Endogenous digitalis-like factors in hypertension and chronic renal insufficiency

Na+/K+-ATPase DR region antibody ameliorated cardiac hypertrophy and fibrosis in rats with 5/6 nephrectomy

The enzyme Na⁺/K⁺-ATPase (NKA) is important in the heart. Reductions in NKA activity and expression have often been observed in chronic kidney disease (CKD)-related heart injury. Previously, our group found that an antibody targeting the NKA1α1 subunit's DR extracellular region (⁸⁹⁷DVEDSYGQQWTYEQR⁹¹¹) stimulated NKA activities and produced cardioprotective effects against ischemic injury and isoproterenol-induced cardiac remodeling. In here, we assessed whether DRm217, a specific DR antibody, exhibits cardioprotective effects in chronic renal failure models. In 5/6 nephrectomy (5/6 Nx) surgery to mimic CKD in Sprague Dawley rat, we observed that NKA activity and expression were depressed in the hearts of 5/6 Nx rats. DRm217, an NKA DR region antibody, alleviated heart hypertrophy and cardiac fibrosis under 5/6 Nx conditions. Further studies revealed that DRm217 inhibited Src activation and reduced reactive oxygen species (ROS) levels in hearts under 5/6 Nx conditions. Our findings imply that NKA could be a treatment target in CKD-related cardiac diseases. Prevention of CKD-induced myocardial injury by DRm217 provides an appealing therapeutic alternative.

The role of autophagy in death of cardiomyocytes

Autophagy mediates cellular quality control mechanisms and energy homeostasis through lysosomal degradation. Autophagy is typically viewed as an adaptive process that allows cells to survive against stress, such as nutrient deprivation and hypoxia. However, autophagy also mediates cell death during development and in response to stress. Cell death accompanied by autophagy activation and accumulation of autophagosomes has been classified as type II programmed cell death. Compared to the wealth of knowledge regarding the adaptive role of autophagy, however, the molecular mechanisms through which autophagy induces cell death and its functional significance are poorly understood. Autophagy is activated excessively under some conditions, causing uncontrolled degradation of cellular materials and cell death. An imbalance between autophagosome formation and lysosomal degradation causes a massive accumulation of autophagosomes, which subsequently causes cellular dysfunction and death. Dysregulation of autophagy induces a unique form of cell death, termed autosis, with defined morphological and biochemical features distinct from other forms of programmed cell death, such as apoptosis and necrosis. In the heart, dysregulated autophagy induces death of cardiomyocytes and actively mediates cardiac injury and dysfunction in some conditions, including reperfusion injury, doxorubicin cardiomyopathy, and lysosomal storage disorders. The goal in this review is to introduce the concept of autophagic cell death and discuss its functional significance in various cardiac conditions.

The Role of Na, K-Atpase Inhibitors in Hypertension and End-Stage Renal Disease

Objective

To review the role of Na, K-ATPase inhibitors in the pathogenesis of essential hypertension and hypertension associated with end-stage renal disease.

Data Sources

MEDLINE search, 1966 to 1997.

Results

There is a suggestive physiologic and epidemiologic relationship between Na, K-ATPase inhibition and hypertension. However, clearance data cannot support the hypothesis that differential metabolism of this family of compounds explains the improved hypertensive control seen in patients on peritoneal dialysis compared to those on hemodialysis.

Conclusions

As a result of the complex methodologies involved, it is unclear whether Na, K-ATPase inhibitors playa significant role in the hypertension of endstage renal disease in general and peritoneal dialysis in particular.

Augmented Cardiopulmonary Baroreflex Sensitivity in Intradialytic Hypertension

Introduction

End-stage renal disease (ESRD) patients with a paradoxical increase in blood pressure (BP) during hemodialysis (HD), termed intradialytic hypertension (ID-HTN), are at significantly increased risk for mortality and adverse cardiovascular events. ID-HTN affects up to 15% of all HD patients, and the pathophysiologic mechanisms remain unknown. We hypothesized that ESRD patients prone to ID-HTN have heightened volume-sensitive cardiopulmonary baroreflex sensitivity (BRS) that leads to exaggerated increases in sympathetic nervous system (SNS) activation during HD.

Methods

We studied ESRD patients on maintenance HD with ID-HTN (n = 10) and without ID-HTN (controls, n = 12) on an interdialytic day, 24 to 30 hours after their last HD session. We measured continuous muscle sympathetic nerve activity (MSNA), beat-to-beat arterial BP, and electrocardiography (ECG) at baseline, and during graded lower body negative pressure (LBNP). Low-dose LBNP isolates cardiopulmonary BRS, whereas higher doses allow assessment of physiologic responses to orthostatic stress.

Results

The ID-HTN patients had significantly higher pre- and post-HD BP, and greater interdialytic fluid weight gain compared to controls. There was a significantly greater increase in MSNA burst incidence (P = 0.044) during graded LBNP in the ID-HTN group, suggesting heightened cardiopulmonary BRS. The ID-HTN group also had a trend toward increased diastolic BP response during LBNP, and had significantly greater increases in BP during the cold pressor test.

Conclusion

Patients with ID-HTN have augmented cardiopulmonary BRS that may contribute to increased SNS activation and BP response during HD.

Cardiovascular dysfunctions and sympathovagal imbalance in hypertension and prehypertension: physiological perspectives

Hypertension (HTN) and prehypertension (pre-HTN) have been identified as independent risk factors for adverse cardiovascular events. Recently, increased psychosocial stress and work stress have contributed to the increased prevalence of HTN and pre-HTN, in addition to the contribution of obesity, diabetes, poor food habits and physical inactivity. Irrespective of the etiology, sympathetic overactivity has been recognized as the main pathophysiologic mechanism in the genesis of HTN and pre-HTN. Sympathovagal imbalance owing to sympathetic overactivity and vagal withdrawal is reported to be the basis of many clinical disorders. However, the role played by vagal withdrawal has been under-reported. In this review, we have analyzed the pathophysiologic involvement of sympathovagal imbalance in the development of HTN and pre-HTN, and the link of sympathovagal imbalance to cardiovascular dysfunctions. We have emphasized that adaptation to a healthier lifestyle will help improve sympathovagal homeostasis and prevent the occurrence of HTN and pre-HTN.

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.

Renal insufficiency as a predictor of adverse events and mortality after renal artery stent placement

BACKGROUND

Renal artery stenosis (RAS) is associated with substantial morbidity and mortality. This relationship is strongest in the presence of renal insufficiency (RI). The goal of this study is to assess the relationship between RI, mortality, and adverse events in the setting of renovascular disease.

METHODS

Two hundred sixty-one patients with significant RAS treated with endovascular stenting were followed up prospectively for 21 +/- 18 months (range, 0 to 85 months). Blood pressure (BP), renal function, medication use, and comorbidities were assessed. Death was verified through the Social Security Death Master Index File, and cause of death was derived from death certificates. Medical records of 230 subjects subsequently were reviewed to identify adverse cardiovascular and renal events.

RESULTS

Overall, 37% of patients experienced at least 1 adverse event postprocedure. Myocardial infarction (MI) and congestive heart failure (CHF) events increased with degree of baseline RI. Seventy-three deaths (28%) occurred postprocedure (range, 13 to 2,457 days). Baseline characteristics associated with mortality included advanced age, decreased use of beta-blockers, increased use of diuretics, increased serum creatinine (Cr) level, decreased Cr clearance (CrCl), bilateral stenoses or stenosis of a solitary kidney, history of CHF, and history of MI. Follow-up characteristics associated with mortality included lower systolic and diastolic BP, increased serum Cr level, and decreased CrCl. RI at baseline and follow-up remained associated with mortality after adjusting for other clinically and statistically significant variables. Patients in whom renal function improved after stenting appeared to show improved survival over those without improved renal function (45% versus 0% cumulative survival, P < 0.05).

CONCLUSION

In patients with RAS undergoing stent therapy, baseline RI is associated with an increased incidence of adverse events, as well as decreased survival, independent of other baseline clinical factors. Importantly, improvement in renal function appears to be associated with increased survival.

Serum Potassium Handling at Pre- and Posthemodialysis in Patients with End-Stage Renal Disease

The current study simultaneously measured serum and red blood cell (RBC) K+ as well as plasma pH, bicarbonate, serum insulin, and aldosterone at pre-, end-, and at 5, 11, and 19 hours posthemodialysis in 25 patients with end-stage renal disease and evaluated the factors influencing serum K+ levels during those periods. Our patients were studied under their current dietary conditions. At end-dialysis, serum K+ levels decreased, but RBC K+ levels were not changed. At this time point, the decrease in serum K+ levels was exclusively caused by removal of K+ by dialysis. At 5 hours postdialysis, serum K+ rapidly increased, whereas RBC K+ decreased. The 5 hour postdialysis serum K+ increase negatively correlated with the decrease in serum insulin levels. At 11 hours postdialysis, serum K+ slowly but significantly increased, and RBC K+ increased to those levels at pre- and end-dialysis. At 19 hours postdialysis, serum K+ further increased, but RBC K+ remained stable. From 5 to 19 hours postdialysis, the increase in serum K+ was independent of changes in plasma pH, bicarbonate, insulin, and aldosterone but was associated with both the predialysis serum K+ levels and the magnitude of the decrease in serum K+ at end-dialysis.

Effects of urea on taurine efflux and cell volume in incubated rat cerebral cortical minislices

The effects of urea on the rate of efflux of preloaded taurine and volume regulation have been examined in incubated minislices from rat superficial cerebral cortex. As external urea was increased in the range 0-100 mmol/L, there was a concentration-dependent slowing of cellular taurine efflux. Cell volumes progressively increased over the range 0-50 mmol/L urea, but decreased slightly in 100 mmol/L. Urea had no effect on cell volume in the absence of taurine. Retardation of efflux, and cell swelling in the presence of 50 mmol/L urea were entirely abolished by trimethylamine (100 mumol/L). TMA had no effect on either variable in the absence of urea. It is suggested that impaired loss of taurine and accompanying cell swelling may be factors contributing to the neurological disturbances accompanying uremia.

Physiological and pharmacological properties of an endogenous sodium pump inhibitor

To investigate on Na+, K+-ATPase behavior in chronic uremia, pre and postdialysis serum from 10 chronic dialysis patients and 10 healthy subjects was pooled and subjected to reverse phase C-18 HPLC. Only one fraction, isolated from pre and postdialysis sera, eluting at 28 min (F1), was found to display significant effects on electrophysiological and transepithelial 22Na flux pattern of rabbit distal colon mucosa mounted in Ussing type chambers; indeed, serosal addition of uremic F1 to colonic mucosa resulted in a slow, but constant, decline in short-circuit current (Isc) (deltaIsc = 1.55+/-0.16 microEq h(-1) cm(-2), mean +/- S.E.M., n=12, p<0.01) and transepithelial conductance (G(T)) (from 4.50+/-0.23 to 3.71+/-0.33 mS cm(-2), p<0.01, n=12). Measurement of transepithelial 22Na fluxes in the presence of pre or postdialysis sera also showed a significant Na+ absorption rate decrease (from 1.3+/-0.22 to 0.48+/-0.30 microEq h(-1) cm(-2), mean +/- S.E.M., n=4, p<0.01), mainly due to a decrease in mucosal-to-serosal Na+ flux. By contrast, assays of peaks isolated from healthy sera did not inhibit Isc and transepithelial Na+ transport. The incubation of highly purified basolateral membranes with F1 for 1 min produced a approximately 26% inhibition of Na+, K+-ATPase. These findings are consistent with the presence of an endogenous inhibitor of sodium pump activity in uremic plasma; it is of pharmacological interest in that it may participate in the development of unpredictable responsiveness to digitalis therapy in pathophysiologic states.

Increased lymphocytic Na+/H+ exchange activity after hemodialysis: evidence for an endogenous inhibitor of Na+/H+ exchange in patients with end-stage renal failure

The Na+/H+ exchange antiport activity was measured in lymphocytes from 16 patients with end-stage renal failure pre- and postdialysis. In addition the effect of the patients' plasma on lymphocytes from healthy subjects was tested. Resting pH (pHi) was not significantly different in lymphocytes pre- and postdialysis. On the other hand, the Na+/H+ exchange activity was significantly lower in lymphocytes before hemodialysis (6.22 +/- 0.73 x 10(-3) pHi/s) than after hemodialysis (9.32 +/- 1.58 x 10(-3) pHi/s; n = 16; p < 0.05). The buffer capacity was not significantly different before and after hemodialysis. The incubation of lymphocytes from healthy control subjects with plasma from patients with end-stage chronic renal failure significantly reduced the lymphocytic Na+/H+ exchange activity. The addition of ultrafiltrate also significantly reduced the Na+/H+ exchange activity in lymphocytes from healthy control subjects. The study indicates the existence of an endogenous inhibitor of the Na+/H+ exchange that is accumulated in plasma from patients with end-stage chronic renal failure.

Altered Na(+)-K+ ATPase activity in uraemic adolescents

The Na(+)-K+ ATPase enzyme plays an essential role in the regulation of cell composition and volume. Enzyme activity itself is regulated by substrate availability and several hormones. In adult uraemic patients red blood cell Na(+)-K+ ATPase activity is decreased. However, it is unknown if children with uraemia exhibit the same phenomenon. Therefore, in the present study we examined whether endogenous digoxin-like factors (EDLF) and physicochemical membrane properties play a role in the regulation of erythrocyte Na(+)-K+ ATPase activity in uraemic children and adolescents. Healthy age-matched children were used as controls. Enzyme activity was measured in detergent-pretreated red blood cells and erythrocyte ghosts. Na(+)-K+ ATPase activity (2204 +/- 538 nmol Pi ml erythrocyte-1 h-1 in detergent pretreated erythrocytes; 204 +/- 56 nmol Pi mg protein-1 h-1 in ghosts) in adolescents with uraemia was lower compared to controls (3245 +/- 362 nmol Pi ml erythrocyte-1 h-1; 266 +/- 37 nmol Pi mg protein-1 h-1, p < 0.001, p < 0.05, respectively). Plasma levels of EDLF were elevated in uraemic patients (0.30 +/- 0.05 versus 0.21 +/- 0.04 ng ml-1, p < 0.01). Furthermore, the membrane lipid component was decreased in patients with uraemia, while the cholesterol/phospholipid ratio and membrane fluidity were similar in both groups. No correlation was found between the decrease in Na(+)-K+ ATPase and the increase in EDLF concentration and altered membrane lipid components. Our results demonstrate, that similar to the findings of adults, the activity of Na(+)-K+ ATPase is diminished in uraemic adolescent patients, and that uraemia-associated elevation in EDLF and altered membrane components do not play a role in the down-regulation of Na(+)-K+ ATPase. Therefore other factors (presence of other inhibitors and/or reduced number of enzyme molecules) should contribute to the lower activity of the Na(+)-K+ pump.

Mechanisms that cause protein and amino acid catabolism in uremia

Anorexia and/or a protein- and calorie-restricted diet can cause protein wasting by limiting the intake of essential amino acids (EAA) and, hence, protein synthesis. By this mechanism plus the effects of inadequate calories, restricted diets could contribute to the loss of lean body mass of uremic patients. Uremia also impairs the normal metabolic responses that must be activated to preserve body protein, thereby augmenting the adverse effects of anorexia. The responses impaired are those that conserve EAA and protein, which results in catabolism of EAA and muscle protein. An important factor that initiates abnormal adaptive responses in uremia is metabolic acidosis, because acidosis stimulates muscle protein degradation and increases the activity of branched-chain ketoacid dehydrogenase and, hence, the catabolism of branched-chain amino acids (BCAA). The effects of acidosis could be mediated by impaired regulation of intracellular pH and/or an increase in glucocorticoid production. Research directed at identifying the specific proteolytic pathways that are activated by metabolic acidosis has excluded a major role for Ca(2+)-activated or lysosomal proteases and suggests activation of an adenosine triphosphate (ATP)- and ubiquitin-dependent proteolytic pathway. The mechanism of activation of this pathway includes an increase in mRNA for enzymes involved in protein and amino acid catabolism.

A circulating inhibitor of the RBC membrane calcium pump in chronic renal failure

A humoral inhibitor of the membrane calcium pump was studied in plasma from 28 normal controls, 33 patients receiving long-term hemodialysis, and 26 with chronic renal failure (CRF; creatinine clearance range was 6 to 97 ml/min). Calcium pump activity was measured as the rate of Sr2+ efflux in normal erythrocytes (RBCs) loaded with Sr2+ (a substitute of Ca2+ in the calcium pump). Plasma, and plasma ultrafiltrates from hemodialysis patients strongly inhibited calcium pump activity compared with controls without plasma (36 +/- 18 vs. 25 +/- 12, %INHIBITION/CONTROL, P < 0.05). Inhibition markedly decreased with acute hemodialysis (16 +/- 12 vs. 5 +/- 14, %INHIBITION/NORMAL PLASMA, N = 15, P < 0.001). In CRF, degree of inhibition correlated with the serum creatinine concentration (r = 0.75, P < 0.001). A kinetic study showed that plasma decreased the maximal rate of the Ca2+ pumps (Vmax) without affecting the apparent affinity for internal cations (KSr). Moreover, the plasma inhibitory factor had a low molecular weight, and was dialyzable and heat stable. In conclusion, we found evidence for an RBC membrane calcium pump inhibitor in uremic plasma, which correlates with the degree of renal insufficiency. Possibly, it may increase calcium content in RBCs and other cells and could thus be related to uremic toxicity and/or hypertension.

The contribution of steroids to digoxin-like immunoreactivity in cord blood

Samples of cord serum from 29 healthy neonates were analysed for digoxin-like immunoreactive substance (DLIS), cortisol, 17 beta-oestradiol, progesterone, dehydroepiandrosterone-sulphate (DHEAS), 17 alpha-hydroxyprogesterone (17OHP), androstenedione, oestriol and ouabain-like activity (OLA; by inhibition of Na+, K+ATPase activity). The mean serum concentration of DLIS was 0.91 (SD = 0.19) nmol/L and the mean OLA was 26.1 (SD = 11.5) nmol/L. There was no correlation between DLIS and OLA. DLIS correlated significantly with oestriol (r = 0.521), progesterone (r = 0.534) and 17OHP (r = 0.43). Stepwise multiple regression analysis showed that 17 beta-oestradiol, progesterone and androstenedione contributed to DLIS and the intercept was 0.64 (SD = 0.127). The concentrations of steroids (17 beta-oestradiol, progesterone, androstenedione) required to displace digoxin by 50% in the digoxin immunoassay and inhibit Na+,K+ATPase in the OLA assay were 10(3)-10(4)-fold higher than those found in cord serum. We conclude that the contribution of these steroids to DLIS is small and that DLIS and OLA measure different compounds.

Reduction of erythrocyte (Na(+)-K+) ATPase activities in non-insulin-dependent diabetic patients with hyperkalemia

To elucidate the mechanism of hyperkalemia in diabetic patients without renal failure, we investigated (Na(+)-K+) adenosine triphosphatase (ATPase) activity in erythrocyte membrane, erythrocyte Na+ and K+ content, and plasma endogenous digitalis-like substance in control subjects (n = 16) and non-insulin-dependent diabetes mellitus (NIDDM) patients (n = 62). NIDDM patients were divided into normokalemic patients (NKDM, n = 48) and hyperkalemic patients (HKDM, n = 14). There was no difference in plasma glucose or hemoglobin A1c (HbA1c) levels, plasma renin activity (PRA), and plasma aldosterone concentrations (PAC) between NKDM and HKDM patients. (Na(+)-K+)ATPase activities in NIDDM patients were significantly reduced compared with those in control subjects (0.336 +/- 0.016 mumol-inorganic phosphate [Pi]/mg protein/h, mean +/- SEM, P less than .05), and (Na(+)-K+)ATPase activities in HKDM patients (0.243 +/- 0.015 mumol Pi/mg protein/h) were significantly reduced compared with those in NKDM patients (0.295 +/- 0.008 mumol Pi/mg protein/h, P less than .01). Plasma K+ content had a significant negative correlation with (Na(+)-K+)ATPase activity in diabetic patients (r = -.365, P less than .01). Erythrocyte Na+ content had a significant negative correlation with (Na(+)-K+)ATPase activity in control subjects (r = -.619, P less than .05). There was no difference in plasma endogenous digitalis-like substance among the three groups. (Na(+)-K+)ATPase activity was not significantly correlated with plasma endogenous digitalis-like substance in control subjects and diabetic patients. These findings suggest that the reduction of (Na(+)-K+)ATPase activity, which was not related to plasma digitalis-like substance, may be partly responsible for hyperkalemia in diabetic patients.

Elevated endogenous digitalis-like substance in hypertensive diabetic patients with a family history of hypertension

Endogenous digital-like substance (DLS) is increased in patients with essential hypertension and is hypothesized to play a role in the pathogenesis of high blood pressure. Whether an increase in DLS in diabetic patients with hypertension is associated with a family history of hypertension or diabetic nephropathy was investigated. Plasma DLS was measured as Na(+)-K(+)-ATPase inhibitory activity (ATPI) in 100 Type 2 diabetic patients. Ouabain was used as a standard of Na-K-ATPase inhibition. Diabetic patients with hypertension demonstrated a greater ATPI level than normotensive diabetic patients (p less than 0.05). In patients with hypertension groups, the positive family history group had a higher ATPI level than the negative family history group (p less than 0.01). Microalbuminuria was not correlated with the ATPI level in diabetic patients. These results suggest that ATPI might play a role in the pathogenesis of hereditary hypertension associated with diabetes mellitus, but not have etiologic significance in diabetic nephropathy.

Effects of acute and chronic uremia on active cation transport in rat myocardium

As abnormalities of active cation transport could contribute to the genesis of uremic cardiomyopathy, we investigated myocardial sodium pump function in rats with acute renal failure (ARF) and with a model of experimental chronic renal failure (CRF) that has metabolic similarities to advanced chronic uremia in humans. CRF rats were hypertensive and had left ventricular hypertrophy (33% higher heart:body weight ratio; P less than 0.01) at four weeks compared to pair-fed sham-operated rats. Importantly, both ouabain- and furosemide-sensitive 86Rb uptake rates were unchanged in left ventricular myocardial slices from CRF, and the intracellular sodium concentration was not different from that of control rats even though skeletal muscle sodium was increased, as we found previously (J Clin Invest 81:1197, 1988). Insulin-stimulated, ouabain-sensitive 86Rb influx was also preserved. There also were no abnormalities in myocardium cation transport in rats with ARF. However, [3H]ouabain binding was decreased 45% in CRF rats (P less than 0.01); it was unchanged in acute uremia. Decreased ouabain binding in chronic uremia was due entirely to fewer low affinity [3H]ouabain binding sites (the binding affinity for ouabain was unaffected). We conclude that in chronic, (but not acute) renal failure, sodium pump number is reduced in myocardium but intracellular sodium is unchanged and active cation flux rates are maintained. These results emphasize that in rats with chronic uremia, intracellular sodium homeostasis is preserved in myocardium, despite the presence of marked abnormalities of active cation transport in skeletal muscle that are characteristic of chronic uremia.

Modulatory effect of some steroid hormones, their glucuronides and ouabain-like compounds on Cavia cobaya kidney Na+,K(+)-ATPase activity

1. Ouabain-like compounds (approx. mol. wt 700, 2,000 and 4,000 Da) were purified from plasma of essential hypertensive patients. 2. Dose-response experiments performed with (a) steroid hormones, (b) their glucuronides and (c) ouabain-like compounds, emphasize a modulatory effect [activation of the Na,K-ATPase at very low concentrations of ligand, inhibition at higher levels; apparent Ki: (a) between 1 and 0.5 mM; (b) between 1 and 0.5 microM; and (c) between 10 and 1 nM; maximum enhancement of the enzymatic activity: (a) +20%; (b) +45%; and (c) +100%]. 3. Displacement experiments of [3H]ouabain evidence a high competition of the ligands towards the cardioglycoside. The relative I50s are: (a) between 1 and 0.5 mM; (b) between 10 and 1 microM; and (c) between 10 and 0.01 nM.

Lipids as endogenous Na,K-ATPase inhibitors in plasma of healthy individuals and in dialysis dependent patients

We assayed plasma Na,K-ATPase inhibitory activity due to total lipids and lipid fractions. The effect of dialysis on the Na,K-ATPase inhibitory activity was also studied. Plasma lipid extracts from 11 healthy volunteers and 9 dialysis-dependent patients (pre and post dialysis) were separated into neutral lipids and phospholipids. Further fractionation was by thin layer chromatography. These lipid fractions were analyzed for Na,K-ATPase inhibitory activity by displacement of [3H]-ouabain from hog brain Na,K-ATPase. Total inhibitory activity was significantly increased (p less than 0.001) in the post-dialysis plasma compared to pre-dialysis plasma of the same patient group and to controls (482, 85 and 78 nmol/L respectively; means of the groups in digoxin equivalents). The major inhibitory activity was associated with non-esterified fatty acids with modest contributions from four other lipid fractions. Our results show that endogenous lipids are major plasma Na,K-ATPase inhibitors in vitro under these assay conditions.

Review: mechanisms for abnormal protein metabolism in uremia

Loss of protein stores, reflected by negative nitrogen balance and accelerated accumulation if nitrogenous breakdown products, is an important factor in the morbidity of chronic renal failure and the high mortality rate of acute renal failure. Low protein intake intensifies the suppressed protein synthesis that results from impaired insulin-stimulated protein anabolism. The metabolic acidosis of uremia contributes to tissue loss, both by increasing muscle protein degradation, and by raising the requirements for essential amino acids. Correcting metabolic acidosis improves the nitrogen balance and reduces tissue wasting. It is important to ensure adequate nutrient intakes, rather than the low protein diet often prescribed to slow loss of renal function.

The search for the endogenous digitalis: an alternative hypothesis

The universal presence of a binding site for cardiac glycosides on Na+-K+-ATPase has engendered speculation as to whether it also serves as a receptor for an endogenous digitalis-like hormone or autacoid. If such a hormone were to exist, it could play a role in sodium homeostasis and in the pathophysiology of primary hypertension and uremia. However, we believe that this hypothesis rests on unproven assumptions. Although typical of many toxins and drugs, binding to a single protein that acts as both its receptor and effector mechanism at the cell membrane, thereby directly affecting transmembrane ion flux, would be unusual for a hormone or autacoid. As an alternative hypothesis for the evolutionary conservation of the cardiac glycoside binding site, we suggest that its endogenous ligand may exist within the cell. After cotranslational insertion of the alpha- and beta-subunits into the membrane of the rough endoplasmic reticulum, Na+-K+-ATPase, like most integral membrane proteins, 1) must be targeted through a complex network of intracellular organelles to the correct plasmalemmal domain, 2) must be monitored for appropriate protein conformation and subunit assembly, and perhaps 3) could have its catalytic function regulated before insertion in the cell membrane. Because the lumina of the endoplasmic reticulum, Golgi, and other organelles and vesicles are topologically equivalent to the outside of the cell, all three functions could be subserved by an intraorganellar ligand for the cardiac glycoside binding site.

Digitalislike immunoreactive substances and extracellular fluid volume status in chronic hemodialysis patients

Circulating digitalislike immunoreactive substances (DLIS) may represent a class of volume-sensitive natriuretic factors. Chronic renal failure patients are known to have elevated levels of circulating natriuretic activity and also to have detectable DLIS. Using digoxin radioimmunoassay, DLIS levels were measured in desalted, deproteinized plasma from 15 stable hemodialysis patients. Predialysis DLIS was 109.3 +/- 6.3 pg/mL (digoxin equivalents) and decreased to 97.5 +/- 5.9 pg/mL following dialysis (P less than 0.001). Predialysis DLIS correlated with weight gain from the prior dialysis (P less than 0.01). The degree of extracellular fluid volume expansion predialysis also correlated with predialysis DLIS (P less than 0.01). Postdialysis DLIS also correlated with postdialysis extracellular fluid volume status (P less than 0.01). DLIS levels in dialysis patients were higher than in 50 normal subjects (30.0 +/- 1.2 pg/mL; P less than 0.001). Also, the changes in DLIS with dialysis were paralleled by similar changes in simultaneously measured human alpha-atrial natriuretic peptide (ANP) levels. These results demonstrate that (1) DLIS levels are higher in hemodialysis patients than in normal individuals; (2) with dialysis, DLIS levels increase and decrease in association with extracellular fluid volume expansion and removal, respectively; (3) DLIS levels correlate with the degree of extracellular fluid volume expansion in dialysis patients; and (4) DLIS levels change in parallel with levels of another class of natriuretic factor ANP. These characteristics are consistent with the hypothesis that DLIS represents a volume-sensitive factor.

Hemodialysis and red cell cation transport in uremia: role of membrane free fatty acids

Active and facilitated cation transport in erythrocytes of uremic patients may be improved acutely by hemodialysis, although the mechanisms remain unknown. As nonesterified fatty acids (NEFA) can affect Na+ pump activity in vitro, changes in plasma and red cell membrane NEFA content following a single hemodialysis procedure were examined and compared with acute changes in erythrocyte cation flux rates in 34 hemodialysis patients. In nonsodium-loaded cells, small changes in Na+ pump flux with dialysis did correlate with changes in intracellular Na+ content (r = 0.59; N = 17; P less than 0.01). On average, neither maximal Na+ pump activity nor Na+/Li+ counter-transport flux improved with dialysis, but Na+/K+/Cl- cotransport rates rose 25% post-dialysis (P less than 0.02). Plasma NEFA levels rose 87% following hemodialysis but erythrocyte membrane NEFA content declined by 23% (P less than 0.001). Importantly, 24 of the 34 subjects studied had a decrease in erythrocyte membrane NEFA content of greater than 10%, and in these patients, the fall in membrane NEFA correlated with an increase in ouabain-sensitive Na+ efflux (r = 0.564; P less than 0.01). The effects of hemodialysis on both erythrocyte NEFA content and Na+ pump flux could be reproduced by incubating pre-dialysis cells in fatty acid-free albumin. We conclude that acute changes in membrane NEFA may modulate active cation transport in uremic erythrocytes.

Endogenous digoxin-like material in normotensive and hypertensive strains of rats

We measured the concentration of endogenous digoxin-like materials (EDLM) in the serum of spontaneously hypertensive rats (SHR) and three normotensive rat strains at four stages during growth using a sensitive RIA. In the SHR, there was a significant peak in the EDLM level between 0.057-0.087 ngE/mL at 6 to 8 weeks of age, shortly after the onset of hypertension. The EDLM concentration returned to normal levels by 20 weeks of age. Sprague-Dawley and Wistar-Kyoto rats had EDLM levels below 0.050 ngE/mL at all time points studied. In contrast, Fischer 344 rats displayed persistently elevated serum EDLM concentrations that exceeded 0.124 ngE/mL from 3 to 20 weeks of life. We conclude that (1) there are significant interstrain differences in serum EDLM levels in rats; and (2) the SHR has a unique peak in serum EDLM levels at 6 to 8 weeks of age, indicating a possible role for the substance in the inception of hypertension.

Endogenous inhibition of red blood cell Na,K-ATPase in essential and pregnancy-induced hypertension

Digoxin-like inhibitors of Na,K-ATPase have been implicated in the pathophysiology of essential (EH) and pregnancy-induced hypertension (PIH). A technique that enhances dissociation of digoxin from red blood cells (RBC) was used to displace endogenous digoxin-like substances from RBCs. RBC membranes were preincubated in Na and ATP (Release) or Na,K,Mg and ATP (Retention) prior to measuring ATPase activity. Groups studied were: 39 men with EH and 34 controls plus 10 women with PIH and 17 normotensive controls. All displayed similar increases in Na,K-ATPase activity (24.0 +/- 7.9%) following Release. Plasma digoxin immunoreactivity (DI) was measured in pregnant women, m = 0.25 +/- 0.07 ng/ml. No DI was detected in nonpregnant women, but RBCs from these women demonstrated the same increase in Na,K-ATPase activity after Release. The 24% increase in activity achieved by Na and ATP preincubation can be reversed by adding K and Mg to the Release suspension. However, after RBC-bound digoxin is displaced by Release preincubation, addition of K and Mg cannot promote renewed binding and pump inhibition. Thus, the observed endogenous inhibition is not due to displacement of a digoxin-like substance but probably is related to alteration of the enzyme-membrane interaction. Furthermore, even though pregnant women demonstrate DI, an inhibitory substance with digoxin-like binding could not be recognized using this technique.

Progesterone, some progesterone derivatives and urinary digoxin-like substances from pregnant women in radioimmuno- and 86Rb-uptake assays of digoxin

Progesterone and some derivatives were tested in a radioimmunoassay (RIA) of digoxin and in a bioassay measuring the 86Rb-uptake into red blood cells as an index of Na+, K+-ATPase activity. The digitalis-like activity of the hormones was compared with that found in chromatographic fractions of material extracted from the urines of pregnant women at term. Progesterone at concentrations greater than 10(-6) M cross-reacted in the RIA, and at 10(-3) M it decreased 86Rb-uptake by 18%. The anaesthetic progesterone derivates 5 alpha-pregnane-3 alpha-ol-20-one and 5 alpha-pregnane-3,20-dione crossreacted to a lesser degree in the RIA and lacked effect in the bioassay. Similar results were obtained with pregnandiol-glucuronide, the major urinary metabolite of progesterone. In contrast, several fractions of the urinary material had significant effects in both assays. It is concluded that the digitalis-like activity of progesterone is not coupled to properties associated with its anaesthetic effects. Furthermore, although progesterone may account for a part of the endogenous digoxin-like substances in serum of neonates and pregnant women, neither progesterone proper nor pregnandiol-glucuronide explains the great amount of digoxin-like substances found in the urines.

Circulating digitalis-like factors

The ability of extracts of mammalian plasma and tissue to mimic the biologic activities of the digitalis glycosides has suggested the existence of endogenous regulators for Na, K ATPase. Purification of plasma extracts has identified several classes of circulating lipids with digitalis-like activity including free fatty acids, lysophospholipids, and arachidonic acid metabolites of the lipoxygenase pathway. Circulating steroids with digitalis-like activity include dehydroepiandrosterone sulfate and hydrocortisone. Evidence for other, more unique compounds has also been published although their structure has not yet been determined. Analysis of tissue suggests that hypothalamus contains a unique, low molecular digitalis-like factor (DLF) which also circulates in plasma. Some studies suggest that the hypothalamic factor is also present in other parts of the brain and in the adrenal. Some of these endogenous DLF may function as modulators of cardiovascular function by regulating renal sodium excretion and peripheral vascular resistance in both physiological and pathophysiological situations.

Endogenous cardiac glycosidelike compounds

The possibility that endogenous inhibitors of the sodium pump exist and bind to the cardiac glycoside binding site on Na+,K+-adenosine triphosphatase (ATPase) has been a source of much controversy. Although numerous hormones and inorganic ions that modulate Na+,K+-ATPase activity have been described, most of these affect the sodium pump indirectly by varying the intracellular sodium concentration or by increasing the number of enzyme units. None of these endogenous compounds has been shown conclusively to modulate sodium pump activity by binding to the cardiac glycoside binding site on Na+,K+-ATPase. However, the near-universal presence of three high-affinity binding sites on the alpha-subunit of the enzyme has engendered much speculation that endogenous ligands for these receptors must exist. In addition, none of the hormones known to indirectly affect sodium pump activity in vivo has been shown to modulate Na+,K+-ATPase activity in response to extracellular volume expansion or to play a role in the pathogenesis of hypertension or chronic renal failure, conditions in which a circulating inhibitor of Na+,K+-ATPase has been implicated. This report presents a condensed history of the search for endogenous inhibitors of Na+,K+-ATPase and describes recent advances in the field. Despite progress in identifying and characterizing compounds that could affect Na+,K+-ATPase activity in vivo, definitive proof for the existence of endogenous ligands for the cardiac glycoside binding site remains elusive.