Genetics of primary hypertension: The clinical impact of adducin polymorphisms

The usefulness of the results so far published on genetics of primary hypertension for establishing the clinical impact of candidate gene polymorphisms is weakened by the scanty information regarding: a) the functional effect of the gene variants of interest in humans; b) the regulatory genetic network (RGN) where the gene is operating with all the interacting environmental–biological factors and the respective hierarchical organization; c) the consistency between the natural history of the established pathophysiological mechanisms underlying hypertension and the new molecular mechanism detected with genetics; d) the limitations regarding the translation of animal data to human due to the differences among species of the genetic molecular mechanisms underlying similar organ function changes in the different species. Of course, not all these information are available for adducin polymorphisms. In this review, being aware of their importance, the evaluation of the clinical impact of adducin has been focused on data obtained together with the interacting genetic-environmental or biological factors. Adducin polymorphisms and endogenous ouabain (EO) were detected by a top-down approach in rodents after having demonstrated, at cellular and kidney level, that an increase in tubular Na reabsorption could underlies the transition from normotension to hypertension both in rodents and humans. Therefore, we hypothesized that adducin polymorphisms and EO may operate within the triggering RGN that initiates the increase in blood pressure in both species. The distinction between triggering RGN and the secondary RGN is important both to limit the level of genetic complexity arising from secondary changes, and to detect the molecular target to develop tailored therapeutic approach. The pharmacogenomic approach, both in rodents or humans, with newly discovered and never treated hypertension, may be useful to strengthen the “causation” of genetic mechanism. Mutant adducin increases tubular reabsorption: diuretics, because of their effect on overall tubular reabsorption, or rostafuroxin, because of its selective inhibition of the adducin and ouabain effects, may be used for this purpose. Indeed the pharmacogenomic approach with both drugs have provided data consistent with the role of adducin and EO. Taken together, all these findings indicate a clear impact of adducin polymorphism and EO in a subset of patients when the appropriate environmental, biological or genetic context is taken into account. The size of this impact is variable and affected by the context.     

Erythrocyte ouabain binding and intracellular Na+ in normotensive obese women and obese women receiving medication for hypertension

People with "primary obesity" may be hypertensive because they have lost their ability to compensate for the effect of low Na+-K+-ATPase levels on blood pressure. In obese patients receiving hypertensive medication (n = 13), but not in normotensive nonmedicated patients (n = 42), diastolic blood pressure was inversely correlated with erythrocyte ouabain binding (P less than 0.02) and directly correlated with intracellular Na+ concentration (P less than 0.01). Moreover, there was a stronger inverse relationship between ouabain binding and intracellular Na+ in patients receiving medication for hypertension (P less than 0.01) than in normotensive patients (P less than 0.05). These data suggest that patients receiving hypertensive medication may be less able to compensate than normotensive patients, (a) for the potential effect of Na+-K+-ATPase levels on intracellular Na+ and (b) for the potential effect of intracellular Na+ concentration on diastolic blood pressure. We propose that obese people with low levels of ouabain binding (primary obesity) may have an increased risk of developing hypertension if their compensatory mechanisms fail.     

Endogenous ouabain in renal Na+ handling and related diseases

The Na⁺ pump and its Endogenous modulator Ouabain (EO) can be considered as an ancestral enzymatic system, conserved among species ranging from Drosophila to humans, related to Na handling. In this review, we examine how EO is linked with vascular function in hypertension and if it impacts the pathogenesis of heart and renal failure. Moreover, the molecular mechanism of endogenous ouabain-linked hypertension involves the sodium pump/sodium–calcium exchanger duet. Biosynthesis of EO occurs in adrenal glands and is under the control of angiotensin II, ACTH and epinephrine. Elevated concentrations of EO and in the sub-nanomolar 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. Experimental data suggest that the Na/K-ATPase α₂-catalytic subunit causes EO-induced vasoconstriction. Finally, maneuvers that promote Na depletion, as diuretic therapy or reduced Na intake, raise the EO levels. Taken together, these findings suggest a key role for EO in body Na homeostasis.     

How does salt retention raise blood pressure?

A critical question in hypertension research is: How is long-term blood pressure controlled? Excessive NaCl ingestion or NaCl retention by the kidneys and the consequent tendency toward plasma volume expansion lead to hypertension. Nevertheless, the precise mechanisms linking salt to high blood pressure are unresolved. The discovery of endogenous ouabain, an adrenocortical hormone, provided an important clue. Ouabain, a selective Na+ pump inhibitor, has cardiotonic and vasotonic effects. Plasma endogenous ouabain levels are significantly elevated in approximately 40% of patients with essential hypertension and in animals with several forms of salt-dependent hypertension. Also, prolonged ouabain administration induces hypertension in rodents. Mice with mutant Na+ pumps or Na/Ca exchangers (NCX) and studies with a ouabain antagonist and an NCX blocker are revealing the missing molecular mechanisms. These data demonstrate that alpha2 Na+ pumps and NCX1 participate in long-term regulation of vascular tone and blood pressure. Pharmacological agents or mutations in the alpha2 Na+ pump that interfere with the action of ouabain on the pump, and reduced NCX1 expression or agents that block NCX all impede the development of salt-dependent or ouabain-induced hypertension. Conversely, nanomolar ouabain, reduced alpha2 Na+ pump expression, and smooth muscle-specific overexpression of NCX1 all induce hypertension. Furthermore, ouabain and reduced alpha2 Na+ pump expression increase myogenic tone in isolated mesenteric small arteries in vitro, thereby tying these effects directly to the elevation of blood pressure. Thus, endogenous ouabain, and vascular alpha2 Na+ pumps and NCX1, are critical links between salt and hypertension. New pharmacological agents that act on these molecular links have potential in the clinical management of hypertension.     

Insulin and exercise stimulate muscle alpha-aminoisobutyric acid transport by a Na+-K+-ATPase independent pathway

Sodium ions are required for the active transport of amino acids such as alpha-aminoisobutyric acid (AIB) into skeletal muscle. To examine the role of Na+-K+-ATPase in this phenomenon, studies were carried out using the isolated perfused rat hindquarter preparation. Perfusion for 30 min with ouabain at a dose sufficient to inhibit the Na+-K+ pump (10(-4) M) inhibited the basal rate of AIB uptake in all muscles studied by up to 80%. However, it failed to inhibit the stimulation of AIB uptake, either by insulin (200 microU/ml) or electrically-induced muscle contractions. The increase in K+ release by the hindquarter in the presence of ouabain was the same under all conditions suggesting comparable inhibition of the Na+-K+ pump. These studies suggest that the basal, but not insulin or exercise-stimulated AIB transport into muscle is acutely dependent on a functional Na+-K+ pump. They also suggest that stimulated and basal uptake of AIB involve different mechanisms.     

Salt intake and depletion increase circulating levels of endogenous ouabain in normal men

High-salt diets elevate circulating Na+ pump inhibitors, vascular resistance, and blood pressure. Ouabain induces a form of hypertension mediated via the alpha2-Na+ pump isoform and the calcium influx mode of the vascular sodium calcium exchanger (NCX). Whereas elevated levels of an endogenous ouabain (EO) and NCX have been implicated in salt-sensitive hypertension, acute changes in sodium balance do not affect plasma EO. This study investigated the impact of longer-term alterations in sodium balance on the circulating levels and renal clearance of EO in normal humans. Thirteen normal men consumed a normal diet, high-salt diet, and hydrochlorothiazide (HCTZ), each for 5-day periods to alter sodium balance. EO and other humoral and urinary variables were determined daily. On a normal diet, urinary sodium excretion (140 +/- 16 meq/day), plasma EO (0.43 +/- 0.08 nmol/l) and urinary EO excretion (1.04 +/- 0.13 nmol/day) were at steady state. On the 3rd day of a high-salt diet, urine sodium excretion (315 +/- 28 meq/day), plasma EO (5.8 +/- 2.2 nmol/l), and the urinary EO excretion (1.69 +/- 0.27 nmol/day) were significantly increased, while plasma renin activity and aldosterone levels were suppressed. The salt-evoked increase in plasma EO was greater in older individuals, in subjects whose baseline circulating EO was higher, and in those with low renal clearance. During HCTZ, body weight decreased and plasma renin activity, aldosterone, and EO (1.71 +/- 0.77 nmol/l) rose, while urinary EO excretion remained within the normal range (1.44 +/- 0.31 nmol/day). Blood pressure fell in one subject during HCTZ. HPLC of the plasma extracts showed one primary peak of EO immunoreactivity with a retention time equivalent to ouabain. High-salt diets and HCTZ raise plasma EO by stimulating EO secretion, and a J-shaped curve relates sodium balance and EO in healthy men. Under normal dietary conditions, approximately 98% of the filtered load of EO is reabsorbed by the kidney, and differences in the circulating levels of EO are strongly influenced by secretion and urinary excretion of EO. The dramatic impact of high-salt diets on plasma EO is consistent with its proposed role as a humoral vasoconstrictor that links salt intake with vascular function in hypertension.     

Characterization of contractions in enzymatically isolated rat ventricular myocytes: effects of ouabain and rubidium

The role of sarcolemma and especially sodium pump activity in the control of phasic contractile activity of Ca2+ tolerant myocytes was studied using ouabain and rubidium as sodium pump inhibitors. Initially, ouabain increased both the amplitude of shortening and the frequency of phasic contractions. Later, the amplitude began to decline whereas the frequency of beating continued to rise, often terminating in a steady contracture of the myocyte. Rubidium caused a rapid rise of beating frequency, which reached its full effect within 1-5 min and remained steady after that. The stimulation of contraction frequency and the inhibition of Na+-K+ ATPase were correlated in the case of ouabain but not in the case of rubidium. The results suggest that the stimulation of phasic contractions may be caused by increased uptake of cellular calcium through Na+-Ca+ exchange as a consequence of sodium pump inhibition and (or) depolarization of the sarcolemma by ouabain and rubidium.     

Na+-K+ pump activation inhibits endothelium-dependent relaxation by activating the forward mode of Na+/Ca2+ exchanger in mouse aorta

The effect of Na+-K+ pump activation on endothelium-dependent relaxation (EDR) and on intracellular Ca2+ concentration ([Ca2+]i) was examined in mouse aorta and mouse aortic endothelial cells (MAECs). The Na+-K+ pump was activated by increasing extracellular K+ concentration ([K+]o) from 6 to 12 mM. In aortic rings, the Na+ ionophore monensin evoked EDR, and this EDR was inhibited by the Na+/Ca2+ exchanger (NCX; reverse mode) inhibitor KB-R7943. Monensin-induced Na+ loading or extracellular Na+ depletion (Na+ replaced by Li+) increased [Ca2+]i in MAECs, and this increase was inhibited by KB-R7943. Na+-K+ pump activation inhibited EDR and [Ca2+]i increase (K+-induced inhibition of EDR and [Ca2+]i increase). The Na+-K+ pump inhibitor ouabain inhibited K+-induced inhibition of EDR. Monensin (>0.1 microM) and the NCX (forward and reverse mode) inhibitors 2'4'-dichlorobenzamil (>10 microM) or Ni2+ (>100 microM) inhibited K+-induced inhibition of EDR and [Ca2+]i increase. KB-R7943 did not inhibit K+-induced inhibition at up to 10 microM but did at 30 microM. In current-clamped MAECs, an increase in [K+]o from 6 to 12 mM depolarized the membrane potential, which was inhibited by ouabain, Ni2+, or KB-R7943. In aortic rings, the concentration of cGMP was significantly increased by acetylcholine and decreased on increasing [K+]o from 6 to 12 mM. This decrease in cGMP was significantly inhibited by pretreating with ouabain (100 microM), Ni2+ (300 microM), or KB-R7943 (30 microM). These results suggest that activation of the forward mode of NCX after Na+-K+ pump activation inhibits Ca2+ mobilization in endothelial cells, thereby modulating vasomotor tone.     

Phorbol esters augment polyamine transport by influencing Na(+)-K+ pump in murine leukemia cells.

In this report, we elucidate the role of Na(+)-K+ pump in the regulation of polyamine spermidine (Spd) transport in murine leukemia (L 1210) cells in culture. Ouabain, known to bind extracellularly to the alpha-subunit of the Na(+)-K+ pump, inhibits the pump activity. The L 1210 cells were found to possess ouabain binding sites at 7.5 fmol/10(6) cells. Ouabain significantly inhibited the Spd uptake in a dose-dependent manner. The maximum inhibition of Spd uptake by ouabain was observed beyond 200 microM. Spd transport was inversely correlated with the [3H]ouabain binding to L 1210 cells: an increase in the saturation of ouabain binding to L 1210 cells resulted in a decrease of the Spd uptake process. Treatment of L 1210 cells with protein kinase C activator phorbol esters increased the Spd transport and, also, ouabain-sensitive 86Rb+ uptake, a measure of the activity of the Na(+)-K+ pump. H-7, a protein kinase C inhibitor, significantly inhibited the ouabain-sensitive 86Rb+ uptake by L 1210 cells. Phorbol esters stimulated the level, but not the rate, of 22Na+ influx. Addition of H-7 to L 1210 cells inhibited the 22Na+ influx process. A concomitant phorbol ester-induced increase in 22Na+ influx, [14C]Spd uptake, together with the functioning of Na(+)-K+ pump, indicates the role of the "Na+ cycle" in the regulation of the polyamine transport process.     

beta-Adrenergic effect on Na+-K+ transport in rat skeletal muscle.

1. Intact rat extensor digitorum longus muscles soaked in L-isoproterenol plus 10(-5) M ouabain gained less sarcoplasmic Na+ than did muscles soaked in ouabain alone. Half maximal effect was produced by 10(-8) M L-isoproterenol. 2. D-Isoproterenol and oxidized L-isoproterenol were only 3 and 1%, respectively, as potent as L-isoproterenol. Other catechols tested had no effect. 3. The effect of L-isoproterenol on sarcoplasmic Na+ content appears to be a beta-adrenergic function in that it was blocked by propranolol, but not by phentolamine, and could be mimicked by dibutyryl cyclic AMP or by caffeine. 4. Reduced gain in sarcoplasmic Na+ was accompanied by reduced loss of sarcoplasmic K+. 5. L-Isoproterenol increased loss of sarcoplasmic Na+ in the absence of ouabain, in muscles recovering from cold treatment. 6. Results suggest that the beta-adrenergic system stimulates a coupled Na-K+ pump. 7. A model is proposed in which stimulation of the Na+-K+ pump in response to beta-adrenergic agents involves a number of intermediate steps, identified tentatively.     

Sodium-potassium pump inhibitor in the mechanism of one-kidney, one wrap hypertension in dogs.

Using ouabain sensitive 86Rb uptake by the vessel wall, we previously showed that sodium-potassium pump activity is decreased in the arteries and veins, and that the sodium-potassium pump inhibitor (SPI) is increased in the plasma of dogs with one-kidney, one wrap (1-K, 1W) hypertension, a low renin model of hypertension. We also showed in rats with a similar type of hypertension that the membrane potential of vascular smooth muscle cells in arteries is decreased, and that this decrease can be reproduced in arterial cells in arteries from normal rats by applying plasma from the hypertensive animals. One endogenous SPI in human plasma has been reported to be ouabain or its isomer. In this study, we used a newly available Dupont ouabain enzyme immunoassay kit to examine plasma and kidneys for SPI in dogs with 1-K, 1W hypertension. We also examined 1) the inhibiting activity of plasma of Na+, K(+)-ATPase obtained from normal kidneys, and 2) the Na+, K(+)-ATPase activity of the kidneys from these hypertensive animals. 1-K, 1W hypertension was produced in dogs by wrapping the left kidney in a silk bag and removing the right kidney. The removed kidney was kept at -70 degrees C till assayed. After 4 weeks of hypertension, the remaining kidney was removed and stored at -70 degrees C till assayed. Blood samples were drawn before and at weeks 3 and 4 of hypertension. Plasma levels of "ouabain" and Na+, K(+)-ATPase inhibitory activity were increased at weeks 3 and 4 of hypertension, compared to pre-hypertension levels. Renal tissue "ouabain" levels were also increased at week 4 of hypertension. However, renal Na+, K(+)-ATPase activity was unchanged. These findings, using two different assays, confirm our 1980 conclusion that SPI is elevated in the plasma of dogs with 1-K, 1W hypertension. The absence of renal Na+, K(+)-ATPase inhibition, despite increased plasma and renal SPI in these animals, may have important implications for the development of this type of hypertension.     

Reversal by EGTA of the enhanced secretory responsiveness of mast cells due to treatment with ouabain

The effect of EGTA on the enhancement by ouabain of compound 48/80-induced secretion from mast cells was compared with the effect on the Na(+)-K+ pump activity. The time-dependent secretory enhancement by ouabain was blocked by addition of EGTA to the cell suspension concomitantly with the addition of ouabain, and EGTA caused a large increase in the pump activity. Addition of 10 microM EGTA to ouabain-treated cells stopped but did not reverse the enhancement. The experiments show that the effect of ouabain was due to changes in a calcium pool utilized in compound 48/80-induced secretion following changes in the Na+,K+ pump activity.     

Signaling mechanisms that link salt retention to hypertension: Endogenous ouabain,the Na+ pump,the Na+/Ca2+ exchanger and TRPC proteins

Salt retention as a result of chronic, excessive dietary salt intake, is widely accepted as one of the most common causes of hypertension. In a small minority of cases, enhanced Na⁺ reabsorption by the kidney can be traced to specific genetic defects of salt transport, or pathological conditions of the kidney, adrenal cortex, or pituitary. Far more frequently, however, salt retention may be the result of minor renal injury or small genetic variation in renal salt transport mechanisms. How salt retention actually leads to the increase in peripheral vascular resistance (the hallmark of hypertension) and the elevation of blood pressure remains an enigma. Here we review the evidence that endogenous ouabain (an adrenocortical hormone), arterial smooth muscle α2 Na⁺ pumps, type-1 Na/Ca exchangers, and receptor- and store-operated Ca²⁺ channels play key roles in the pathway that links salt to hypertension. We discuss cardenolide structure–function relationships in an effort to understand why prolonged administration of ouabain, but not digoxin, induces hypertension, and why digoxin is actually anti-hypertensive. Finally, we summarize recent observations which indicate that ouabain upregulates arterial myocyte Ca²⁺ signaling mechanisms that promote vasoconstriction, while simultaneously downregulating endothelial vasodilator mechanisms. In sum, the reports reviewed here provide novel insight into the molecular mechanisms by which salt retention leads to hypertension.     

Effect of inhibition of Na+-K+ ATPase on the prostacyclin generation of cultured human vascular endothelial cells

Prostacyclin (PGI2) generation of cultured human vascular endothelial cells (VEC) was observed coincidentally with the increase of 45Ca net influx. Ca ionophore A23187 enhanced not only PGI2 generation and 45Ca net influx but also 45Ca efflux. PGI2 generation was completely abolished by the pretreatment with Ca++ immobilizer, TMB-8. A Na+-K+ ATPase inhibitor, ouabain increased 45Ca net influx, but decreased 45Ca efflux, and enhanced PGI2 generation. These observation indicate that PGI2 generation of VEC may be regulated by not only Ca++ but also Na+, and it was suggested that enhanced PGI2 generation by ouabain might be derived from the increased cytosolic Ca++concentration by the decreased Ca++ efflux, and it was considered to be originated from the suppression of Na+-Ca++ exchange systems by the increased intracellular Na+ concentration via inhibition of Na+-K+ ATPase activity by ouabain. Enhancement of PGI2 generation of VEC by the increased ouabain like substances (OLS) in hypertension is suspected to be beneficial on the maintenance of vascular homeostasis.     

Organ hypertrophic signaling within caveolae membrane subdomains triggered by ouabain and antagonized by PST 2238

In addition to inhibition of the Na-K ATPase, ouabain activates a signal transduction function, triggering growth and proliferation of cultured cells even at nanomolar concentrations. An isomer of ouabain (EO) circulates in mammalians at subnanomolar concentrations, and increased levels are associated with cardiac hypertrophy and hypertension. We present here a study of cardiac and renal hypertrophy induced by ouabain infused into rats for prolonged periods and relate this effect to the recently described ouabain-induced activation of the Src-EGFr-ERK signaling pathway. Ouabain infusion into rats (15 microg/kg/day for 18 weeks) doubled plasma ouabain levels from 0.3 to 0.7 nm and increased blood pressure by 20 mm Hg (p < 0.001), cardiac left ventricle (+11%, p < 0.05), and kidney weight (+9%, p < 0.01). These effects in vivo are associated with a significant enrichment of alpha1, beta1, gammaa Na-K ATPase subunits together with Src and EGFr in isolated renal caveolae membranes and activation of ERK1/2. In caveolae, direct Na-K ATPase/Src interactions can be demonstrated by co-immunoprecipitation. The interaction is amplified by ouabain, at a high affinity binding site, detectable in caveolae but not in total rat renal membranes. The high affinity site for ouabain is associated with Src-dependent tyrosine phosphorylation of rat alpha1 Na-K ATPase. The antihypertensive compound, PST 2238, antagonized all ouabain-induced effects at 10 microg/kg/day in vivo or 10(-10)-10(-8) m in vitro. These findings provide a molecular mechanism for the in vivo pro-hypertrophic and hypertensinogenic activity of ouabain, or by analogy those of EO in humans. They also explain the pharmacological basis for PST 2238 treatment.     

Effects of 20-HETE on Na+ transport and Na+ -K+ -ATPase activity in the thick ascending loop of Henle

Previous studies have indicated that 20-hydroxyeicosatetraenoic acid (20-HETE) inhibits Na+ transport in the medullary thick ascending loop of Henle (mTALH), but the mechanisms involved remain uncertain. The present study compared the effects of 20-HETE with those of ouabain and furosemide on intracellular Na+ concentration ([Na+]i), Na+ -K+ -ATPase activity, and 86Rb+ uptake, an index of Na+ transport, in mTALH isolated from rats. Ouabain (2 mM) increased, whereas furosemide (100 microM) decreased, [Na+]i in the mTALH of rats. Ouabain and furosemide inhibited 86Rb+ uptake by 91 and 30%, respectively. 20-HETE (1 microM) had a similar effect as ouabain and increased [Na+]i from 19 +/- 1 to 30 +/- 1 mM. 20-HETE reduced Na+ -K+ -ATPase activity by 30% and 86Rb+ uptake by 37%, but it had no effect on 86Rb+ uptake or [Na+]i in the mTALH of rats pretreated with ouabain. 20-HETE inhibited 86Rb+ uptake by 12% and increased [Na+]i by 19 mM in mTALH pretreated with furosemide. These findings indicate that 20-HETE secondarily inhibits Na+ transport in the mTALH of the rat, at least, in part by inhibiting the Na+ -K+ -ATPase activity and raising [Na+]i.     

Role of extracellular Ca2+ in diaphragmatic contraction: effects of ouabain, monensin, and ryanodine.

The effects of zero extracellular Ca2+ on the contractility of rat diaphragmatic strips in vitro were studied in conjunction with various pharmacological agents known to influence the intracellular Ca2+ concentration: the Na+ ionophore, monensin, and the Na(+)-K+ pump inhibitor, ouabain, which enhance [Ca2+]i, caffeine, which induces Ca2+ release from the sarcoplasmic reticulum (SR), and ryanodine, which prevents Ca2+ retention by the SR. The effect of increasing [Ca2+]i on diaphragmatic contraction was assessed by comparing contractions induced by 120 mM K+ in the small muscle strips before and after the addition of ouabain or monensin. Monensin (20 microM) and ouabain (1-100 microM) augmented contractions up to threefold. Treatment of diaphragm strips with 3 nM ryanodine increased baseline tension 360% above the original resting tension but only if the diaphragm was electrically stimulated concurrently; 100 microM ryanodine induced contracture in quiescent tissue. High K+ contractures were of greater magnitude in the presence of ryanodine compared with control, and relaxation time was prolonged by greater than 200%. Ca(2+)-free conditions ameliorated these actions of ryanodine. Ryanodine reduced contractions induced by 10 mM caffeine and nearly abolished them in Ca(2+)-free solution. The data demonstrate that extracellular Ca2+ is important in certain types of contractile responses of the diaphragm and suggest that the processes necessary to utilize extracellular Ca2+ are present in the diaphragm.     

Ouabain,endogenous ouabain and ouabain-like factors: The Na+ pump/ouabain receptor,its linkage to NCX,and its myriad functions

In this brief review we discuss some aspects of the Na⁺ pump and its roles in mediating the effects of ouabain and endogenous ouabain (EO): i) in regulating the cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) via Na/Ca exchange (NCX), and ii) in activating a number of protein kinase (PK) signaling cascades that control a myriad of cell functions. Importantly, [Ca²⁺]_CYT and the other signaling pathways intersect at numerous points because of the influence of Ca²⁺ and calmodulin in modulating some steps in those other pathways. While both mechanisms operate in virtually all cells and tissues, this article focuses primarily on their functions in the cardiovascular system, the central nervous system (CNS) and the kidneys.     

An endogenous digitalis-like compound extracted from human urine: biochemical and chemical studies

Plasma and urine levels of an endogenous digitalis-like compound (EDLC) are increased in low renin Na+-dependent experimental hypertension, in some normotensive offspring of hypertensive patients and in some essential hypertensive patients. Urine-drived EDLC was purified from 550 L of urine from essential hypertensive patients (n = 8) and from normotensive subjects with a family history of hypertension (n = 27), using flash chromatography on C18 reversed-phase, anion exchange chromatography and various reversed-phase high performance liquid chromatographies. The mechanism of Na+-K+ ATPase inhibition and the related effects of semipurified urine-derived EDLC were studied and compared with those of ouabain. Its action was similar to that of ouabain in 8 out of 10 of the tests applied. The main effects of such a compound were the depression of Na+-K+ pump activity of human erythrocytes, the inhibition of 5-hydroxytryptamine reuptake by human platelets, and the induction of natriuresis in urethanized rats. Therefore, EDLC may be considered as one of the natriuretic hormones whose mechanism of action closely resembles that of ouabain.