Spironolactone attenuates experimental uremic cardiomyopathy by antagonizing marinobufagenin

Spironolactone has been noted to attenuate cardiac fibrosis. We have observed that the cardiotonic steroid marinobufagenin plays an important role in the diastolic dysfunction and cardiac fibrosis seen with experimental renal failure. We performed the following studies to determine whether and how spironolactone might ameliorate these changes. First, we studied rats subjected to partial nephrectomy or administration of exogenous marinobufagenin. We found that spironolactone (20 mg/kg per day) attenuated the diastolic dysfunction as assessed by ventricular pressure-volume loops and essentially eliminated cardiac fibrosis as assessed by trichrome staining and Western blot. Next, we examined the effects of spironolactone and its major metabolite, canrenone (both 100 nM), on marinobufagenin stimulation of rat cardiac fibroblasts. Both spironolactone and canrenone prevented the stimulation of collagen production by 1 nM marinobufagenin but not 100 nM marinobufagenin, as assessed by proline incorporation and procollagen 1 expression, as well as signaling through the sodium-potassium-ATPase, as evidenced by protein kinase C isoform delta translocation and extracellular signal regulated kinase 1/2 activation. Both spironolactone and canrenone also altered ouabain binding to cultured porcine cells in a manner consistent with competitive inhibition. Our data suggest that some of the antifibrotic effects of spironolactone may be attributed to antagonism of marinobufagenin signaling through the sodium-potassium-ATPase.

Marinobufagenin induces increases in procollagen expression in a process involving protein kinase C and Fli-1: implications for uremic cardiomyopathy

The cardiotonic steroid marinobufagenin (MBG) has been implicated in the pathogenesis of experimental uremic cardiomyopathy, which is characterized by progressive cardiac fibrosis. We examined whether the transcription factor Friend leukemia integration-1 (Fli-1) might be involved in this process. Fli-1-knockdown mice demonstrated greater cardiac collagen-1 expression and fibrosis compared with wild-type mice; both developed increased cardiac collagen expression and fibrosis after 5/6 nephrectomy. There was a strong inverse relationship between the expressions of Fli-1 and procollagen in primary culture of rat cardiac and human dermal fibroblasts as well as a cell line derived from renal fibroblasts and MBG-induced decreases in nuclear Fli-1 as well as increases in procollagen-1 expression in these cells. Transfection of a Fli-1 expression vector prevented increased procollagen-1 expression from MBG. MBG exposure induced a rapid translocation of the delta-isoform of protein kinase C (PKCdelta) to the nucleus. This translocation was prevented by pharmacological inhibition of phospholipase C, and MBG-induced increases in procollagen-1 expression were prevented with a PKCdelta- but not a PKCalpha-specific inhibitor. Finally, immunoprecipitation studies strongly suggest that MBG induced phosphorylation of Fli-1. We feel these data support a causal relationship with MBG-induced translocation of PKCdelta, which results in phosphorylation of as well as decreases in nuclear Fli-1 expression, which, in turn, leads to increases in collagen production. Should these findings be confirmed, we speculate that this pathway may represent a therapeutic target for uremic cardiomyopathy as well as other conditions associated with excessive fibrosis.

Partial nephrectomy as a model for uremic cardiomyopathy in the mouse

Because of the plethora of genetic manipulations available in the mouse, we performed a partial nephrectomy in the mouse and examined whether the phenotypical features of uremic cardiomyopathy described in humans and rats were also present in the murine model. A 5/6 nephrectomy was performed using a combination of electrocautory to decrease renal mass on the left kidney and right surgical nephrectomy. This procedure produced substantial and persistent hypertension as well as increases in circulating concentrations of marinobufagenin. Invasive physiological measurements of cardiac function demonstrated that the 5/6 nephrectomy resulted in impairment of both active and passive left ventricular relaxation at 4 wk whereas tissue Doppler imaging detected changes in diastolic function after 6 wk. Morphologically, hearts demonstrated enlargement and progressive fibrosis, and biochemical measurements demonstrated downregulation of the sarcoplasmic reticulum calcium ATPase as well as increases in collagen-1, fibronectin, and vimentin expression. Our results suggest that partial nephrectomy in the mouse establishes a model of uremic cardiomyopathy which shares phenotypical features with the rat model as well as patients with chronic renal failure.

Marinobufagenin stimulates fibroblast collagen production and causes fibrosis in experimental uremic cardiomyopathy

We have observed recently that experimental renal failure in the rat is accompanied by increases in circulating concentrations of the cardiotonic steroid, marinobufagenin (MBG), and substantial cardiac fibrosis. We performed the following studies to examine whether MBG might directly stimulate cardiac fibroblast collagen production. In vivo studies were performed using the 5/6th nephrectomy model of experimental renal failure (PNx), MBG infusion (MBG), PNx after immunization against MBG, and concomitant PNx and adrenalectomy. Physiological measurements with a Millar catheter and immunohistochemistry were performed. In vitro studies were then pursued with cultured isolated cardiac fibroblasts. We observed that PNx and MBG increased MBG levels, blood pressure, heart size, impaired diastolic function, and caused cardiac fibrosis. PNx after immunization against MBG and concomitant PNx and adrenalectomy had similar blood pressure as PNx but less cardiac hypertrophy, diastolic dysfunction, and cardiac fibrosis. MBG induced increases in procollagen-1 expression by cultured cardiac fibroblasts at 1 nM concentration. These increases in procollagen expression were accompanied by increases in collagen translation and increases in procollagen-1 mRNA without any demonstrable increase in procollagen-1 protein stability. The stimulation of fibroblasts with MBG could be prevented by administration of inhibitors of tyrosine phosphorylation, Src activation, epidermal growth factor receptor transactivation, and N-acetyl cysteine. Based on these findings, we propose that MBG directly induces increases in collagen expression by fibroblasts, and we suggest that this may be important in the cardiac fibrosis seen with experimental renal failure.

Ouabain decreases sarco(endo)plasmic reticulum calcium ATPase activity in rat hearts by a process involving protein oxidation

The effect of cardiac glycosides to increase cardiac inotropy by altering Ca(2+) cycling is well known but still poorly understood. The studies described in this report focus on defining the effects of ouabain signaling on sarcoplasmic reticulum Ca(2+)-ATPase function. Rat cardiac myocytes treated with 50 microM ouabain demonstrated substantial increases in systolic and diastolic Ca(2+) concentrations. The recovery time constant for the Ca(2+) transient, tau(Ca(2+)), was significantly prolonged by ouabain. Exposure to 10 microM H(2)O(2), which causes an increase in intracellular reactive oxygen species similar to that of 50 microM ouabain, caused a similar increase in tau(Ca(2+)). Concurrent exposure to 10 mM N-acetylcysteine or an aqueous extract from green tea (50 mg/ml) both prevented the increases in tau(Ca(2+)) as well as the changes in systolic or diastolic Ca(2+) concentrations. We also observed that 50 microM ouabain induced increases in developed pressure in addition to diastolic dysfunction in the isolated perfused rat heart. Coadministration of ouabain with N-acetylcysteine prevented these increases. Analysis of sarcoplasmic reticulum Ca(2+)-ATPase protein revealed increases in both the oxidation and nitrotyrosine content in the ouabain-treated hearts. Liquid chromatography-mass spectrometric analysis confirmed that the sarcoplasmic reticulum Ca(2+)-ATPase protein from ouabain-treated hearts had modifications consistent with oxidative and nitrosative stress. These data suggest that ouabain induces oxidative changes of the sarcoplasmic reticulum Ca(2+)-ATPase structure and function that may, in turn, produce some of the associated changes in Ca(2+) cycling and physiological function.

Central role for the cardiotonic steroid marinobufagenin in the pathogenesis of experimental uremic cardiomyopathy

Patients with chronic renal failure develop a "uremic" cardiomyopathy characterized by diastolic dysfunction, cardiac hypertrophy, and systemic oxidant stress. Patients with chronic renal failure are also known to have increases in the circulating concentrations of the cardiotonic steroid marinobufagenin (MBG). On this background, we hypothesized that elevations in circulating MBG may be involved in the cardiomyopathy. First, we observed that administration of MBG (10 microg/kg per day) for 4 weeks caused comparable increases in plasma MBG as partial nephrectomy at 4 weeks. MBG infusion caused increases in conscious blood pressure, cardiac weight, and the time constant for left ventricular relaxation similar to partial nephrectomy. Decreases in the expression of the cardiac sarcoplasmic reticulum ATPase, cardiac fibrosis, and systemic oxidant stress were observed with both MBG infusion and partial nephrectomy. Next, rats were actively immunized against a MBG-BSA conjugate or BSA control, and partial nephrectomy was subsequently performed. Immunization against MBG attenuated the cardiac hypertrophy, impairment of diastolic function, cardiac fibrosis, and systemic oxidant stress seen with partial nephrectomy without a significant effect on conscious blood pressure. These data suggest that the increased concentrations of MBG are important in the cardiac disease and oxidant stress state seen with renal failure.

Salt loading induces redistribution of the plasmalemmal Na/K-ATPase in proximal tubule cells

BACKGROUND

We have reported that digitalis-like substances (cardiotonic steroids), including marinobufagenin (MBG), induce endocytosis of the plasmalemmal Na/K-ATPase in LLC-PK1 cells. The current report addresses the potential relevance of plasmalemmal Na/K-ATPase redistribution to in vivo salt handling.

METHODS

Male Sprague-Dawley rats were given 1 week of a high salt (4.0% NaCl) or normal salt (0.4% NaCl) diet. Urinary sodium excretion, as well as MBG excretion, was monitored, and proximal tubules were isolated using a Percoll gradient method. Tubular (86)Rb uptake, Na/K-ATPase enzymatic activity, and Na/K-ATPase alpha1 subunit density were determined.

RESULTS

The high salt diet increased urinary sodium (17.8 +/- 1.8 vs. 2.5 +/- 0.3 mEq/day, P < 0.01) and MBG excretion (104 +/- 12 vs. 26 +/- 4 pmol/day), and decreased proximal tubular (86)Rb uptake (0.44 +/- 0.07 vs. 1.00 +/- 0.10, P < 0.01) and Na/K-ATPase enzymatic activity (5.1 +/- 1.1 vs. 9.9 +/- 1.6 micromol/mg pr/hr, P < 0.01) relative to the normal diet. Proximal tubular Na/K-ATPase alpha1 protein density was decreased in the plasmalemma fraction but increased in both early and late endosomes following the high salt diet. In rats fed a high salt diet, anti-MBG antibody caused a 60% reduction in urinary sodium excretion, substantial increases in proximal tubule (86)Rb uptake, and Na/K-ATPase enzymatic activity, as well as significant decreases in the early and late endosomal Na/K-ATPase alpha1 protein content.

CONCLUSION

These data suggest that redistribution of the proximal tubule Na/K-ATPase in response to endogenous cardiotonic steroids plays an important role in renal adaptation to salt loading.

Ouabain induces endocytosis of plasmalemmal Na/K-ATPase in LLC-PK1 cells by a clathrin-dependent mechanism

BACKGROUND

We have demonstrated that ouabain causes dose- and time-dependent decreases in (86)Rb uptake in porcine proximal tubular (LLC-PK1) cells. The present study addresses the molecular mechanisms involved in this process.

METHODS

Studies were performed with cultured LLC-PK1 and Src family kinase deficient (SYF) cells.

RESULTS

We found that 50 nmol/L ouabain applied to the basal, but not apical, aspect for 12 hours caused decreases in the plasmalemmal Na/K-ATPase. This loss of plasmalemmal Na/K-ATPase reverses completely within 12 to 24 hours after removal of ouabain. Ouabain also increased the Na/K-ATPase content in both early and late endosomes, activated phosphatidylinositol 3-kinase (PI(3)K), and also caused a translocation of some Na/K-ATPase to the nucleus. Immunofluorescence demonstrated that the Na/K-ATPase colocalized with clathrin both before and after exposure to ouabain, and immunoprecipitation experiments confirmed that ouabain stimulated interactions among the Na/K-ATPase, adaptor protein-2 (AP-2), and clathrin. Potassium (K) depletion, chlorpromazine, or PI(3)K inhibition all significantly attenuated this ouabain-induced endocytosis. Inhibition of the ouabain-activated signaling process through Src by 4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) significantly attenuated ouabain-induced endocytosis. Moreover, experiments performed in SYF cells demonstrated that ouabain induced increases in the endocytosis of the Na/K-ATPase when Src was reconstituted (SYF+), but not in the Src-deficient (SYF-) cells.

CONCLUSION

These data demonstrate that ouabain stimulates a clathrin-dependent endocytosis pathway that translocates the Na/K-ATPase to intracellular compartments, thus suggesting a potential role of endocytosis in ouabain-induced signal transduction as well as proximal tubule sodium handling.

Effect of green tea extract on cardiac hypertrophy following 5/6 nephrectomy in the rat

BACKGROUND

Left ventricular hypertrophy commonly complicates chronic renal failure. We have observed that at least one pathway of left ventricular hypertrophy appears to involve signaling through reactive oxygen species (ROS). Green tea is a substance that appears to have substantial antioxidant activity, yet is safe and is currently widely used. We, therefore, studied whether green tea supplementation could attenuate the development of left ventricular hypertrophy in an animal model of chronic renal failure.

METHODS

Male Sprague-Dawley rats were subjected to sham or remnant kidney surgery and given green tea extract (0.1% and 0.25%) or plain drinking water for the next 4 weeks. Heart weight, body weight, and cardiac Na-K-ATPase activity were measured at the end of this period. To further test our hypothesis, we performed studies in cardiac myocytes isolated from adult male Sprague-Dawley rats. We measured the generation of ROS using the oxidant sensitive dye dichlorofluorescein (DCF) as well as (3H)phenylalanine incorporation following exposure to cardiac glycosides with and without green tea extract.

RESULTS

Administration of green tea extract at 0.25% resulted in attenuation of left ventricular hypertrophy, hypertension, and preserved cardiac Na-K-ATPase activity in rats subjected to remnant kidney surgery (all P < 0.01). In subsequent studies performed in isolated cardiac myocytes, both ouabain and marinobufagenin (MBG) were both found to increase ROS production and (3H)phenylalanine incorporation at concentrations substantially below their inhibitor concentration (IC) 50 for the sodium pump. Addition of green tea extract prevented increases in ROS production as well as (3H)phenylalanine incorporation in these isolated cardiac myocytes.

CONCLUSION

Green tea extract appears to block the development of cardiac hypertrophy in experimental renal failure. Some of this effect may be related to the attenuation of hypertension, but a direct effect on cardiac myocyte ROS production and growth was also identified. Clinical studies of green tea extract in chronic renal failure patients may be warranted.

Effect of chronic renal failure on cardiac contractile function, calcium cycling, and gene expression of proteins important for calcium homeostasis in the rat

Patients with chronic renal failure frequently develop cardiac hypertrophy and diastolic dysfunction; however, the mechanisms by which this occurs are still unclear. Male Sprague-Dawley rats were subjected to 5/6 nephrectomy and studied for their isolated myocyte function, calcium cycling, and gene expression of proteins important in calcium homeostasis after 4 wk. Comparable rats subjected to suprarenal aortic banding for the same duration were used for comparison. Rats subjected to 5/6 nephrectomy and aortic banding developed comparable hypertension; however, rats subjected to 5/6 nephrectomy experienced a greater degree of cardiac hypertrophy and downregulation of cardiac sodium potassium ATPase (Na+/K+ -ATPase) activity than rats subjected to aortic banding. Moreover, cells isolated from the 5/6 nephrectomy rat hearts displayed impaired contractile function and altered calcium cycling compared with cells isolated from control or aortic constriction rat hearts. The 5/6 nephrectomy rat heart cells displayed a prolonged time constant for calcium recovery following stimulation, which corresponded to decreases in homogenate sarcoplasmic reticulum calcium ATPase-2a (SERCA2a) activity, protein density, and mRNA for SERCA2a. In conclusion, chronic renal failure leads to alterations in cardiac gene expression, which produces alterations in cardiac calcium cycling and contractile function. These changes cannot be explained only by the observed increases in BP.

Effects of cardiac glycosides on sodium pump expression and function in LLC-PK1 and MDCK cells

BACKGROUND

The decreases in proximal tubule sodium reabsorption seen with chronic renal failure and volume expansion have been ascribed to circulating digitalis-like substances (DLS). However, the circulating concentrations of DLS do not acutely inhibit the sodium pump to a degree consistent with the observed changes in proximal tubule sodium reabsorption.

METHODS

We examined how cell lines that simulated proximal (LLC-PK1) and distal tubule (MDCK) cells responded to acute (30 min) and long-term (up to 12 hours) Na+,K+-ATPase inhibition with DLS.

RESULTS

In LLC-PK1, but not MDCK cells, low concentrations of ouabain decreased 86Rb uptake profoundly in a time and dose dependent manner. In LLC-PK1 cells grown to confluence, transcellular 22Na flux was markedly reduced in concert with the decreases in 86Rb uptake. Similar findings were observed with marinobufagenin (MBG) and deproteinated extract of serum derived from patients with chronic renal failure. However, inhibition of the Na+,K+-ATPase with low extracellular potassium concentrations did not produce any of these effects. Western and Northern blots detected no change in alpha1 Na+,K+-ATPase protein and message RNA, respectively, in LLC-PK1 cells treated with ouabain for 12 hours. However, the decrease in enzymatic activity of Na+,K+-ATPase of these cells was comparable to observed decreases in 86Rb uptake. Differential centrifugation as well as biotinylation experiments demonstrated a shift of the Na+,K+-ATPase from the plasmalemma with prolonged ouabain treatment.

CONCLUSIONS

The results show that binding of cardiac glycosides by proximal (but not distal) tubular cells results in internalization of Na+,K+-ATPase with the net effect to amplify inhibition of the Na+,K+-ATPase. As the circulating concentrations of DLS increase with chronic renal failure and volume expansion, we suggest that this phenomenon explains some of the decreased sodium reabsorption by the proximal tubule seen in these conditions.

Effects of uremic serum on isolated cardiac myocyte calcium cycling and contractile function

BACKGROUND

Diastolic dysfunction occurs in patients with chronic renal failure. Moreover, serum from uremic patients contains one or more inhibitors of the plasmalemmal Na,K-ATPase (sodium pump). We hypothesized that a circulating substance present in uremic sera contributes to both sodium pump inhibition and diastolic dysfunction.

METHODS

Serum samples were obtained from six patients with chronic renal failure and diastolic dysfunction.

RESULTS

Their serum samples caused marked inhibition of Na,K-ATPase purified from dog kidney at all concentrations studied (all P < 0.01) and also impaired ouabain-sensitive rubidium uptake by myocytes isolated from Sprague-Dawley rats (P < 0.01). These cardiac myocytes were studied for their contractile function with video-edge detection and calcium metabolism with indo-1 fluorescence spectroscopy after exposure to these uremic sera. These uremic sera caused increases in myocyte fractional shortening (P < 0.01) as well as an increase in the time constant of relengthening (P < 0.01). Examining the calcium transient, the time constant for calcium recovery was also increased (P < 0.01). Exposure of these cells to sera from age- and sex-matched healthy subjects did not result in significant changes in contraction or calcium cycling. Extracts of uremic serum samples inhibited isolated Na,K-ATPase whereas extracts of normal serum samples did not. The effect of uremic serum extracts on contractile function and calcium cycling were quite similar to that of intact serum or the addition of ouabain. Co-incubation of uremic serum extract with an antibody fragment directed against digoxin markedly attenuated the inhibition of Na,K-ATPase activity and completely prevented any effects on calcium cycling or contractile function.

CONCLUSION

These data show that one or more substances are present in uremic sera that acutely cause increased force of contraction and impaired recovery of cardiac myocyte calcium concentration as well as impaired relaxation. As these effects are similar to that seen with ouabain and can be prevented by co-incubation with an antibody fragment to digitalis, which also attenuates the sodium pump inhibitory effect, we suggest that this (these) substance(s) circulating in uremic sera and inhibiting the sodium pump also causes the acute diastolic dysfunction seen in our system.

Effects of hypokalemia on cardiac growth

In neonatal myocytes grown in culture, reductions in extracellular potassium concentration produced a hypertrophic response as assessed by induction of early response genes, atrial natriuretic peptide and skeletal actin, and repression of the alpha3 isoform of the sodium pump in a dose dependent manner. The degree of alpha3 repression appeared to be dose dependent with decreases in media (K). Similarly, decreases in media potassium concentrations caused increases in cytosolic calcium concentration in a dose dependent manner; moreover these increases in cytosolic calcium concentration correlated quite well with repression of alpha3 expression. In contrast, although moderate reductions of potassium concentration induced upregulation of skACT and ANP, severely reduced potassium concentrations caused repression of skACT and ANP expression. In parallel studies performed in vivo, 3-5 weeks dietary K restriction induced molecular phenotypical changes similar to that seen in the neonatal myocyte model without demonstrable growth as assessed by the heart weight/body weight ratio. However, when rates subjected to dietary K restriction were subsequently subjected to acute aortic constriction, cardiac growth was greater than in rats fed a control diet. These data suggest that hypokalemia may produce molecular phenotypic alterations consistent with cardiac hypertrophy as well as contribute to hypertrophy in an in vivo model.

Similarities and differences between the properties of native and recombinant Na+/K+-ATPases

Progress of mutagenesis studies on the relation of the structure of Na+/K+-ATPase to its reaction mechanism has been impeded by the paucity of information on the properties of small amounts of impure recombinant enzyme obtained in the currently available expression systems, and the uncertainty of whether expression in a new environment alters the various catalytic activities of this membrane enzyme. Hence, our aim was to make a detailed comparison of the properties of the extensively studied canine kidney Na+/K+-ATPase with those of its alpha1,beta1 subunits expressed in the baculovirus-infected Sf-9 cells. The active fraction of the recombinant enzyme, containing 10-20% of the expressed a subunits, was found to have normal molar activity, all the partial reactions, and the ability to catalyze ATP-dependent Na+/K+ exchange after reconstitution into proteoliposomes. Comparison of steady-state kinetics of the hydrolytic activities of recombinant and native enzymes showed that (a) ATP and Na+ plots of Na+-ATPase were the same in the two preparations; (b) apparent K+ affinity of K+-phosphatase of recombinant enzyme was lower than that of kidney enzyme; and (c) for Na+/K+- ATPase activity, apparent K+ affinity of recombinant enzyme was lower, and its apparent Na+ and ATP affinities were higher than those of kidney enzyme. The two enzymes had similar ADP- and K+-sensitive phosphointermediates, identical affinities for ouabain, and similar ligand sensitivities of dissociation rates of ouabain-enzyme complexes. Evidently, the recombinant enzyme has reduced affinity at cytoplasmic K+ sites, but no changes at multiple Na+, ATP, and ouabain binding sites. Likely causes of this selective change include altered glycosylation state of beta and interactions among active and inactive recombinant enzymes. The present results provide the necessary database for the appropriate use of an expression system in structure-function studies on canine alpha1,beta1 isoform of Na+/K+-ATPase, and indicate the need for similar studies on recombinant Na+/K+-ATPases obtained in other expression systems.

Functional coupling of phosphorylation and nucleotide binding sites in the proteolytic fragments of Na+/K(+)-ATPase

Cleavage of the alpha-subunit of Na+/K(+)-ATPase by trypsin at Arg438-Ala439 causes enzyme inhibition which has been suggested to be due to altered alignment of phosphorylation site on the 48-kDa N-terminal fragment with nucleotide binding site on the 64-kDa C-terminal fragment. Our aims were to test this hypothesis and to assess the effect of the cleavage on the enzyme's two ATP sites. Na(+)-dependent phosphorylation of the partially cleaved enzyme by ATP showed that K0.5 values of ATP for phosphorylations of intact alpha and 48-kDa peptide were the same (0.4 microM). Unchanged interactions among the residues across the cleavage site were also indicated by data showing that reaction of fluorescein isothiocyanate with the 64-kDa peptide blocked phosphorylation of the 48-kDa peptide by ATP. ATP is known to block the reaction of fluorescein isothiocyanate with the enzyme. Experiments on the partially cleaved enzyme showed that K0.5 of ATP for protection of alpha was 30-60 microM, and the value for the protection of interacting 48-kDa and 64-kDa peptides was 1-3 mM. Evidently, while the cleavage does not affect the high affinity catalytic site, it disrupts the allosteric low affinity ATP site. Experiments on reconstituted preparations showed that the cleavage abolished ATP-dependent Na+/K+ exchange, Pi+ATP-dependent Rb+/Rb+ exchange, ATP-dependent Na+/Na+ exchange, and ADP+ATP-dependent Na+/Na+ exchange activities. Selective disruption of the low affinity ATP site accounts for the inhibitions of all functions involving K+(Rb+), based on the established role of this site in the control of K+ access channels. Cleavage-induced inhibitions of other activities, however, suggest additional roles of the low affinity ATP site in the reaction cycle.

Different oxidant sensitivities of the alpha 1 and alpha 2 isoforms of Na+/K(+)-ATPase expressed in baculovirus-infected insect cells

Inhibition of Na+/K(+)-ATPase by partially reduced oxygen metabolites is an early event in the course of cell injury caused by oxidative stress. We showed before that isoforms of the enzyme obtained from different sources have different oxidant sensitivities. To evaluate the role of tissue-specific impurities in this difference, cDNAs of alpha 1 and alpha 2 isoforms were expressed in Sf-9 insect cells, and the effects of H2O2 on the resulting isolated enzymes were studied. The expressed alpha 2 was significantly more sensitive than alpha 1 to H2O2. These findings, together with our previous data showing different oxidant sensitivities of alpha 1 and alpha 3 in a cardiac enzyme preparation, indicate that differential oxidant sensitivities of Na+/K(+)-ATPase isoforms of various tissues are dictated by the primary sequences of alpha 1, alpha 2, and alpha 3 subunits.

Reconstitution and partial purification of cardiac sarcolemmal Na+/H+ (K+/H+) antiporter

Most of the plasma membrane Na+/H+ antiporters that have been well characterized do not transport K+. Our previous studies showed that cardiac plasma membrane contains a nonselective alkali cation/H+ antiporter that transports either Na+ or K+. The purpose of this work was to develop a convenient assay for the cardiac antiporter to be used in the course of its purification. Sarcolemmal membranes prepared from bovine heart were solubilized with Triton X-100 in the presence of a mixture of phospholipids, and the solubilized proteins were passed through Bio-Beads to form proteoliposomes. Exchange activities were assayed either by measuring H+ gradient-dependent uptakes of 22Na+ and 86Rb+ by proteoliposomes or by recording H+ release from proteoliposomes as reported by the fluorescence of a pH probe that was trapped in the proteoliposomes during reconstitution. Properties of the reconstituted nonselective antiporter were found to be similar to those of the native sarcolemmal antiporter. Using a DEAE-cellulose column, 20-fold purification of the antiporter was achieved, demonstrating the suitability of the reconstitution assay procedures for further purification of the functional antiporter.

Inhibition of cardiac sarcolemmal Na+/H+ antiporter by opioids

In our routine screening of chemicals that would inhibit cardiac sarcolemmal Na+/H+ antiporter, we discovered that some of the opioids produced inhibition of cardiac sarcolemmal Na+/H+ antiporter in micromolar concentrations. Using U-50,488H, a selective kappa-opioid agonist, we characterized the nature of interaction between opioids and the Na+/H+ antiporter. The inhibitory effect of U-50,488H on Na+/H+ antiporter was immediate and reversible, and was not mediated through the interaction with the opioid receptors but due to the direct interaction of U-50,488H with the Na+/H+ antiporter. The kinetic data show that in the presence of U-50,488H the Km for Na+ was increased from 2.5 +/- 0.2 to 5.0 +/- 0.3 mM, while the Vmax (52.0 +/- 5.0 nmol.mg-1.min-1) remained the same. These results suggest that U-50,488H and Na+ compete for the same site on the antiporter. When testing the effect of U-50,488H on other transport systems of cardiac sarcolemma, we found that U-50,488H also inhibited Na+/Ca2+ antiporter and Na+/K+ pump but at much higher concentrations suggesting that U-50,488H shows some degree of selectivity for cardiac sarcolemmal Na+/H+ antiporter. When we compared the inhibitory potency of U-50,488H with amiloride and its analog, namely 5-(N,N-hexamethylene)amiloride, we found that U-50,488H (IC50 = 100 +/- 15 microM) was threefold more potent than amiloride (IC50 = 300 +/- 20 microM) but it was three-fold less potent than the amiloride analog (IC50 = 30 +/- 10 microM) in inhibiting cardiac sarcolemmal Na+/H+ antiporter. These results show that although U-50,488H is more potent than amiloride, the inhibitory characteristics of U-50,488H on cardiac sarcolemmal Na+/H+ antiporter are similar to amiloride.

Ion specificity of cardiac sarcolemmal Na+/H+ antiporter

In bovine cardiac sarcolemmal vesicles, an outward H+ gradient stimulated the initial rate of amiloride-sensitive uptake of 22Na+, 42K+, or 86Rb+. Release of H+ from the vesicles was stimulated by extravesicular Na+, K+, Rb+, or Li+ but not by choline or N-methylglucamine. Uptakes of Na+ and Rb+ were half-saturated at 3 mM Na+ and 3 mM Rb+, but the maximal velocity of Na+ uptake was 1.5 times that of Rb+ uptake. Na+ uptake was inhibited by extravesicular K+, Rb+, or Li+, and Rb+ uptake was inhibited by extravesicular Na+ or Li+. Amiloride-sensitive uptake of Na+ or Rb+ increased with increase in extravesicular pH and decrease in intravesicular pH. In the absence of pH gradient, there were stimulations of Na+ uptake by intravesicular Na+ and K+ and of Rb+ uptake by intravesicular Rb+ and Na+. Similarly, there were trans stimulations of Na+ and Rb+ efflux by extravesicular alkali cations. The data suggest the existence of a nonselective antiporter catalyzing either alkali cation/H+ exchange or alkali cation/alkali cation exchange. Since increasing Na+ caused complete inhibition of Rb+/H+ exchange, but saturating K+ caused partial inhibitions of Na+/H+ exchange and Na+/Na+ exchange, the presence of a Na(+)-selective antiporter is also indicated. Although both antiporters may be involved in pH homeostasis, a role of the nonselective antiporter may be in the control of Na+/K+ exchange across the cardiac sarcolemma.

Sodium regulation of alpha 2-adrenoreceptors in Dahl rats. Effect of feeding a low or high salt diet

Sodium regulation of alpha 2-adrenoreceptors was investigated in inbred salt-sensitive (S) and inbred salt-resistant (R) rats fed a high or low salt diet. The systolic blood pressure was higher in S rats than in R rats, and this difference was obviously greater on a high salt diet. In rats fed a low or high salt diet, S rats had higher alpha 2-adrenoreceptor density in the kidneys compared with R rats as measured by [3H]yohimbine binding and Scatchard analysis. The affinity of the receptors in the kidney for the antagonist, yohimbine, was nearly the same in these two strains either on a low or high salt diet. In the brain, the affinities or the numbers of receptors were not significantly different whether these two strains were fed a low or high salt diet. Inclusion of NaCl up to 80 mM in the assay medium did not alter the in vitro binding of [3H]yohimbine in the kidney or brain. On the other hand, inclusion of NaCl in the assay medium reduced the ability of epinephrine in competing with [3H]yohimbine for the receptor sites in the kidney and in the brain, and this effect of NaCl was the same in a given tissue between S and R rats, whether they were fed a low or high salt diet. These results suggest that: (1) in the kidneys, the receptor density and not the receptor affinity was different between S and R strains whether they were fed a low or high salt diet; (2) in the brain, the receptor density and affinity were the same between S and R rats regardless of the diet (low or high salt), indicating that the sodium salt diet modulates the peripheral but not the central alpha 2-adrenoreceptors; and this modulatory effect was observed only in S rats; (3) Na+ was able to reduce the affinity of the agonist (epinephrine) for the receptors in both S and R rats, and this effect of Na+ on central and peripheral alpha 2-adrenoreceptors was similar in prehypertensive rats and rats with salt-induced hypertension; and (4) the resistance of R rats to salt-induced hypertension was not due to the absence of Na+ binding component involved in the regulation of alpha 2-adrenoreceptor-adenylate cyclase complex.

Sodium regulation of alpha 2-adrenoceptor-agonist interactions in renal membranes of young and old Dahl rats

Systolic blood pressure, the affinity and density of the alpha 2-adrenoceptors, Na+ effect on alpha 2-adrenoceptors as measured by specific [3H]yohimbine binding, and Na+ regulation of alpha 2-adrenoceptor-agonist interactions were studied in 6 week and 9 month old inbred salt sensitive (S) and inbred salt-resistant (R) rats. The systolic blood pressure of 6 week old S and R rats was not significantly different, while the systolic blood pressure of 9 month old S rats was higher than the age-matched R rats. The affinity of the alpha 2-adrenoceptors for yohimbine was the same in the 6 week old S and R rats but it was 2-fold less in the 9 month old S rats compared to the age-matched R rats. In the 6 week old rats, the number of alpha 2-adrenoceptors (Bmax) was significantly higher in the S rats than in the R rats; whereas, in the 9 month old rats, the alpha 2-adrenoceptors were 2.6 times higher in the S rats than the age-matched R rats. Na+, up to 80 mM, did not modify the number of alpha 2-adrenoceptors in the 6 week and 9 month old S and R rats. However, Na+ reduced the affinity of the alpha 2-adrenoceptors for epinephrine and this effect of Na+ was found to be the same in the 6 week and 9 month old S and R rats. These findings suggest that Na+ regulation of alpha 2-adrenoceptor-agonist interactions was not altered with age and it was found to be similar in prehypertensive rats and rats with severe hypertension; further, the resistance of R rats to salt-induced hypertension was not due to the absence of Na+ binding component in the alpha 2-adrenoceptor-adenylate cyclase complex.

Interaction of amiloride with alpha-adrenoreceptors: evidence from radioligand binding studies

We investigated the effect of amiloride on alpha-adrenoreceptors (alpha 1 and alpha 2) using radioligand binding techniques. Amiloride inhibited [3H]yohimbine and [3H]prazosin binding to alpha 2- and alpha 1-adrenoreceptors, respectively, from various tissues in a concentration-dependent manner. Amiloride was approximately 9-12 times more potent in inhibiting [3H]yohimbine binding to alpha 2-adrenoreceptors from rat tissues than from other mammalian tissues. However, it had almost the same potency in inhibiting [3H]prazosin binding to alpha 1-adrenoreceptors from rat as well as other mammalian tissues. Further, in rat tissues, amiloride was approximately 10 times more potent in inhibiting [3H]yohimbine than [3H]prazosin binding. Amiloride inhibited [3H]yohimbine binding noncompetitively and [3H]prazosin binding competitively. The inhibition of [3H]yohimbine and [3H]prazosin binding by amiloride was reversible. Since amiloride has been shown to be an inhibitor of Na+-H+ exchanger protein, we believe that it regulates the alpha 2-adrenoreceptors by binding to Na+ -H+ exchanger protein. Triamterene, a compound similar to amiloride in regard to diuretic effect, had very little effect on [3H]yohimbine and [3H]prazosin binding to rat kidney membranes, suggesting that the alpha-adrenoreceptor antagonistic properties of amiloride are not related to its antikaliuretic effect. The results of the present study suggest that some of the pharmacological actions of amiloride (antihypertensive and diuretic effects) can be explained in part by its regulatory effect on both alpha 1- and alpha 2-adrenoreceptors.

Elimination of Na+ and GTP regulation of alpha 2-adrenoceptor-agonist interactions in rat kidney membranes by 4,4'-diisothiocyano-2,2'-stilbene disulfonic acid (DIDS)

Pretreatment of rat kidney membranes with 4,4'-diisothiocyano-2,2'-stilbene disulfonic acid (DIDS) did not affect the Bmax but reduced the affinity of the receptors for antagonist by less than 2-fold. However, such treatment reduced the affinity of the receptors for agonist by several fold by preventing the formation of agonist-receptor-Gi-protein complex. The modulatory effects of GTP and Na+ on alpha 2-adrenoceptor-agonist interactions were also abolished. These results suggest that DIDS acts at multiple sites on alpha 2-adrenoceptor-adenylate cyclase complex.

Sodium regulation of alpha 2-adrenoreceptors of human platelets: inactivation by 4,4'-diisothiocyano-2,2'-stilbene disulfonic acid (DIDS)

In the present study, we investigated the effect of DIDS on Na+ regulation of alpha 2-adrenoreceptors in human platelets. Pretreatment of platelet membranes at 23 degrees C for 60 min with DIDS produced a reduction in the affinity of the receptors for both antagonist and agonist in a concentration related manner; however, there was a marked difference in the degree of reduction in the affinity of the receptors for antagonist and agonist. Thus, at 1 mM concentration of DIDS, the affinity of the receptors for antagonist was reduced by 2-fold while the affinity of the receptors for agonist was reduced by 14-fold. Furthermore, this concentration of DIDS abolished the ability of Na+ in reducing the affinity of the receptors for agonist. We suggest that the effect of DIDS is via Na+ binding component of the alpha 2-adrenoreceptor-adenylate cyclase complex.

[3H]Yohimbine binding to human platelet membranes: evidence for high and low affinity binding sites with negative cooperativity

Evidence is presented that [3H]yohimbine binding to human platelet membranes does not follow the simple mass action kinetics. Although [3H]yohimbine binding was saturable and stereospecific, Scatchard analysis of the equilibrium binding data produced a curvilinear plot. Competitive displacement of [3H]yohimbine from the binding sites by unlabeled yohimbine and other alpha 2-antagonists produced shallow inhibition curves. Further, the apparent Hill coefficients of equilibrium binding and competitive displacement data were found to be less than unity. Factors such as binding to nonreceptor sites or to alpha1-adrenoceptors, dopamine receptors, or 5-HT receptors that may explain the curvilinear curve were excluded. The rank order for inhibiting [3H]yohimbine was rauwolscine greater than yohimbine greater than phentolamine greater than clonidine much greater than prazosin, suggesting that the binding sites had the characteristics of alpha2-adrenoreceptors. The affinity of the alpha2-antagonist for the receptor was enhanced by Na+ but not by guanine nucleotide, suggesting that the binding of the antagonist is modulated only by Na+. Graphic analysis of the specific binding data resulted in two components: one with high affinity and low capacity sites, and second with low affinity and high capacity sites. The experiments on dissociation kinetics, however, suggest that the observed deviation of [3H]yohimbine binding from the simple mass action kinetics is most likely due to negative cooperative interactions among alpha2-adrenoceptor sites.

Subunit associations of (Na+ + K+)-dependent adenosine triphosphatase. Chemical cross-linking studies

Cross-linking reactions of the alpha- and beta-subunits of the purified membrane-bound enzyme with several reagents were studied. In the presence of 1,5-difluoro-2,4-dinitrobenzene, formation of a cross-linked alpha, beta-dimer was affected specifically by K+ + ATP or enzyme phosphorylation. The same conditions affected the formation of cross-linked alpha, alpha-dimer in the presence of 4,4'-difluoro-3,3'-dinitrodiphenyl sulfone or o-phenanthroline-Cu2+. Since noncovalent alpha, beta-association has been established, the data suggest K+ + ATP-induced or phosphorylation-induced changes in alpha, beta-domain and alpha, alpha-domain of an oligomer of alpha, beta-dimer. When the formation of cross-linked alpha, beta-dimer or alpha, alpha-dimer was induced by phosphorylation, only half of the subunits were cross-linked, suggesting the existence of a cooperative tetramer of alpha, beta-dimer. When microsomes or red cell membranes were exposed to 32Pi under phosphorylation-induced cross-linking conditions, the only products were alpha, beta-dimer and alpha, alpha-dimer, indicating the existence of an oligomer of alpha, beta-dimer in crude membranes. Subunits of the enzyme solubilized with octaethylene glycol dodecyl ether, by methods that have been suggested to yield unassociated alpha, beta-dimers, underwent spontaneous cross-linking that was not affected by enzyme dilution. Since the largest product was alpha 2 beta 2, the solubilized enzyme is at least a dimer of alpha, beta-dimer. The findings establish that the membrane-bound enzyme is an oligomer of alpha, beta-dimer. Whether or not a single alpha, beta-dimer is capable of catalytic and transport functions has not been determined.

Origins of the different sensitivities of (Na+ + K+)-dependent adenosinetriphosphatase preparations to ouabain

Properties of the Na+, K+-ATPase preparations from rat and dog kidney medullae were compared. The two enzymes, with a 1000-fold difference in ouabain sensitivities, had similar subunit compositions and similar K0.5 values and Hill coefficients for substrates and activators of several catalytic activities; suggesting that the structural differences of the two are limited to the ouabain binding domains. Experiments on the interactions of ouabain, Pi, and Mg2+ with the enzymes showed that the two enzymes differed (a) in their inherent affinities for ouabain; and (b) in that Mg2+ binding increased affinity for ouabain to a greater extent in the dog enzyme than in the rat enzyme.