Isoform specificity of cardiac glycosides binding to human Na+,K+-ATPase alpha1beta1, alpha2beta1 and alpha3beta1

Intrathecal Digoxin Administration During Spinal Anesthesia: A Case Report Outlining Recovery and Rehabilitation

This case report highlights the rehabilitation outcomes of a 24-year-old female who received inadvertent intrathecal digoxin during a routine cesarean section, leading to encephalomeningitis, vestibulocochlear neuritis, and incomplete paraplegia. Despite initial neurological deficits, the patient demonstrated significant improvement in both cognition and functional mobility during a one-month inpatient rehabilitation program, ultimately achieving ambulation with assistive devices. This case underscores the potential for rehabilitation of neurological sequela following accidental intrathecal digoxin administration.

Inadvertent Intrathecal Administration of Digoxin, with Review of the Literature

While the systemic effects of digoxin have been studied, limited data exist on the effects of neuraxial administration. Prior case reports document how digoxin and lidocaine share indistinguishable vials and were inadvertently stocked together in spinal and epidural anesthesia kits, necessitating a need for further implementation of safety measures. Here, we report the poor progression and brain death of a postpartum woman after accidental administration of intrathecal digoxin during a routine elective cesarean section (C-section). It is imperative that quality improvement and safety measures are taken to avoid the recurrence of this medical error.

Relationship among surface electric double layer of cardiomyocyte membrane and toxicology of digoxin and opening of ion channels

We applied a new idea that the potential effect can change the ion adsorption structure on the cell surface to explore the mechanism of digoxin poisoning and the regulation of ion channels. The effects of digoxin on the electrophoretic mobility and behaviors (non-contraction or contraction or autorhythmicity) of cardiomyocytes were observed by single-cell electrophoresis technique (imitate the opening method of in vivo channel) and the method of decomposing surface potential components on the cells. As well as affect the association with electrical activity. The results suggested that the increase of cardiomyocytes transmembrane potential and the Na⁺-K⁺ exchange on the cell surface of the action potential phase 4 caused by the poisoning dose of digoxin, leading to the oscillation of adsorbed ions on the cell surface and the incomplete channel structure, which were the mechanism of cardiac ectopic beats. The results revealed that the opening of ion channels is regulated by the surface electric double layer of the cell membrane.

Cardiac glycosides stimulate endocytosis of GLUT1 via intracellular Na+ ,K+ -ATPase α3-isoform in human cancer cells

Glucose transporter GLUT1 plays a primary role in the glucose metabolism of cancer cells. Here, we found that cardiac glycosides (CGs) such as ouabain, oleandrin, and digoxin, which are Na⁺ ,K⁺ -ATPase inhibitors, decreased the GLUT1 expression in the plasma membrane of human cancer cells (liver cancer HepG2, colon cancer HT-29, gastric cancer MKN45, and oral cancer KB cells). The effective concentration of ouabain was lower than that for inhibiting the activity of Na⁺ ,K⁺ -ATPase α1-isoform (α1NaK) in the plasma membrane. The CGs also inhibited [³ H]2-deoxy- d-glucose uptake, lactate secretion, and proliferation of the cancer cells. In intracellular vesicles of human cancer cells, Na⁺ ,K⁺ -ATPase α3-isoform (α3NaK) is abnormally expressed. Here, a low concentration of ouabain inhibited the activity of α3NaK. Knockdown of α3NaK significantly inhibited the ouabain-decreased GLUT1 expression in HepG2 cells, while the α1NaK knockdown did not. Consistent with the results in human cancer cells, CGs had no effect on GLUT1 expression in rat liver cancer dRLh-84 cells where α3NaK was not endogenously expressed. Interestingly, CGs decreased GLUT expression in the dRLh-84 cells exogenously expressing α3NaK. In HepG2 cells, α3NaK was found to be colocalized with TPC1, a Ca²⁺ -releasing channel activated by nicotinic acid adenine dinucleotide phosphate (NAADP). The CGs-decreased GLUT1 expression was significantly inhibited by a Ca²⁺ chelator, a Ca²⁺ -ATPase inhibitor, and a NAADP antagonist. The GLUT1 decrease was also attenuated by inhibitors of dynamin and phosphatidylinositol-3 kinases (PI3Ks). In conclusion, the binding of CGs to intracellular α3NaK elicits the NAADP-mediated Ca²⁺ mobilization followed by the dynamin-dependent GLUT1 endocytosis in human cancer cells.

Oleandrin: A Systematic Review of its Natural Sources, Structural Properties, Detection Methods, Pharmacokinetics and Toxicology

Oleandrin is a highly lipid-soluble cardiac glycoside isolated from the plant Nerium oleander (Apocynaceae) and is used as a traditional herbal medicine due to its excellent pharmacological properties. It is widely applied for various disease treatments, such as congestive heart failure. Recently, oleandrin has attracted widespread attention due to its extensive anti-cancer and novel anti-viral effects. However, oleandrin has a narrow therapeutic window and exhibits various toxicities, especially typical cardiotoxicity, which is often fatal. This severe toxicity and low polarity have significantly hindered its application in the clinic. This review describes natural sources, structural properties, and detection methods of oleandrin. Based on reported poisoning cases and sporadic animal experiments, the pharmacokinetic characteristics of oleandrin are summarized, so as to infer some possible phenomena, such as enterohepatic circulation. Moreover, the relevant factors affecting the pharmacokinetics of oleandrin are analyzed, and some research approaches that may ameliorate the pharmacokinetic behavior of oleandrin are proposed. With the toxicology of oleandrin being thoroughly reviewed, the development of safe clinical applications of oleandrin may be possible given potential research strategies to decrease toxicity.

Structure and Function of Na,K-ATPase-The Sodium-Potassium Pump

Na,K-ATPase is an ubiquitous enzyme actively transporting Na-ions out of the cell in exchange for K-ions, thereby maintaining their concentration gradients across the cell membrane. Since its discovery more than six decades ago the Na-pump has been studied extensively and its vital physiological role in essentially every cell has been established. This article aims at providing an overview of well-established biochemical properties with a focus on Na,K-ATPase isoforms, its transport mechanism and principle conformations, inhibitors, and insights gained from crystal structures. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Application of carbohydrates in approved small molecule drugs: A review

Carbohydrates are an important energy source and play numerous key roles in all living organisms. Carbohydrates chemistry involved in diagnosis and treatment of diseases has been attracting increasing attention. Carbohydrates could be one of the major focuses of new drug discovery. Currently, however, carbohydrate-containing drugs account for only a small percentage of all drugs in clinical use, which does not match the important roles of carbohydrates in the organism. In other words, carbohydrates are a relatively untapped source of new drugs and therefore may offer exciting novel therapeutic opportunities. Here, we presented an overview of the application of carbohydrates in approved small molecule drugs and emphasized and evaluated the roles of carbohydrates in those drugs. The potential development direction of carbohydrate-containing drugs was presented after summarizing the advantages and challenges of carbohydrates in the development of new drugs.

Oleandrin: A bioactive phytochemical and potential cancer killer via multiple cellular signaling pathways

Nerium oleander, a member of family Apocynaceae, is commonly known as Kaner in various countries of Asia and Mediterranean region. This plant has been renowned to possess significant therapeutic potential due to its various bioactive compounds which have been isolated from this plant e.g., cardiac glycosides, oleandrin, α-tocopherol, digitoxingenin, urosolic acid, quercetin, odorosides, and adigoside. Oleandrin, a saponin glycoside is one of the most potent and pharmacologically active phytochemicals of N. oleander. Its remarkable pharmacotherapeutic potential have been interpreted as anticancer, anti-inflammatory, anti-HIV, neuroprotective, antimicrobial and antioxidant. This particular bioactive entity is known to target the multiple deregulated signaling cascades of cancer such as NF-κB, MAPK, and PI3K/Akt. The main focus of the current study is to comprehend the action mechanisms of oleandrin against various pathological conditions. The current review is a comprehensive summary to facilitate the researchers to understand the pharmacological position of the oleandrin in the arena of drug discovery, representing this compound as a new drug candidate for further researches. Moreover, in vivo and in silico based studies are required to explore the mechanistic approaches regarding the pharmacokinetics and biosafety profiling of this compound to completely track its candidature status in natural drug discovery.

In Vitro Cell Sensitivity to Palytoxin Correlates with High Gene Expression of the Na+/K+-ATPase β2 Subunit Isoform

The marine polyether palytoxin (PLTX) is one of the most toxic natural compounds, and is involved in human poisonings after oral, inhalation, skin and/or ocular exposure. Epidemiological and molecular evidence suggest different inter-individual sensitivities to its toxic effects, possibly related to genetic-dependent differences in the expression of Na+/K+-ATPase, its molecular target. To identify Na+/K+-ATPase subunits, isoforms correlated with in vitro PLTX cytotoxic potency, sensitivity parameters (EC50: PLTX concentration reducing cell viability by 50%; Emax: maximum effect induced by the highest toxin concentration; 10-7 M) were assessed in 60 healthy donors' monocytes by the MTT (methylthiazolyl tetrazolium) assay. Sensitivity parameters, not correlated with donors' demographic variables (gender, age and blood group), demonstrated a high inter-individual variability (median EC50 = 2.7 × 10-10 M, interquartile range: 0.4-13.2 × 10-10 M; median Emax = 92.0%, interquartile range: 87.5-94.4%). Spearman's analysis showed significant positive correlations between the β2-encoding ATP1B2 gene expression and Emax values (rho = 0.30; p = 0.025) and between Emax and the ATP1B2/ATP1B3 expression ratio (rho = 0.38; p = 0.004), as well as a significant negative correlation between Emax and the ATP1B1/ATP1B2 expression ratio (rho = -0.30; p = 0.026). This toxicogenetic study represents the first approach to define genetic risk factors that may influence the onset of adverse effects in human PLTX poisonings, suggesting that individuals with high gene expression pattern of the Na+/K+-ATPase β2 subunit (alone or as β2/β1 and/or β2/β3 ratio) could be highly sensitive to PLTX toxic effects.

Closing the Gap Between Therapeutic Use and Mode of Action in Remedial Herbs

The ancient tradition of taking parts of a plant or preparing plant extracts for treating certain discomforts and maladies has long been lacking a scientific rationale to support its preparation and still widespread use in several parts of the world. In an attempt to address this challenge, we collected and integrated data connecting metabolites, plants, diseases, and proteins. A mechanistic hypothesis is generated when a metabolite is known to be present in a given plant, that plant is known to be used to treat a certain disease, that disease is known to be linked to the function of a given protein, and that protein is finally known or predicted to interact with the original metabolite. The construction of plant–protein networks from mutually connected metabolites and diseases facilitated the identification of plausible mechanisms of action for plants being used to treat analgesia, hypercholesterolemia, diarrhea, catarrh, and cough. Additional concrete examples using both experimentally known and computationally predicted, and subsequently experimentally confirmed, metabolite–protein interactions to close the connection circle between metabolites, plants, diseases, and proteins offered further proof of concept for the validity and scope of the approach to generate mode of action hypotheses for some of the therapeutic uses of remedial herbs.

Evaluation of Drug-Related Receptors in Children With Dilated Cardiomyopathy

Background: Effective treatments for pediatric dilated cardiomyopathy (DCM) are limited. Currently, pediatric DCM therapy mainly includes supportive heart failure (HF) treatment. While the treatment for child DCM patients is generally the same as that for adult DCM patients, few randomized prospective studies on the clinical efficacy of treatments for pediatric DCM have been published. We explored the appropriate treatments for child patients. Methods: The ultrastructure of pediatric DCM and control hearts was analyzed by electron microscopy and HE staining. Left ventricular tissues from children in the DCM and control groups were subjected to quantitative RT-PCR (qRT-PCR) to study the mRNA expression of receptors related to various treatments, including drugs targeting the renin-angiotensin-aldosterone system (RAAS) system, digoxin, milrinone, and β-receptor blockers, in child patients in the clinic. Furthermore, the differences in drug receptors in heart tissues between children and adults with DCM were analyzed. Results: Compared with the control children, the children in the DCM group showed marked abnormalities in structure and organelles. The mRNA levels of angiotensin-converting enzyme (ACE), REN, prorenin receptor (PRR), NEP, ATP1A1, and phosphodiesterase3 (PDE3A) were higher in the pediatric DCM group than the control group. Interestingly, the mRNA expression of these treatment-related receptors was much higher in children than in adults. Conclusion: ACE inhibitors, PRR or REN receptor inhibitors, PDE3 inhibitors and LCZ696 may be effective in children with DCM. However, β-receptor blockers are not valid treatments for pediatric DCM. Moreover, high receptor expression was observed in children. These data will improve the selection of drugs for DCM patients, enhance treatment, and increase the survival rate.

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

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

Osmoregulation in the Plotosidae Catfish: Role of the Salt Secreting Dendritic Organ

Unlike other marine teleosts, the Plotosidae catfishes reportedly have an extra-branchial salt secreting dendritic organ (DO). Salinity acclimation [brackishwater (BW) 3aaa, seawater (SWcontrol) 34aaa, and hypersaline water (HSW) 60aaa] for 14 days was used to investigate the osmoregulatory abilities of Plotosus lineatus through measurements of blood chemistry, muscle water content (MWC), Na⁺/K⁺-ATPase (NKA) specific activity and ion transporter expression in gills, DO, kidney and intestine. Ion transporter expression was determined using immunoblotting, immunohistochemistry (IHC) and quantitative polymerase chain reaction (qPCR). HSW elevated mortality, plasma osmolality and ions, and hematocrit, and decreased MWC indicating an osmoregulatory challenge. NKA specific activity and protein levels were significantly higher in DO compared to gill, kidney and intestine at all salinities. NKA specific activity increased in kidney and posterior intestine with HSW but only kidney showed correspondingly higher NKA α-subunit protein levels. Since DO mass was greater in HSW, the total amount of DO NKA activity expressed per gram fish was greater indicating higher overall capacity. Gill NKA and V-ATPase protein levels were greater with HSW acclimation but this was not reflected in NKA activity, mRNA or ionocyte abundance. BW acclimation resulted in lower NKA activity in gill, kidney and DO. Cl^- levels were better regulated and the resulting strong ion ratio in BW suggests a metabolic acidosis. Elevated DO heat shock protein 70 levels in HSW fish indicate a cellular stress. Strong NKA and NKCC1 (Na⁺:K⁺:2Cl^- cotransporter1) co-localization was observed in DO parenchymal cells, which was rare in gill ionocytes. NKCC1 immunoblot expression was only detected in DO, which was highest at HSW. Cystic fibrosis transmembrane regulator Cl^- channel (CFTR) localize apically to DO NKA immunoreactive cells. Taken together, the demonstration of high NKA activity in DO coexpressed with NKCC1 and CFTR indicates the presence of the conserved secondary active Cl^- secretion mechanism found in other ion transporting epithelia suggesting a convergent evolution with other vertebrate salt secreting organs. However, the significant osmoregulatory challenge of HSW indicates that the DO may be of limited use under more extreme salinity conditions in contrast to the gill based ionoregulatory strategy of marine teleosts.

Selectivity analyses of γ-benzylidene digoxin derivatives to different Na,K-ATPase α isoforms: a molecular docking approach

Digoxin and other cardiotonic steroids (CTS) exert their effect by inhibiting Na,K-ATPase (NKA) activity. CTS bind to the various NKA isoforms that are expressed in different cell types, which gives CTS their narrow therapeutic index. We have synthesised a series of digoxin derivatives (γ-Benzylidene digoxin derivatives) with substitutions in the lactone ring (including non-oxygen and ether groups), to obtain CTS with better NKA isoform specificity. Some of these derivatives show some NKA isoform selective effects, with BD-3, BD-8, and BD-13 increasing NKA α2 activity, BD-5 inhibiting NKA α1 and NKA α3, BD-10 reducing NKA α1, but stimulating NKA α2 and α3; and BD-14, BD-15, and BD-16 enhancing NKA α3 activity. A molecular-docking approach favoured NKA isoform specific interactions for the compounds that supported their observed activity. These results show that BD compounds are a new type of CTS with the capacity to target NKA activity in an isoform-specific manner.

Comparison of Digitalis Sensitivities of Na+/K+-ATPases from Human and Pig Kidneys

Digitalis drugs are selective inhibitors of the plasma membrane Na⁺/K⁺-ATPase. There are many studies on molecular mechanisms of digitalis interaction with purified pig kidney enzyme, with the tacit assumption that it is a good model of human kidney enzyme. However, previous studies on crude or recombinant human kidney enzymes are limited, and have not resulted in consistent findings on their digitalis sensitivities. Hence, we prepared comparably purified enzymes from human and pig kidneys and determined inhibitory constants of digoxin, ouabain, ouabagenin, bufalin, and marinobufagenin (MBG) on enzyme activity under optimal turnover conditions. We found that each compound had the same potency against the two enzymes, indicating that (i) the pig enzyme is an appropriate model of the human enzyme, and (ii) prior discrepant findings on human kidney enzymes were either due to structural differences between the natural and recombinant enzymes or because potencies were determined using binding constants of digitalis for enzymes under nonphysiological conditions. In conjunction with previous findings, our newly determined inhibitory constants of digitalis compounds for human kidney enzymes indicate that (i) of the compounds that have long been advocated to be endogenous hormones, only bufalin and MBG may act as such at kidney tubules, and (ii) beneficial effects of digoxin, the only digitalis with extensive clinical use, does not involve its inhibitory effect on renal tubular Na⁺/K⁺-ATPase.

Na+/K+-ATPase α1 subunit, a novel therapeutic target for hepatocellular carcinoma

We aimed to identify the expression patterns of Na⁺/K⁺-ATPase (NKA) α subunits in human hepatocellular carcinoma (HCC) samples and evaluate these subunits as potential targets for HCC treatment. The mRNA expression profiles of NKA α subunits in human HCC samples were analyzed. We found that the mRNA expression for NKA α1 subunit (ATP1A1) was higher than that for other NKA α subunits. Also, ATP1A1 gene expression was markedly higher in HCC samples than in adjacent nontumor tissue samples. Western blotting data suggested that 6 of 14 (43%) HCC samples had elevated ATP1A1 protein expression. Furthermore, knockdown of ATP1A1 expression in human HCC HepG2 and MHCC97H cells markedly reduced their proliferation in vitro and suppressed the tumorigenicity of MHCC97H cells in vivo. Downregulation of ATP1A1 expression resulted in cell-cycle arrest at G2/M phase and apoptosis in HepG2 cells as well as decreased migration in Hep3B cells. We further validated that ATP1A1 downregulation caused intracellular accumulation of reactive oxygen species. Pretreatment with N-acetyl cysteine blocked cell-growth inhibition induced by ATP1A1 downregulation. Collectively, these data suggested that targeting ATP1A1 is a novel approach to the treatment of HCC.

Pivotal role of α2 Na+ pumps and their high affinity ouabain binding site in cardiovascular health and disease

Reduced smooth muscle (SM)-specific α2 Na⁺ pump expression elevates basal blood pressure (BP) and increases BP sensitivity to angiotensin II (Ang II) and dietary NaCl, whilst SM-α2 overexpression lowers basal BP and decreases Ang II/salt sensitivity. Prolonged ouabain infusion induces hypertension in rodents, and ouabain-resistant mutation of the α2 ouabain binding site (α2^R/R mice) confers resistance to several forms of hypertension. Pressure overload-induced heart hypertrophy and failure are attenuated in cardio-specific α2 knockout, cardio-specific α2 overexpression and α2^R/R mice. We propose a unifying hypothesis that reconciles these apparently disparate findings: brain mechanisms, activated by Ang II and high NaCl, regulate sympathetic drive and a novel neurohumoral pathway mediated by both brain and circulating endogenous ouabain (EO). Circulating EO modulates ouabain-sensitive α2 Na⁺ pump activity and Ca²⁺ transporter expression and, via Na⁺ /Ca²⁺ exchange, Ca²⁺ homeostasis. This regulates sensitivity to sympathetic activity, Ca²⁺ signalling and arterial and cardiac contraction.

Cardiac glycosides induced toxicity in human cells expressing α1-, α2-, or α3-isoforms of Na-K-ATPase

The Na+-K+-ATPase is specifically inhibited by cardiac glycosides, some of which may also function as endogenous mammalian hormones. Previous studies using Xenopus oocytes, yeast cells, or purified isoforms demonstrated that affinities of various cardiac glycosides for three isoforms of the Na+-K+-ATPase (α1-α3β1) may differ, a finding with potential clinical implication. The present study investigates isoform selectivity and effects of cardiac glycosides on cultured mammalian cells under more physiological conditions. H1299 cells (non-small cell lung carcinoma) were engineered to express only one α-isoform (α1, α2, or α3) by combining stable transfection of isoforms and silencing endogenous α1. Cardiac glycoside binding was measured by displacement of bound 3H-ouabain. The experiments confirm moderate α1/α3:α2 selectivity of ouabain, moderate α2:α1 selectivity of digoxin, and enhanced α2:α1 selectivity of synthetic derivatives (Katz A, Tal DM, Heller D, Haviv H, Rabah B, Barkana Y, Marcovich AL, Karlish SJD. J Biol Chem 289: 21153-21162, 2014). Relative α2:α1 selectivity of digoxin vs. ouabain was also manifested by enhanced internalization of α2 in response to digoxin. Cellular proliferation assays of H1299 cells confirmed the patterns of α2:α1 selectivity for ouabain, digoxin, and a synthetic derivative and reveal a crucial role of surface pump density on sensitivity to cardiac glycosides. Because cardiac glycosides are being considered as drugs for treatment of cancer, effects of ouabain on proliferation of 12 cancer and noncancer cell lines, with variable plasma membrane expression of α1, have been tested. These demonstrated that sensitivity to ouabain indeed depends linearly on the plasma membrane surface density of Na+-K+-ATPase irrespective of status, malignant or nonmalignant.

Oleandrin: A cardiac glycosides with potent cytotoxicity

Cardiac glycosides are used in the treatment of congestive heart failure and arrhythmia. Current trend shows use of some cardiac glycosides in the treatment of proliferative diseases, which includes cancer. Nerium oleander L. is an important Chinese folk medicine having well proven cardio protective and cytotoxic effect. Oleandrin (a toxic cardiac glycoside of N. oleander L.) inhibits the activity of nuclear factor kappa-light-chain-enhancer of activated B chain (NF-κB) in various cultured cell lines (U937, CaOV3, human epithelial cells and T cells) as well as it induces programmed cell death in PC3 cell line culture. The mechanism of action includes improved cellular export of fibroblast growth factor-2, induction of apoptosis through Fas gene expression in tumor cells, formation of superoxide radicals that cause tumor cell injury through mitochondrial disruption, inhibition of interleukin-8 that mediates tumorigenesis and induction of tumor cell autophagy. The present review focuses the applicability of oleandrin in cancer treatment and concerned future perspective in the area.

Na,K-ATPase alpha isoforms at the blood-cerebrospinal fluid-trigeminal nerve and blood-retina interfaces in the rat

Background

Cerebrospinal fluid (CSF) sodium concentration increases during migraine attacks, and both CSF and vitreous humor sodium increase in the rat migraine model. The Na,K-ATPase is a probable source of these sodium fluxes. Since Na,K-ATPase isoforms have different locations and physiological roles, our objective was to establish which alpha isoforms are present at sites where sodium homeostasis is disrupted.

Methods

Specific Na,K-ATPase alpha isoforms were identified in rat tissues by immunohistochemistry at the blood-CSF barrier at the choroid plexus, at the blood-CSF-trigeminal barrier at the meninges, at the blood-retina barrier, and at the blood-aqueous barrier at the ciliary body. Calcitonin gene-related peptide (CGRP), occludin, or von Willibrand factor (vWF) were co-localized with Na,K-ATPase to identify trigeminal nociceptor fibers, tight junctions, and capillary endothelial cells respectively.

Results

The Na,K-ATPase alpha-2 isoform is located on capillaries and intensely at nociceptive trigeminal nerve fibers at the meningeal blood-CSF-trigeminal barrier. Alpha-1 and −3 are lightly expressed on the trigeminal nerve fibers but not at capillaries. Alpha-2 is expressed at the blood-retina barriers and, with alpha-1, at the ciliary body blood aqueous barrier. Intense apical membrane alpha-1 was associated with moderate cytoplasmic alpha-2 expression at the choroid plexus blood-CSF barrier.

Conclusion

Na,K-ATPase alpha isoforms are present at the meningeal, choroid plexus, and retinal barriers. Alpha-2 predominates at the capillary endothelial cells in the meninges and retinal ganglion cell layer.

Branchial ionocyte organization and ion-transport protein expression in juvenile alewives acclimated to freshwater or seawater

The alewife (Alosa pseudoharengus) is a clupeid that undergoes larval and juvenile development in freshwater preceding marine habitation. The purpose of this study was to investigate osmoregulatory mechanisms in alewives that permit homeostasis in different salinities. To this end, we measured physiological, branchial biochemical and cellular responses in juvenile alewives acclimated to freshwater (0.5 p.p.t.) or seawater (35.0 p.p.t.). Plasma chloride concentration was higher in seawater-acclimated than freshwater-acclimated individuals (141 mmol l(-1) vs 134 mmol l(-1)), but the hematocrit remained unchanged. In seawater-acclimated individuals, branchial Na(+)/K(+)-ATPase (NKA) activity was higher by 75%. Western blot analysis indicated that the abundance of the NKA α-subunit and a Na(+)/K(+)/2Cl(-) cotransporter (NKCC1) were greater in seawater-acclimated individuals by 40% and 200%, respectively. NKA and NKCC1 were localized on the basolateral surface and tubular network of ionocytes in both acclimation groups. Immunohistochemical labeling for the cystic fibrosis transmembrane conductance regulator (CFTR) was restricted to the apical crypt of ionocytes in seawater-acclimated individuals, whereas sodium/hydrogen exchanger 3 (NHE3) labeling was present on the apical surface of ionocytes in both acclimation groups. Ionocytes were concentrated on the trailing edge of the gill filament, evenly distributed along the proximal 75% of the filamental axis and reduced distally. Ionocyte size and number on the gill filament were not affected by salinity; however, the number of lamellar ionocytes was significantly lower in seawater-acclimated fish. Confocal z-series reconstructions revealed that mature ionocytes in seawater-acclimated alewives occurred in multicellular complexes. These complexes might reduce paracellular Na(+) resistance, hence facilitating Na(+) extrusion in hypo-osmoregulating juvenile alewives after seaward migration.

Shortening and intracellular Ca2+ in ventricular myocytes and expression of genes encoding cardiac muscle proteins in early onset type 2 diabetic Goto-Kakizaki rats

There has been a spectacular rise in the global prevalence of type 2 diabetes mellitus. Cardiovascular complications are the major cause of morbidity and mortality in diabetic patients. Contractile dysfunction, associated with disturbances in excitation-contraction coupling, has been widely demonstrated in the diabetic heart. The aim of this study was to investigate the pattern of cardiac muscle genes that are involved in the process of excitation-contraction coupling in the hearts of early onset (8-10 weeks of age) type 2 diabetic Goto-Kakizaki (GK) rats. Gene expression was assessed in ventricular muscle with real-time RT-PCR; shortening and intracellular Ca(2+) were measured in ventricular myocytes with video edge detection and fluorescence photometry, respectively. The general characteristics of the GK rats included elevated fasting and non-fasting blood glucose and blood glucose at 120 min following a glucose challenge. Expression of genes encoding cardiac muscle proteins (Myh6/7, Mybpc3, Myl1/3, Actc1, Tnni3, Tnn2, Tpm1/2/4 and Dbi) and intercellular proteins (Gja1/4/5/7, Dsp and Cav1/3) were unaltered in GK ventricle compared with control ventricle. The expression of genes encoding some membrane pumps and exchange proteins was unaltered (Atp1a1/2, Atp1b1 and Slc8a1), whilst others were either upregulated (Atp1a3, relative expression 2.61 ± 0.69 versus 0.84 ± 0.23) or downregulated (Slc9a1, 0.62 ± 0.07 versus 1.08 ± 0.08) in GK ventricle compared with control ventricle. The expression of genes encoding some calcium (Cacna1c/1g, Cacna2d1/2d2 and Cacnb1/b2), sodium (Scn5a) and potassium channels (Kcna3/5, Kcnj3/5/8/11/12, Kchip2, Kcnab1, Kcnb1, Kcnd1/2/3, Kcne1/4, Kcnq1, Kcng2, Kcnh2, Kcnk3 and Kcnn2) were unaltered, whilst others were either upregulated (Cacna1h, 0.95 ± 0.16 versus 0.47 ± 0.09; Scn1b, 1.84 ± 0.16 versus 1.11 ± 0.11; and Hcn2, 1.55 ± 0.15 versus 1.03 ± 0.08) or downregulated (Hcn4, 0.16 ± 0.03 versus 0.37 ± 0.08; Kcna2, 0.35 ± 0.03 versus 0.80 ± 0.11; Kcna4, 0.79 ± 0.25 versus 1.90 ± 0.26; and Kcnj2, 0.52 ± 0.07 versus 0.78 ± 0.08) in GK ventricle compared with control ventricle. The amplitude of ventricular myocyte shortening and the intracellular Ca(2+) transient were unaltered; however, the time-to-peak shortening was prolonged and time-to-half decay of the Ca(2+) transient was shortened in GK myocytes compared with control myocytes. The results of this study demonstrate changes in expression of genes encoding various excitation-contraction coupling proteins that are associated with disturbances in myocyte shortening and intracellular Ca(2+) transport.

Stabilization of the α2 isoform of Na,K-ATPase by mutations in a phospholipid binding pocket

The α2 isoform of Na,K-ATPase plays a crucial role in Ca(2+) handling, muscle contraction, and inotropic effects of cardiac glycosides. Thus, structural, functional, and pharmacological comparisons of α1, α2, and α3 are of great interest. In Pichia pastoris membranes expressing human α1β1, α2β1, and α3β1 isoforms, or using the purified isoform proteins, α2 is most easily inactivated by heating and detergent (α2 ≫ α3 > α1). We have examined an hypothesis that instability of α2 is caused by weak interactions with phosphatidylserine, which stabilizes the protein. Three residues, unique to α2, in trans-membrane segments M8 (Ala-920), M9 (Leu-955), and M10 (Val-981) were replaced by equivalent residues in α1, singly or together. Judged by the sensitivity of the purified proteins to heat, detergent, "affinity" for phosphatidylserine, and stabilization by FXYD1, the triple mutant (A920V/L955F/V981P, called α2VFP) has stability properties close to α1, although single mutants have only modest or insignificant effects. Functional differences between α1 and α2 are unaffected in α2VFP. A compound, 6-pentyl-2-pyrone, isolated from the marine fungus Trichoderma gamsii is a novel probe of specific phospholipid-protein interactions. 6-Pentyl-2-pyrone inactivates the isoforms in the order α2 ≫ α3 > α1, and α2VFP and FXYD1 protect the isoforms. In native rat heart sarcolemma membranes, which contain α1, α2, and α3 isoforms, a component attributable to α2 is the least stable. The data provide clear evidence for a specific phosphatidylserine binding pocket between M8, M9, and M10 and confirm that the instability of α2 is due to suboptimal interactions with phosphatidylserine. In physiological conditions, the instability of α2 may be important for its cellular regulatory functions.

Saccharomyces cerivisiae as a model system for kidney disease: what can yeast tell us about renal function?

Ion channels, solute transporters, aquaporins, and factors required for signal transduction are vital for kidney function. Because mutations in these proteins or in associated regulatory factors can lead to disease, an investigation into their biogenesis, activities, and interplay with other proteins is essential. To this end, the yeast, Saccharomyces cerevisiae, represents a powerful experimental system. Proteins expressed in yeast include the following: 1) ion channels, including the epithelial sodium channel, members of the inward rectifying potassium channel family, and cystic fibrosis transmembrane conductance regulator; 2) plasma membrane transporters, such as the Na(+)-K(+)-ATPase, the Na(+)-phosphate cotransporter, and the Na(+)-H(+) ATPase; 3) aquaporins 1-4; and 4) proteins such as serum/glucocorticoid-induced kinase 1, phosphoinositide-dependent kinase 1, Rh glycoprotein kidney, and trehalase. The variety of proteins expressed and studied emphasizes the versatility of yeast, and, because of the many available tools in this organism, results can be obtained rapidly and economically. In most cases, data gathered using yeast have been substantiated in higher cell types. These attributes validate yeast as a model system to explore renal physiology and suggest that research initiated using this system may lead to novel therapeutics.

Increase in ouabain-sensitive K+-ATPase activity in hepatocellular carcinoma by overexpression of Na+, K+-ATPase alpha 3-isoform

Na(+),K(+)-ATPase is a housekeeping pump in virtually all animal cells. Recently, cardiac glycosides that inhibit Na(+),K(+)-ATPase have been reported to be candidate for novel anticancer drug. Here, we investigated clinical significance of Na(+),K(+)-ATPase alpha1-isoform (alpha 1NaK), alpha2-isoform (alpha 2NaK) and alpha 3-isoform (alpha 3NaK) in hepatocellular carcinoma (HCC). Interestingly, the expression levels of alpha 3NaK protein in HCC tissues were significantly higher than those in the accompanying non-tumor tissues, whereas no significant increases in expression of alpha 1NaK and alpha 2NaK proteins were observed in HCC compared to non-tumor tissues. The ouabain (10 microM)-sensitive K(+)-ATPase activities (Na(+),K(+)-ATPase activities) in the membrane fraction from HCC tissue were significantly higher than those from non-tumor tissues. The Na(+),K(+)-ATPase activity was positively and significantly correlated with the expression level of alpha 3NaK. Apparent affinity for Na(+) in the Na(+),K(+)-ATPase activity in HCC tissues was significantly lower than that in non-tumor tissues, consistent with an elevated expression of alpha 3NaK relative to alpha 1NaK. Our results suggest that overexpression of alpha 3NaK increases the Na(+),K(+)-ATPase activity of HCC cells.

Selectivity of digitalis glycosides for isoforms of human Na,K-ATPase

There are four isoforms of the alpha subunit (alpha1-4) and three isoforms of the beta subunit (beta1-3) of Na,K-ATPase, with distinct tissue-specific distribution and physiological functions. alpha2 is thought to play a key role in cardiac and smooth muscle contraction and be an important target of cardiac glycosides. An alpha2-selective cardiac glycoside could provide important insights into physiological and pharmacological properties of alpha2. The isoform selectivity of a large number of cardiac glycosides has been assessed utilizing alpha1beta1, alpha2beta1, and alpha3beta1 isoforms of human Na,K-ATPase expressed in Pichia pastoris and the purified detergent-soluble isoform proteins. Binding affinities of the digitalis glycosides, digoxin, beta-methyl digoxin, and digitoxin show moderate but highly significant selectivity (up to 4-fold) for alpha2/alpha3 over alpha1 (K(D) alpha1 > alpha2 = alpha3). By contrast, ouabain shows moderate selectivity ( approximately 2.5-fold) for alpha1 over alpha2 (K(D) alpha1 <or= alpha3 < alpha2). Binding affinities for the three isoforms of digoxigenin, digitoxigenin, and all other aglycones tested are indistinguishable (K(D) alpha1 = alpha3 = alpha2), showing that the sugar determines isoform selectivity. Selectivity patterns for inhibition of Na,K-ATPase activity of the purified isoform proteins are consistent with binding selectivities, modified somewhat by different affinities of K(+) ions for antagonizing cardiac glycoside binding on the three isoforms. The mechanistic insight on the role of the sugars is strongly supported by a recent structure of Na,K-ATPase with bound ouabain, which implies that aglycones of cardiac glycosides cannot discriminate between isoforms. In conclusion, several digitalis glycosides, but not ouabain, are moderately alpha2-selective. This supports a major role of alpha2 in cardiac contraction and cardiotonic effects of digitalis glycosides.