this class of enzymes. Effects of 15-lipoxygenase on the hydrolysis of adenosine 5'-triphosphate were investigated in vitro using soybean lipoxygenase and adenosine 5'-[gamma-32P]triphosphate. The amount of inorganic phosphate released from adenosine 5'-triphosphate was dependent upon enzyme as well as substrate concentrations, pH, and the duration of incubation. The ATPase activity with a Vmax value of 3.3 mumol.mg protein-1.h-1 and a Km value of 5.9 mM was noted in the presence of different concentrations of ATP at pH = 7.4. Phenidone, a lipoxygenase inhibitor, had no effect on this reaction. These findings suggest that soybean lipoxygenase catalyzes the release of inorganic phosphate from ATP primarily via hydrolysis.

Brain "ouabain", a neurosteroid, mediates sympathetic hyperactivity in salt-sensitive hypertension

This review addresses recent developments in the neurobiology of an endogenous inhibitor of brain Na+, K+ - ATPase, "ouabain". "Ouabain" is present in hypothalamic and medullary neurons and mediates sympathoexcitatory and pressor responses to acute and chronic increases in cerebrospinal fluid (CSF) sodium concentration as well as mediates the sympathoexcitatory and pressor responses to high dietary sodium intake in SHR and Dahl-S rats, and sympathetic hyperactivity in the congestive heart failure. Some of these actions of "ouabain" in the CNS take place in the median preoptic nucleus and ventral part of the AV3V region. Despite recent advances in unveiling a biological role for "ouabain" its structure, biosynthetic and metabolic pathways as well as actual control mechanisms remain unresolved.

Sympathoexcitatory effect of hypothalamic/hypophysary inhibitory factor in rats

We recorded changes in arterial blood pressure, heart rate, and renal sympathetic nerve activity in response to intracerebroventricular injection of bovine hypothalamic/hypophysary inhibitory factor and ouabain in conscious Wistar rats. Ouabain at 0.3 to 0.6 microgram caused dose-related increases in blood pressure, heart rate, and nerve activity (peak increases: 19 +/- 2 mm Hg, 42 +/- 4 beats per minute, and 48 +/- 4%, respectively; P < .05 versus basal). These responses were all blocked by central antibody Fab fragments, which bind ouabain and related steroids with high affinity. The inhibitory factor significantly increased blood pressure but decreased heart rate and nerve activity. Dose-dependent increases in blood pressure as well as heart rate and nerve activity were observed when the inhibitory factor was injected after intravenous injection of the vasopressin antagonist D-(CH2)5Tyr-(Me)AVP. Central Fab fragments, however, did not affect these responses. Both ouabain and the inhibitory factor inhibited Na+,K+-ATPase activity in vitro. Fab fragments blocked this inhibition by ouabain but not by the inhibitory factor. These data indicate that the ouabainlike sympathoexcitatory effect of this factor is masked probably by a potent central effect on vasopressin release. In contrast to rat brain "ouabain," this factor does not exhibit a high affinity for the Fab fragments, supporting the previous finding that this compound is structurally a nonouabain Na+,K+-ATPase inhibitor.

Endogenous sodium pump inhibition: current status and therapeutic opportunities

One might ask, given the number of false trails that have been pursued, why we, and so many others, have continued to pursue the elusive digitalis-like factor? The answer can be found in the many review articles cited above [4-13]. In animal models of volume-dependent hypertension, evidence favoring sodium pump inhibition as at least a contributing factor, is essentially overwhelming. These observations are supported by multiple lines of less direct evidence in humans which are also compatible with a contribution of a circulating sodium pump inhibitor. Indeed, if multiple premature claims announcing the isolation of the digitalis-like factor had not appeared, this would be one of a large number of interesting scientific areas in which identification of a responsible vector was expected momentarily. The disenchantment so often expressed, we believe, reflects a response to those premature claims. We echo a recent review on the digoxin-like sodium pump inhibitor story from one of the productive groups in this area. "Now that there is little doubt that endogenous digoxin-like inhibitors of sodium transport exist..., the link between these substances, salt intake and vascular tone must be pursued with increasing vigor" [12]. That pursuit, of course, will be easier if the criteria concerning the responsible mediator are employed systematically. Because the current situation resembles so strikingly the situation late in the nineteenth century--when efforts focused on the attempt to identify a specific microorganism as the agent responsible for specific disease--we employed Koch's Postulates as the organizing principle. The challenge faced by Robert Koch over a century ago is identical to the challenge that those of us who are interested in digitalis-like factors face today. Passionate advocacy and equally impassioned denial can be seen as a stage in the scientific process when the problem is important and has proven to be more intractable than anticipated. Substantial, but still circumstantial evidence supports strongly a role for a circulating digitalis-like factor not only in normal sodium homeostasis and in the pathogenesis of salt-sensitive hypertension, but also in the pathogenesis of a wide array of processes that have an uncertain etiology. Although supported by many lines of evidence, this intriguing concept remains controversial, in large part because the responsible factor has proven to be very elusive. Informed opinion today ranges from arguments that the agent does not exist to contrary arguments that the agent has been identified. A very large number of candidates from a wide range of chemical classes have been proposed. Indeed, the large number of candidates, none supported by absolutely definitive evidence, has contributed to the controversy. In this essay, we have attempted to define and illustrate the information that will be required before a candidate becomes widely accepted.

Chronic central versus peripheral ouabain, blood pressure, and sympathetic activity in rats

To assess whether chronic ouabain administration causes hypertension by increasing sympathetic activity, we recorded arterial blood pressure and heart rate at rest and after ganglionic blockade in conscious Wistar rats following 10 to 14 days of central or peripheral administration of ouabain. Intracerebroventricular or intravenous infusion of ouabain (10 micrograms/d for both) as well as subcutaneous ouabain pellets (releasing 25 micrograms ouabain/d per pellet) increased mean arterial pressure by 20 to 30 mm Hg and heart rate by 40 to 60 beats per minute. Ouabain pellets increased blood pressure and heart rate in a dose-related manner. After 2 weeks of all ouabain treatments, ouabainlike activity in plasma was not changed but increased significantly in hypothalamus and adrenals. Ouabainlike activity in the adrenals was increased more by intravenous than subcutaneous or intracerebroventricular ouabain treatment, but the different treatment modes caused similar increases in the hypothalamus. Concomitant central infusion of antibody Fab fragments against ouabain prevented the ouabain pellet-induced increases in blood pressure and heart rate. Ganglionic blockade by intravenous hexamethonium normalized blood pressure and heart rate in ouabain-treated rats. These data suggest that in normotensive rats exogenous ouabain, regardless of the mode of administration, may act centrally to cause sympathoexcitation and thus hypertension.

Role of brain ouabainlike compound in central nervous system-mediated natriuresis in rats

Intracerebroventricular infusion of artificial sodium-rich cerebrospinal fluid induces increases in blood pressure and urinary sodium excretion. To examine the role of brain ouabainlike compound in these central nervous system-mediated responses, we evaluated the effects of prior intracerebroventricular injection of the Fab fragments of digoxin-specific antibody (Digibind, 10 mg/mL, 10 microL) on changes in blood pressure and urinary sodium excretion after intracerebroventricular infusion of high-sodium (323 mmol/L, 150 microL/kg per 15 minutes) cerebrospinal fluid in anesthetized rats. Antiouabain action of Digibind was revealed by the inhibition of a contractile response to ouabain in guinea pig aorta. Similar significant increases in blood pressure were found in rats that received preinjection of Digibind (n = 14) compared with control rats that received injection of saline (n = 5) or normal sheep IgG (n = 8). In rats pretreated with Digibind the natriuretic responses to central high sodium were significantly diminished by 68% (P < .05) or 82% (P < .05) compared with rats treated with saline or normal IgG, respectively. In contrast, Digibind did not affect either pressor or natriuretic responses to intracerebroventricular angiotensin II (600 ng/30 microL per 10 minutes). These data indicate that Digibind significantly inhibits increases in renal sodium excretion in response to high central sodium and suggest that brain ouabainlike compound may be involved in central nervous system-mediated natriuresis with nonpressor mechanisms.

Brain ouabain-like activity and the sympathoexcitatory and pressor effects of central sodium in rats

Intracerebroventricularly infused hypertonic saline elicits sympathoexcitatory and pressor effects. To clarify the mechanisms mediating these effects, we evaluated blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) responses to intracerebroventricular administration of 0.3 M NaCl, ouabain, and rat hypothalamic and pituitary extracts containing ouabain-like activity (OLA) in conscious Wistar rats, before and after intracerebroventricular preinjection of digoxin-specific antibody Fab (DAF) fragments. To exclude modulatory effects of arginine vasopressin (AVP), treatment with DAF fragments was in all experiments preceded by intravenous injection of the AVP antagonist [beta-mercapto-beta,beta-cyclopentamethylenepropionyl1,o- Me-Tyr2,Arg8]AVP. After AVP antagonist pretreatment, 0.3 M NaCl i.c.v. at 3.8 microliters/min for 10 minutes caused simultaneous increases in BP, RSNA, and HR. After AVP antagonist pretreatment, intracerebroventricular injections of 0.3 and 1.0 microgram/l microliter ouabain or the OLA equivalent to 1 microgram ouabain/2 microliters elicited similar significant increases in BP, HR, and RSNA. After pretreatment with AVP antagonist and DAF fragments (66 micrograms/4 microliters i.c.v.), BP, HR, and RSNA responses to 0.3 M NaCl, ouabain, and OLA were all significantly diminished. In contrast, combined AVP blockade and DAF fragments did not affect the BP response to intracerebroventricular angiotensin II, the BP, HR, and RSNA response to intracerebroventricular carbachol and to air stress, or the HR and RSNA responses to intravenous sodium nitroprusside. Intracerebroventricularly injected gamma-globulins (66 micrograms/4 microliters) did not affect the responses to 0.3 M NaCl, ouabain, or OLA.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain ouabain and central effects of dietary sodium in spontaneously hypertensive rats

High sodium intake (HNa) increases brain ouabainlike activity (OLA) in rats. In spontaneously hypertensive rats (SHR), HNa exaggerates development of hypertension and pressor and sympathoexcitatory responses to stress. To investigate whether dietary sodium-induced changes in brain OLA play a functional role, responses of mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) to intracerebroventricular ouabain and to mental stress and intracerebroventricular alpha 2-adrenoceptor agonist guanabenz alone or preceded by intracerebroventricular ouabain were recorded in conscious SHR and Wistar-Kyoto (WKY) rats maintained from 4 to 8 weeks of age on different sodium diets: 1) low sodium intake (LNa, 17 mumol), 2) normal sodium intake (NNa, 101 mumol), and 3) HNa (1,370 mumol). SHR on NNa showed significantly higher MAP and RSNA compared with WKY rats on NNa. HNa or LNa significantly increased or decreased MAP but had no effects on resting RSNA in SHR and had no effects on resting MAP and RSNA in WKY rats. Intracerebroventricular ouabain induced dose-dependent increases in MAP, RSNA, and HR. In both SHR and WKY rats, LNa significantly enhanced these responses. In contrast, HNa significantly attenuated these responses only in SHR. Air stress increased and intracerebroventricular guanabenz decreased MAP, HR, and RSNA. The magnitudes of increases and decreases were significantly larger in SHR than in WKY rats. In WKY rats, dietary sodium did not change these responses. In contrast, in SHR, HNa significantly enhanced MAP, HR, and RSNA responses to air stress or intracerebroventricular guanabenz.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain renin-angiotensin system and the hypothalamic, digitalis-like Na+, K+-ATPase inhibitor in rats

Since angiotensin (ang) II blockers attenuate the centrally-induced pressor responses to a Na+, K+-ATPase inhibitor, ouabain, a brain renin-ang system is assumed to be involved in this pressor mechanism. Centrally-induced pressor responses to hypertonic saline were also blocked with ang II blockers. Thereby, the increase in the plasma level of digoxin-like immunoreactivity was abolished with simultaneous infusions of an ang II analogue or atrial natriuretic polypeptide (ANP). These results indicate that the pressor responses to sodium salts are mediated via inhibition of the brain Na+, K+-ATPase, and that the brain renin-ang system is involved in this mechanism. We noted the existence of a digoxin-like immunoreactive substance (DLI) containing neurons in the hypothalamus (PVN, SON) with the fibers densely distributed in the AV3V area including OVLT, SFO and the median eminence, an area where receptors for ang II are also distributed (1,2). Since pressor responses to intracerebroventricular (ICV) infusions of hypertonic NaCl are abolished with ICV pretreatment with ang II blockers, a brain renin-ang system may be involved in this mechanism. We then searched for a possible relationship between the central effects of NaCl, a brain renin-ang system and an endogenous Na+, K+-ATPase inhibitor in the hypothalamus.