Na+ pump low and high ouabain affinity alpha subunit isoforms are differently distributed in cells

Biological science of headache channels

Several episodic neurological diseases, including familial hemiplegic migraine (FHM) and different types of epilepsy, are caused by mutations in ion channels, and hence classified as channelopathies. The classification of FHM as a channelopathy has introduced a new perspective in headache research and has strengthened the idea of migraine as a disorder of neural excitability. Here we review recent studies of the functional consequences of mutations in the CACNA1A and SCNA1A genes (encoding the pore-forming subunit of Ca(V)2.1 and Na(V)1.1 channels) and the ATPA1A2 gene (encoding the alpha(2) subunit of the Na(+)/K(+) pump), responsible for FHM1, FHM3, and FHM2, respectively. These studies show that: (1) FHM1 mutations produce gain-of-function of the Ca(V)2.1 channel and, as a consequence, increased glutamate release at cortical synapses and facilitation of induction and propagation of cortical spreading depression (CSD); (2) FHM2 mutations produce loss-of-function of the alpha(2) Na(+)/K(+)-ATPase; and (3) the FHM3 mutation accelerates recovery from fast inactivation of Na(V)1.5 channels. These findings are consistent with the hypothesis that FHM mutations share the ability to render the brain more susceptible to CSD, by causing excessive synaptic glutamate release (FHM1) or decreased removal of K(+) and glutamate from the synaptic cleft (FHM2) or excessive extracellular K(+) (FHM3).

Na+ transport in cardiac myocytes; Implications for excitation-contraction coupling

Intracellular Na(+) concentration ([Na(+)](i)) is very important in modulating the contractile and electrical activity of the heart. Upon electrical excitation of the myocardium, voltage-dependent Na(+) channels open, triggering the upstroke of the action potential (AP). During the AP, Ca(2+) enters the myocytes via L-type Ca(2+) channels. This triggers Ca(2+) release from the sarcoplasmic reticulum (SR) and thus activates contraction. Relaxation occurs when cytosolic Ca(2+) declines, mainly due to re-uptake into the SR via SR Ca(2+)-ATPase and extrusion from the cell via the Na(+)/Ca(2+) exchanger (NCX). NCX extrudes one Ca(2+) ion in exchange for three Na(+) ions and its activity is critically regulated by [Na(+)](i). Thus, via NCX, [Na(+)](i) is centrally involved in the regulation of intracellular [Ca(2+)] and contractility. Na(+) brought in by Na(+) channels, NCX and other Na(+) entry pathways is extruded by the Na(+)/K(+) pump (NKA) to keep [Na(+)](i) low. NKA is regulated by phospholemman, a small sarcolemmal protein that associates with NKA. Unphosphorylated phospholemman inhibits NKA by decreasing the pump affinity for internal Na(+) and this inhibition is relieved upon phosphorylation. Here we discuss the main characteristics of the Na(+) transport pathways in cardiac myocytes and their physiological and pathophysiological relevance.

Selective expression of a sodium pump isozyme by cough receptors and evidence for its essential role in regulating cough

We have identified a distinct subtype of airway vagal afferent nerve that plays an essential role in regulating the cough reflex. These afferents are exquisitely sensitive to punctate mechanical stimuli, acid, and decreases in extracellular chloride concentrations, but are insensitive to capsaicin, bradykinin, histamine, adenosine, serotonin, or changes in airway intraluminal pressures. In this study we used intravital imaging, retrograde neuronal tracing, and electrophysiological analyses to characterize the structural basis for their peculiar mechanical sensitivity and to further characterize the regulation of their excitability. In completing these experiments, we uncovered evidence for an essential role of an isozyme of Na(+)-K(+) ATPase in regulating cough. These vagal sensory neurons arise bilaterally from the nodose ganglia and are selectively and brilliantly stained intravitally with the styryl dye FM2-10. Cough receptor terminations are confined and adherent to the extracellular matrix separating the airway epithelium and smooth muscle layers, a site of extensive remodeling in asthma and chronic obstructive pulmonary disease. The cough receptor terminals uniquely express the alpha(3) subunit of Na(+)-K(+) ATPase. Intravital staining of cough receptors by FM2-10, cough receptor excitability in vitro, and coughing in vivo are potently and selectively inhibited by the sodium pump inhibitor ouabain. These data provide the first detailed morphological description of the peripheral terminals of the sensory nerves regulating cough and identify a selective molecular target for their modulation.

Upregulation of Na+ and Ca2+ transporters in arterial smooth muscle from ouabain-induced hypertensive rats

Prolonged ouabain administration (25 microg kg(-1) day(-1) for 5 wk) induces "ouabain hypertension" (OH) in rats, but the molecular mechanisms by which ouabain elevates blood pressure are unknown. Here, we compared Ca(2+) signaling in mesenteric artery smooth muscle cells (ASMCs) from normotensive (NT) and OH rats. Resting cytosolic free Ca(2+) concentration ([Ca(2+)](cyt); measured with fura-2) and phenylephrine-induced Ca(2+) transients were augmented in freshly dissociated OH ASMCs. Immunoblots revealed that the expression of the ouabain-sensitive alpha(2)-subunit of Na(+) pumps, but not the predominant, ouabain-resistant alpha(1)-subunit, was increased (2.5-fold vs. NT ASMCs) as was Na(+)/Ca(2+) exchanger-1 (NCX1; 6-fold vs. NT) in OH arteries. Ca(2+) entry, activated by sarcoplasmic reticulum (SR) Ca(2+) store depletion with cyclopiazonic acid (SR Ca(2+)-ATPase inhibitor) or caffeine, was augmented in OH ASMCs. This reflected an augmented expression of 2.5-fold in OH ASMCs of C-type transient receptor potential TRPC1, an essential component of store-operated channels (SOCs); two other components of some SOCs were not expressed (TRPC4) or were not upregulated (TRPC5). Ba(2+) entry activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol [a measure of receptor-operated channel (ROC) activity] was much greater in OH than NT ASMCs. This correlated with a sixfold upregulation of TRPC6 protein, a ROC family member. Importantly, in primary cultured mesenteric ASMCs from normal rats, 72-h treatment with 100 nM ouabain significantly augmented NCX1 and TRPC6 protein expression and increased resting [Ca(2+)](cyt) and ROC activity. SOC activity was also increased. Silencer RNA knockdown of NCX1 markedly downregulated TRPC6 and eliminated the ouabain-induced augmentation; silencer RNA knockdown of TRPC6 did not affect NCX1 expression but greatly attenuated its upregulation by ouabain. Clearly, NCX1 and TRPC6 expression are interrelated. Thus, prolonged ouabain treatment upregulates the Na(+) pump alpha(2)-subunit-NCX1-TRPC6 (ROC) Ca(2+) signaling pathway in arterial myocytes in vitro as well as in vivo. This may explain the augmented myogenic responses and enhanced phenylephrine-induced vasoconstriction in OH arteries (83) as well as the high blood pressure in OH rats.

Effect of ouabain on sodium pump alpha-isoform expression in an animal model of mania

While the pathophysiologic mechanisms of bipolar illness are unknown, a dysregulation of electrolytes, particularly intracellular sodium (Na) and calcium (Ca), are thought to contribute to the illness. Ouabain, a potent Na pump inhibitor, administered intracerebroventricularly (ICV), has been used previously to model mania. The current study evaluates the effect of ICV ouabain on Na pump isoform expression in rat brain. Animals received 5 microl ICV of either 10(-3) M ouabain or artificial cerebrospinal fluid (aCSF). They were then sacrificed 7 days after the ICV injection and specific brain areas were dissected and frozen until the assay (frontal cortex, hippocampus, and basal ganglia). The three isoforms of the alpha subunit of the Na pump that are expressed in the brain were quantified with immunoblot analysis with actin serving as internal control. The behavioral hyperactivity seen in rats receiving ICV ouabain is associated with an increase of expression of the glial-specific alpha2 isoform in the basal ganglia, and the neuron-specific alpha3 isoforms in the frontal cortex. These findings, in association with human post mortem studies finding that alpha2 is underexpressed in the temporal cortex of bipolar subjects, suggest that Na pump isoform expression may be of interest in the pathophysiology of mania.

Oleandrin-mediated inhibition of human tumor cell proliferation: importance of Na,K-ATPase alpha subunits as drug targets

Cardiac glycosides such as oleandrin are known to inhibit the Na,K-ATPase pump, resulting in a consequent increase in calcium influx in heart muscle. Here, we investigated the effect of oleandrin on the growth of human and mouse cancer cells in relation to Na,K-ATPase subunits. Oleandrin treatment resulted in selective inhibition of human cancer cell growth but not rodent cell proliferation, which corresponded to the relative level of Na,K-ATPase alpha3 subunit protein expression. Human pancreatic cancer cell lines were found to differentially express varying levels of alpha3 protein, but rodent cancer cells lacked discernable expression of this Na,K-ATPase isoform. A correlation was observed between the ratio of alpha3 to alpha1 isoforms and the level of oleandrin uptake during inhibition of cell growth and initiation of cell death; the higher the alpha3 expression relative to alpha1 expression, the more sensitive the cell was to treatment with oleandrin. Inhibition of proliferation of Panc-1 cells by oleandrin was significantly reduced when the relative expression of alpha3 was decreased by knocking down the expression of alpha3 isoform with alpha3 siRNA or increasing expression of the alpha1 isoform through transient transfection of alpha1 cDNA to the cells. Our data suggest that the relative lack of alpha3 (relative to alpha1) in rodent and some human tumor cells may explain their unresponsiveness to cardiac glycosides. In conclusion, the relatively higher expression of alpha3 with the limited expression of alpha1 may help predict which human tumors are likely to be responsive to treatment with potent lipid-soluble cardiac glycosides such as oleandrin.

Isoform specificity of the Na/K-ATPase association and regulation by phospholemman

Phospholemman (PLM) phosphorylation mediates enhanced Na/K-ATPase (NKA) function during adrenergic stimulation of the heart. Multiple NKA isoforms exist, and their function/regulation may differ. We combined fluorescence resonance energy transfer (FRET) and functional measurements to investigate isoform specificity of the NKA-PLM interaction. FRET was measured as the increase in the donor fluorescence (CFP-NKA-alpha1 or CFP-NKA-alpha2) during progressive acceptor (PLM-YFP) photobleach in HEK-293 cells. Both pairs exhibited robust FRET (maximum of 23.6 +/- 3.4% for NKA-alpha1 and 27.5 +/- 2.5% for NKA-alpha2). Donor fluorescence depended linearly on acceptor fluorescence, indicating a 1:1 PLM:NKA stoichiometry for both isoforms. PLM phosphorylation induced by cAMP-dependent protein kinase and protein kinase C activation drastically reduced the FRET with both NKA isoforms. However, submaximal cAMP-dependent protein kinase activation had less effect on PLM-NKA-alpha2 versus PLM-NKA-alpha1. Surprisingly, ouabain virtually abolished NKA-PLM FRET but only partially reduced co-immunoprecipitation. PLM-CFP also showed FRET to PLM-YFP, but the relationship during progressive photobleach was highly nonlinear, indicating oligomers involving >or=3 monomers. Using cardiac myocytes from wild-type mice and mice where NKA-alpha1 is ouabain-sensitive and NKA-alpha2 is ouabain-resistant, we assessed the effects of PLM phosphorylation on NKA-alpha1 and NKA-alpha2 function. Isoproterenol enhanced internal Na(+) affinity of both isoforms (K((1/2)) decreased from 18.1 +/- 2.0 to 11.5 +/- 1.9 mm for NKA-alpha1 and from 16.4 +/- 2.5 to 10.4 +/- 1.5 mm for NKA-alpha2) without altering maximum transport rate (V(max)). Protein kinase C activation also decreased K((1/2)) for both NKA-alpha1 and NKA-alpha2 (to 9.4 +/- 1.0 and 9.1 +/- 1.1 mm, respectively) but increased V(max) only for NKA-alpha2 (1.9 +/- 0.4 versus 1.2 +/- 0.5 mm/min). In conclusion, PLM associates with and modulates both NKA-alpha1 and NKA-alpha2 in a comparable but not identical manner.

Extracellular potassium dependence of the Na+-K+-ATPase in cardiac myocytes: isoform specificity and effect of phospholemman

Cardiac Na(+)-K(+)-ATPase (NKA) regulates intracellular Na(+), which in turn affects intracellular Ca(2+) and contractility via the Na(+)/Ca(2+) exchanger. Extracellular K(+) concentration ([K(+)]) is a central regulator of NKA activity. Phospholemman (PLM) has recently been recognized as a critical regulator of NKA in the heart. PLM reduces the intracellular Na(+) affinity of NKA, an effect relieved by PLM phosphorylation. Here we tested whether the NKA alpha(1)- vs. alpha(2)- isoforms have different external K(+) sensitivity and whether PLM and PKA activation affects the NKA affinity for K(+) in mouse cardiac myocytes. We measured the external [K(+)] dependence of the pump current generated by the ouabain-resistant NKA isoform in myocytes from wild-type (WT) mice (i.e., current due to NKA-alpha(1)) and mice in which the NKA isoforms have swapped ouabain affinities (alpha(1) is ouabain sensitive and alpha(2) is ouabain resistant) to assess current due to NKA-alpha(2). We found that NKA-alpha(1) has a higher affinity for external K(+) than NKA-alpha(2) [half-maximal pump activation (K(0.5)) = 1.5 +/- 0.1 vs. 2.9 +/- 0.3 mM]. The apparent external K(+) affinity of NKA was significantly lower in myocytes from WT vs. PLM-knockout mice (K(0.5) = 2.0 +/- 0.2 vs. 1.05 +/- 0.08 mM). However, PKA activation by isoproterenol (1 microM) did not alter the K(0.5) of NKA for external K(+) in WT myocytes. We conclude that 1) NKA-alpha(1) has higher affinity for K(+) than NKA-alpha(2) in cardiac myocytes, 2) PLM decreases the apparent external K(+) affinity of NKA, and 3) phosphorylation of PLM at the cytosolic domain does not alter apparent extracellular K(+) affinity of NKA.

A review of recent insights into the role of the sarcoplasmic reticulum and Ca entry in uterine smooth muscle

The uterine sacroplasmic reticulum (SR) takes up and stores calcium [Ca], using an ATPase (SERCA) and the Ca-buffering proteins, calsequestrin and calreticulin. This stored Ca can be released via IP(3)-gated Ca channels. Decreases in luminal Ca concentration [Ca] have been directly measured following agonist stimulation. During spontaneous contractions however, there appears to be no involvement of the SR, as Ca entry and efflux across the plasma membrane account for these phasic contractions. After over-viewing current knowledge concerning SR structure and function, we highlight three areas of research which suggest new ways of looking at the role of the SR in the uterus, although they may be controversial or speculative at the moment. Firstly, we review the evidence for the function, if any, of Ca-induced SR Ca release channels, the ryanodine receptor (RyR) and the lack of Ca sparks (the elemental release events from RyRs), in the uterus. Secondly, we ask does regulation of SERCA by the accessory protein, phospholamban, occur in the uterus and what is the effect of knocking out phospholamban on uterine activity? Thirdly, we address the question of when and how store-operated Ca entry occurs in the myometrium. By analogy with other, usually less excitable tissues, is there a mechanism that links store Ca depletion to plasma membrane Ca entry in smooth muscle cells within intact uterus and is it physiologically relevant and regulated? Are the recently described proteins ORAI and STIM-1 involved in uterine store-operated Ca entry? We end the review by integrating these new insights with previous data to present a new working model of the SR in the uterus.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

The Na-K-ATPase and calcium-signaling microdomains

The Na-K-ATPase is an energy-transducing ion pump that converts the free energy of ATP into transmembrane ion gradients. It also serves as a functional receptor for cardiotonic steroids such as ouabain and digoxin. Binding of ouabain to the Na-K-ATPase can activate calcium signaling in a cell-specific manner. The exquisite calcium modulation via the Na-K-ATPase is achieved by the ability of the pump to integrate signals from numerous protein and non-protein molecules, including ion transporters, channels, protein kinases/phosphatases, as well as cellular Na+. This review focuses on the unique properties of the Na-K-ATPase and its role in the formation of different calcium-signaling microdomains.

Ca2+ clearance and contractility in vascular smooth muscle: evidence from gene-altered murine models

The central importance of calcium clearance proteins, and their regulators, in the modulation of myocardial contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) has long been established. Key players identified include the Na(+)-Ca(2+) exchanger, the Na(+)-K(+) ATPase, the sarco(endo)plasmic reticulum Ca(2+)-ATPase and associated phospholamban. Gene-targeted and transgenic murine models have been critical in the elucidation of their function. The study of these proteins in the regulation of contractile parameters in vascular smooth muscle, on the other hand, is less well studied. More recently, gene-targeted and transgenic models have expanded our knowledge of Ca(2+) clearance proteins and their role in both tonic and phasic smooth muscle contractility. In this review, we will briefly treat the mechanisms which underlie Ca(2+) clearance in smooth muscle. These will be addressed in light of studies using gene-modified mouse models, the results of which will be compared and contrasted with those in the cardiomyocyte. The recently identified human mutations in phospholamban, which lead to dilated cardiomyopathy, are also present in vascular and other smooth muscle. Given the importance of these Ca(2+) clearance systems to modulation of smooth muscle, it is likely that mutations will also lead to smooth muscle pathology.

Gap junctions couple astrocytes and oligodendrocytes

In vertebrates, a family of related proteins called connexins form gap junctions (GJs), which are intercellular channels. In the central nervous system (CNS), GJs couple oligodendrocytes and astrocytes (O/A junctions) and adjacent astrocytes (A/A junctions), but not adjacent oligodendrocytes, forming a "glial syncytium." Oligodendrocytes and astrocytes each express different connexins. Mutations of these connexin genes demonstrate that the proper functioning of myelin and oligodendrocytes requires the expression of these connexins. The physiological function of O/A and A/A junctions, however, remains to be illuminated.

Ouabain-induced stimulation of sodium-hydrogen exchange in rat optic nerve astrocytes

Sodium-dependent transporters are inhibited indirectly by the Na-K-ATPase inhibitor ouabain. Here we report stimulation of sodium-hydrogen exchange (NHE) in ouabain-treated cells. BCECF was used to measure cytoplasmic pH in cultured rat optic nerve astrocytes. Ammonium chloride was applied to acid load the cells. On removal of ammonium chloride, cytoplasmic pH fell abruptly, then gradually recovered toward baseline. Ouabain (1 microM) did not change cell sodium content, but the rate of pH recovery increased by 68%. Ouabain speeded pH recovery both in the presence and absence of bicarbonate. In bicarbonate-free medium, dimethylamiloride, an NHE inhibitor, eliminated the effect of 1 microM ouabain on pH recovery. Western blot analysis showed an NHE1 immunoreactive band but not NHE2, NHE3, or NHE4. Immunoprecipitation studies showed phosphorylation of NHE1 in cells treated with 1 microM ouabain. Ouabain evoked an increase of cAMP, and the effect of 1 microM ouabain on pH recovery was abolished by H-89, a protein kinase A inhibitor. 8-Bromoadenosine-cAMP increased the pH recovery rate, and this recovery was not further increased by ouabain. Although 1 microM ouabain did not alter cytoplasmic calcium concentration, it stimulated calcium entry after store depletion, a response abolished by 2-APB. Ouabain-induced stimulation of pH recovery was suppressed by inhibitors of capacitative calcium entry, SKF-96365, and 2-APB, as well as the cytoplasmic calcium chelator BAPTA. The cAMP increase in ouabain-treated cells was abolished by BAPTA and 2-APB. Taken together, the results are consistent with increased capacitative calcium entry and subsequent cAMP-PKA-dependent stimulation of NHE1 in ouabain-treated cells.

Thermodynamic regulation of NKCC1-mediated Cl- cotransport underlies plasticity of GABA(A) signaling in neonatal neurons

In the adult brain, chloride (Cl-) influx through GABA(A) receptors is an important mechanism of synaptic inhibition. However, under a variety of circumstances, including acquired epilepsy, neuropathic pain, after trains of action potentials or trauma, and during normal early brain development, GABA(A) receptor activation excites neurons by gating Cl- efflux because the intracellular Cl- concentration (Cl(i)) is elevated. These findings require an inducible, active mechanism of chloride accumulation. We used gramicidin-perforated patch recordings to characterize Cl- transport via NKCC1, the principal neuronal Cl- accumulator, in neonatal CA1 pyramidal neurons. NKCC1 activity was required to maintain elevated Cl(i) such that GABA(A) receptor activation was depolarizing. Kinetic analysis of NKCC1 revealed reversible transmembrane Cl- transport characterized by a large maximum velocity (vmax) and high affinity (Km), so that NKCC1 transport was limited only by the net electrochemical driving force for Na+, K+, and Cl-. At the steady-state Cl(i), NKCC1 was at thermodynamic equilibrium, and there was no evidence of net Cl- transport. Trains of action potentials that have been previously shown to induce persistent changes in neuronal E(Cl) (reversal potential for Cl-) did not alter vmax or Km of NKCC1. Rather, action potentials shifted the thermodynamic set point, the steady-state Cl(i) at which there was no net NKCC1-mediated Cl- transport. The persistent increase in Cl(i) required intact alpha2/alpha3 Na+-K+-ATPase activity, indicating that trains of action potentials reset the thermodynamic equilibrium for NKCC1 transport by lowering Na(i). Activity-induced changes in Na+-K+-ATPase activity comprise a novel mechanism for persistent alterations in synaptic signaling mediated by GABA.

Clearance of store-released Ca2+ by the Na+-Ca2+ exchanger is diminished in aortic smooth muscle from Na+-K+-ATPase alpha 2-isoform gene-ablated mice

Two alpha-isoforms of the Na+-K+-ATPase are expressed in vascular smooth muscle cells (VSMCs). The alpha 1-isoform is proposed to serve a cytosolic housekeeping role, whereas the alpha 2-isoform modulates Ca2+ storage via coupling to the Na+-Ca2+ exchanger (NCX) in a subsarcolemmal compartment. To evaluate the ramifications of this proposed interaction, Ca2+-store load and the contributions of the primary Ca2+ transporters to Ca2+ clearance were studied in aortic VSMCs from embryonic wild-type (WT) and Na+-K+-ATPase alpha 2-isoform gene-ablated, homozygous null knockout (alpha 2-KO) mice. Ca2+ stores were unloaded by inhibiting the sarco(endo)plasmic reticulum Ca2+-ATPase with cyclopiazonic acid (CPA) in Ca2+-free media to limit Ca2+ influx. Ca2+ clearance by the plasma membrane Ca2+-ATPase (PMCA), NCX, or mitochondria was selectively inhibited. In WT VSMCs, NCX accounted for 90% of the Ca2+ efflux. In alpha 2-KO VSMCs, preferential clearance of store-released Ca2+ by NCX was lost, whereas PMCA activity was increased. Selective inhibition of the alpha 2-isoform (0.5 microM ouabain for 20 min), before treatment with CPA enhanced the store load in VSMCs from WT, but not alpha 2-KO mice. A subsequent analysis of capacitative Ca2+ entry (CCE) indicated that the magnitude of Ca2+ influx was significantly greater in alpha 2-KO cells. Our findings support the concept of a subsarcolemmal space where the alpha 2-isoform coupled with NCX modulates Ca2+-store function and, thereby, CCE.

Progesterone binding to the alpha1-subunit of the Na/K-ATPase on the cell surface: insights from computational modeling

Progesterone triggers the resumption of meiosis in the amphibian oocyte through a signaling system at the plasma membrane. Analysis of [(3)H]ouabain and [(3)H]progesterone binding to the plasma membrane of the Rana pipiens oocyte indicates that progesterone competes with ouabain for a low affinity ouabain binding site on a 112kDa alpha1-subunit of the membrane Na/K-ATPase. Published amino acid sequences from both low and high affinity ouabain binding alpha1-subunits are compared, together with published site-directed mutagenesis studies of ouabain binding. We propose that the progesterone binding site is located in the external loop (23 amino acids) between the M1-M2 transmembrane helices. Analysis of loop topology and the countercurrent hydrophobicity/polarity gradients within the M1-M2 loop further suggest that the polar beta and hydrophobic alpha surfaces of the planar progesterone molecule interact with opposite sides of the amino acid loop. The 19-angular methyl group of progesterone is essential for activity; it could bind to the C-terminal region of the M1-M2 loop. Maximum biological activity requires formation of hydrogen-bond networks between the 3-keto group of progesterone and Arg(118), Asp(129) and possibly Glu(122-124) in the C-terminal region of the loop. The 20-keto group hydrogen may in turn hydrogen bond to Cys(111) near the M1 helix. Peptide flexibility undergoes a maximal transition near the midway point in the M1-M2 loop, suggesting that folding occurs within the loop, which further stabilizes progesterone binding.

Store-operated Ca2+ entry in astrocytes: different spatial arrangement of endoplasmic reticulum explains functional diversity in vitro and in situ

Ca(2+) signaling is the astrocyte form of excitability and the endoplasmic reticulum (ER) plays an important role as an intracellular Ca(2+) store. Since the subcellular distribution of the ER influences Ca(2+) signaling, we compared the arrangement of ER in astrocytes of hippocampus tissue and astrocytes in cell culture by electron microscopy. While the ER was usually located in close apposition to the plasma membrane in astrocytes in situ, the ER in cultured astrocytes was close to the nuclear membrane. Activation of metabotropic receptors linked to release of Ca(2+) from ER stores triggered distinct responses in cultured and in situ astrocytes. In culture, Ca(2+) signals were commonly first recorded close to the nucleus and with a delay at peripheral regions of the cells. Store-operated Ca(2+) entry (SOC) as a route to refill the Ca(2+) stores could be easily identified in cultured astrocytes as the Zn(2+)-sensitive component of the Ca(2+) signal. In contrast, such a Zn(2+)-sensitive component was not recorded in astrocytes from hippocampal slices despite of evidence for SOC. Our data indicate that both, astrocytes in situ and in vitro express SOC necessary to refill stores, but that a SOC-related signal is not recorded in the cytoplasm of astrocytes in situ since the stores are close to the plasma membrane and the refill does not affect cytoplasmic Ca(2+) levels.

Ouabain modulation of cellular calcium stores and signaling

Ouabain-like factors modulate intracellular Ca2+ concentrations and Ca2+ stores. Recently, a role for Na+-K+-ATPase Na+ transport inhibition as a pivotal event in ouabain signaling was questioned (Kaunitz JD. Am J Physiol Renal Physiol 290: F995–F996, 2006). In the present study, we used a mathematical model of Ca2+ trafficking in cytoplasm and subplasmalemmal microdomains to simulate the pathways through which ouabain can affect Ca2+ signaling: inhibition of active transport by Na+-K+-ATPase α1- and α2-isoforms, activation of inositol trisphosphate (IP3) production, and increased IP3 receptor (IP3R) conductance. A fundamental prediction is that Na+-K+-ATPase inhibition favors sarcoplasmic reticulum Ca2+ store loading, whereas Src-mediated increases in IP3 production and IP3R sensitization favor store depletion. The model predicts that α2-isoform inhibition generates a peak-and-plateau pattern of cytosolic Ca2+ concentration ([Ca2+]cyt) elevation, whereas α1-isoform inhibition yields a monophasic rise. The effects of ouabain-mediated increases in IP3 production or IP3R conductance on [Ca2+]cyt depend on their relative distributions between cellular microdomains and the bulk cytoplasm. Simulations suggest that the intracellular localization of IP3 production is a pivotal determinant of the changes in compartmental Ca2+ concentrations that can be induced by ouabain. As a consequence of sequestration of the ouabain-sensitive α2-isoform into microdomains, inhibition of the α2-isoform in rodents is not predicted to significantly affect cytosolic Na+ concentration. Model simulations support the hypothesis that ouabain can enhance agonist-evoked [Ca2+]cyt transients when its predominant effect is to inhibit α2-isoform Na+ transport and, thereby, increase Ca2+ loading into sarcoplasmic reticulum stores.

Association analysis of chromosome 1 migraine candidate genes

BACKGROUND

Migraine with aura (MA) is a subtype of typical migraine. Migraine with aura (MA) also encompasses a rare severe subtype Familial Hemiplegic Migraine (FHM) with several known genetic loci. The type 2 FHM (FHM-2) susceptibility locus maps to chromosome 1q23 and mutations in the ATP1A2 gene at this site have recently been implicated. We have previously provided evidence of linkage of typical migraine (predominantly MA) to microsatellite markers on chromosome 1, in the 1q31 and 1q23 regions. In this study, we have undertaken a large genomic investigation involving candidate genes that lie within the chromosome 1q23 and 1q31 regions using an association analysis approach.

METHODS

We have genotyped a large population of case-controls (243 unrelated Caucasian migraineurs versus 243 controls) examining a set of 5 single nucleotide polymorphisms (SNPs) and the Fas Ligand dinucleotide repeat marker, located within the chromosome 1q23 and 1q31 regions.

RESULTS

Several genes have been studied including membrane protein (ATP 1 subtype A4 and FasL), cytoplasmic glycoprotein (CASQ 1) genes and potassium (KCN J9 and KCN J10) and calcium (CACNA1E) channel genes in 243 migraineurs (including 85% MA and 15% of migraine without aura (MO)) and 243 matched controls. After correction for multiple testing, chi-square results showed non-significant P values (P > 0.008) across all SNPs (and a CA repeat) tested in these different genes, however results with the KCN J10 marker gave interesting results (P = 0.02) that may be worth exploring further in other populations.

CONCLUSION

These results do not show a significant role for the tested candidate gene variants and also do not support the hypothesis that a common chromosome 1 defective gene influences both FHM and the more common forms of migraine.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Characterization of an extracellular epitope antibody to the neuronal K-Cl cotransporter, KCC2

1. Ion gradients across the cell membrane are important for proper cellular communication and homeostasis. With the exception of erythrocytes, chloride (Cl), one of the most important free anions in animal cells, is not distributed at thermodynamic equilibrium across the plasma membrane. The K-Cl cotransporter (COT), consisting of at least four isoforms, utilizes the larger outwardly directed chemical driving force of K to expel Cl from the cell against its inwardly directed chemical gradient and has been implicated recently as one of the main Cl extruders in developing neurons. 2. Previous in situ hybridization studies have indicated widespread mRNA distribution of the neuronal-specific K-Cl COT isoform (KCC2) throughout the rat central nervous system (CNS). However, immunohistochemical studies have been limited owing to the availability of a more selective antibody to KCC2. The goal of the present study was to develop a new molecular tool for the immunohistochemical identification and neuronal distribution of KCC2. 3. Herein, we present evidence of immunohistochemical corroboration of the widespread KCC2 mRNA expression using a novel extracellular anti-peptide antibody directed against the second extracellular loop (ECL2) of KCC2. Immunoperoxidase and immunofluorescent labelling revealed widespread post-synaptic somatic and dendritic localization of KCC2 in multiple neuronal populations in the cerebral cortex, hippocampus, brainstem, lumbar spinal cord and cerebellum. We also demonstrate that binding of the antibody to an extracellular epitope within ECL2 does not alter cotransporter function. In essence, the present study reports on a new molecular tool for structural and functional studies of KCC2.

Functional analysis of Na+/K+-ATPase isoform distribution in rat ventricular myocytes

The Na+/K+-ATPase (NKA) is the main route for Na+ extrusion from cardiac myocytes. Different NKA α-subunit isoforms are present in the heart. NKA-α1 is predominant, although there is a variable amount of NKA-α2 in adult ventricular myocytes of most species. It has been proposed that NKA-α2 is localized mainly in T-tubules (TT), where it could regulate local Na+/Ca2+ exchange and thus cardiac myocyte Ca2+. However, there is controversy as to where NKA-α1 vs. NKA-α2 are localized in ventricular myocytes. Here, we assess the TT vs. external sarcolemma (ESL) distribution functionally using formamide-induced detubulation of rat ventricular myocytes, NKA current (IPump) measurements and the different ouabain sensitivity of NKA-α1 (low) and NKA-α2 (high) in rat heart. Ouabain-dependent IPump inhibition in control myocytes indicates a high-affinity NKA isoform (NKA-α2, K1/2 = 0.38 ± 0.16 μM) that accounts for 29.5 ± 1.3% of IPump and a low-affinity isoform (NKA-α1, K1/2 = 141 ± 17 μM) that accounts for 70.5% of IPump. Detubulation decreased cell capacitance from 164 ± 6 to 120 ± 8 pF and reduced IPump density from 1.24 ± 0.05 to 1.02 ± 0.05 pA/pF, indicating that the functional density of NKA is significantly higher in TT vs. ESL. In detubulated myocytes, NKA-α2 accounted for only 18.2 ± 1.1% of IPump. Thus, ∼63% of IPump generated by NKA-α2 is from the TT (although TT are only 27% of the total sarcolemma), and the NKA-α2/NKA-α1 ratio in TT is significantly higher than in the ESL. The functional density of NKA-α2 is ∼4.5 times higher in the T-tubules vs. ESL, whereas NKA-α1 is almost uniformly distributed between the TT and ESL.

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

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

Time resolved kinetics of the guinea pig Na–Ca exchanger (NCX1) expressed in Xenopus oocytes: voltage and Ca2+ dependence of pre-steady-state current investigated by photolytic Ca2+concentration jumps

Kinetic properties of the Na-Ca exchanger (guinea pig NCX1) expressed in Xenopus oocytes were investigated by patch clamp techniques and photolytic Ca(2+) concentration jumps. Current measured in oocyte membranes expressing NCX1 is almost indistinguishable from current measured in patches derived from cardiac myocytes. In the Ca-Ca exchange mode, a transient inward current is observed, whereas in the Na-Ca exchange mode, current either rises to a plateau, or at higher Ca(2+) concentration jumps, an initial transient is followed by a plateau. No comparable current was observed in membrane patches not expressing NCX1, indicating that photolytic Ca(2+) concentrations jumps activate Na-Ca exchange current. Electrical currents generated by NCX1 expressed in Xenopus oocytes are about four times larger than those obtained from cardiac myocyte membranes enabling current recording with smaller concentration jumps and/or higher time resolution. The apparent affinity for Ca(2+) of nonstationary exchange currents (0.1 mM) is much lower than that of stationary currents (6 muM). Measurement of the Ca(2+) dependence of the rising phase provides direct evidence that the association rate constant for Ca(2+) is about 5 x 10(8) M(-1) s(-1) and voltage independent. In both transport modes, the transient current decays with a voltage independent but Ca(2+)-dependent rate constant, which is about 9,000 s(-1) at saturating Ca(2+) concentrations. The voltage independence of this relaxation is maintained for Ca(2+) concentrations far below saturation. In the Ca-Ca exchange mode, the amount of charge translocated after a concentration jump is independent of the magnitude of the jump but voltage dependent, increasing at negative voltages. The slope of the charge-voltage relation is independent of the Ca(2+) concentration. Major conclusions are: (1) Photolytic Ca(2+) concentration jumps generate current related to NCX1. (2) The dissociation constant for Ca(2+) at the cytoplasmic transport binding site is about 0.1 mM. (3) The association rate constant of Ca(2+) at the cytoplasmic transport sites is high (5 x 10(-8) M(-1)s(-1)) and voltage independent. (4) The minimal five-state model (voltage independent binding reactions, one voltage independent conformational transition and one very fast voltage dependent conformational transition) used before to describe Ca(2+) translocation at saturating Ca(2+) concentrations is valid for Ca(2+) concentrations far below saturation.

Na,K-ATPase generates calcium oscillations in hippocampal astrocytes

Na,K-ATPase maintains not only ionic homeostasis, but also participates in a multiprotein complex mediating intracellular signalling. We show that ouabain, a specific ligand for Na,K-ATPase, evokes calcium oscillations in hippocampal astrocytes in primary cultures. Coimmunoprecipitation studies suggest that the mechanism underlying these calcium oscillations involves a multiprotein complex consisting of ankyrin-B, the inositol 1,4,5-trisphosphate receptor and Na,K-ATPase. The ouabain/Na,K-ATPase multi-protein complex induced calcium-dependent downstream activation of the transcription factor nuclear factor-kappaB. Calcium oscillations and nuclear factor-kappaB activation were blocked following intracellular calcium store depletion. Thus, the specific Na,K-ATPase ligand ouabain induced inositol 1,4,5-trisphosphate receptor-dependent calcium oscillations in hippocampal astrocytes, which mediates nuclear factor-kappaB activation.

A quantitative model for linking Na+/Ca2+ exchanger to SERCA during refilling of the sarcoplasmic reticulum to sustain [Ca2+] oscillations in vascular smooth muscle

We have developed a quantitative model for the creation of cytoplasmic Ca2+ gradients near the inner surface of the plasma membrane (PM). In particular we simulated the refilling of the sarcoplasmic reticulum (SR) via PM-SR junctions during asynchronous [Ca2+]i oscillations in smooth muscle cells of the rabbit inferior vena cava. We have combined confocal microscopy data on the [Ca2+]i oscillations, force transduction data from cell contraction studies and electron microscopic images to build a basis for computational simulations that model the transport of calcium ions from Na+/Ca2+ exchangers (NCX) on the PM to sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) pumps on the SR as a three-dimensional random walk through the PM-SR junctional cytoplasmic spaces. Electron microscopic ultrastructural images of the smooth muscle cells were elaborated with software algorithms to produce a very clear and dimensionally accurate picture of the PM-SR junctions. From this study, we conclude that it is plausible and possible for enough Ca2+ to pass through the PM-SR junctions to replete the SR during the regenerative Ca2+ release, which underlies agonist induced asynchronous Ca2+ oscillations in vascular smooth muscle.

Familial hemiplegic migraine

Familial hemiplegic migraine (FHM) is a rare and genetically heterogeneous autosomal dominant subtype of migraine with aura. Mutations in the genes CACNA1A and SCNA1A, encoding the pore-forming alpha(1) subunits of the neuronal voltage-gated Ca2+ channels Ca(V)2.1 and Na+ channels Na(V)1.1, are responsible for FHM1 and FHM3, respectively, whereas mutations in ATP1A2, encoding the alpha2 subunit of the Na+, K+ adenosinetriphosphatase (ATPase), are responsible for FHM2. This review discusses the functional studies of two FHM1 knockin mice and of several FHM mutants in heterologous expression systems (12 FHM1, 8 FHM2, and 1 FHM3). These studies show the following: (1) FHM1 mutations produce gain-of-function of the Ca(V)2.1 channel and, as a consequence, increased Ca(V)2.1-dependent neurotransmitter release from cortical neurons and facilitation of in vivo induction and propagation of cortical spreading depression (CSD: the phenomenon underlying migraine aura); (2) FHM2 mutations produce loss-of-function of the alpha2 Na+,K+-ATPase; and (3) the FHM3 mutation accelerates recovery from fast inactivation of Na(V)1.5 (and presumably Na(V)1.1) channels. These findings are consistent with the hypothesis that FHM mutations share the ability of rendering the brain more susceptible to CSD by causing either excessive synaptic glutamate release (FHM1) or decreased removal of K+ and glutamate from the synaptic cleft (FHM2) or excessive extracellular K+ (FHM3). The FHM data support a key role of CSD in migraine pathogenesis and point to cortical hyperexcitability as the basis for vulnerability to CSD and to migraine attacks. Hence, they support novel therapeutic strategies that consider CSD and cortical hyperexcitability as key targets for preventive migraine treatment.

Interaction between Na+/K+-pump and Na+/Ca2+-exchanger modulates intercellular communication

Ouabain, a specific inhibitor of the Na(+)/K(+)-pump, has previously been shown to interfere with intercellular communication. Here we test the hypothesis that the communication between vascular smooth muscle cells is regulated through an interaction between the Na(+)/K(+)-pump and the Na(+)/Ca(2+)-exchanger leading to an increase in the intracellular calcium concentration ([Ca(2+)](i)) in discrete areas near the plasma membrane. [Ca(2+)](i) in smooth muscle cells was imaged in cultured rat aortic smooth muscle cell pairs (A7r5) and in rat mesenteric small artery segments simultaneously with force. In A7r5 coupling between cells was estimated by measuring membrane capacitance. Smooth muscle cells were uncoupled when the Na(+)/K(+)-pump was inhibited either by a low concentration of ouabain, which also caused a localized increase of [Ca(2+)](i) near the membrane, or by ATP depletion. Reduction of Na(+)/K(+)-pump activity by removal of extracellular potassium ([K(+)](o)) also uncoupled cells, but only after inhibition of K(ATP) channels. Inhibition of the Na(+)/Ca(2+)-exchange activity by SEA0400 or by a reduction of the equilibrium potential (making it more negative) also uncoupled the cells. Depletion of intracellular Na(+) and clamping of [Ca(2+)](i) at low concentrations prevented the uncoupling. The experiments suggest that the Na(+)/K(+)-pump may affect gap junction conductivity via localized changes in [Ca(2+)](i) through modulation of Na(+)/Ca(2+)-exchanger activity.

Modification of cytosolic calcium signaling by subplasmalemmal microdomains

To investigate the hypothesis that Na(+) concentration in subplasmalemmal microdomains regulates Ca(2+) concentrations in cellular microdomains ([Ca](md)), the cytosol ([Ca](cyt)), and sarcoplasmic reticulum (SR; [Ca](sr)), we modeled transport events in those compartments. Inputs to the model were obtained from published measurements in descending vasa recta pericytes and other smooth muscle cells. The model accounts for major classes of ion channels, Na(+)/Ca(2+) exchange (NCX), and the distributions of Na(+)-K(+)-ATPase alpha(1)- and alpha(2)-isoforms in the plasma membrane. Ca(2+) release from SR stores is assumed to occur via ryanodine (RyR) and inositol trisphosphate (IP(3)R) receptors. The model shows that the requisite existence of a significant Na(+) concentration difference between the cytosol ([Na](cyt)) and microdomains ([Na](md)) necessitates restriction of intercompartmental diffusion. Accepting the latter, the model predicts resting ion concentrations that are compatible with experimental measurements and temporal changes in [Ca](cyt) similar to those observed on NCX inhibition. An important role for NCX in the regulation of Ca(2+) signaling is verified. In the resting state, NCX operates in "forward mode," with Na(+) entry and Ca(2+) extrusion from the cell. Inhibition of NCX respectively raises and reduces [Ca](cyt) and [Na](cyt) by 40 and 30%. NCX translates variations in Na(+)-K(+)-ATPase activity into changes in [Ca](md), [Ca](sr), and [Ca](cyt). Taken together, the model simulations verify the feasibility of the central hypothesis that modulation of [Na](md) can influence both the loading of Ca(2+) into SR stores and [Ca(2+)](cyt) variation.

Endogenous and exogenous cardiac glycosides and their mechanisms of action

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

Ouabain-induced isoform-specific localization change of the Na+, K+-ATPase alpha subunit in the synaptic plasma membrane of rat brain

Na+, K+-ATPase is one of major membrane proteins that has two subunits, alpha and beta. The alpha subunit has the ATPase activity and the ouabain binding site. Among four isoforms of the alpha subunit, expression of alpha1, alpha2, and alpha3, but not alpha4, is observed in matured rat brain. Ouabain is one of cardiac glycosides, and endogenous ouabain-like compounds have been recognized as a new class of steroid hormone. The alpha subunit is considered as their endogenous receptor. Recent studies envisaged the importance of membrane microdomains (MDs) as signaling platforms, which are recovered as a detergent-resistant membrane microdomain fraction (DRM). Although this ATPase has been considered as a non-DRM protein, some amount of the alpha subunit was found to be a component of the DRM prepared from the synaptic plasma membrane fraction (SPM) of rat brain. Ouabain treatment increased the amount of alpha3 isoform, but not alpha1, in the DRM derived from synaptosome fraction and SPM. These results suggest that the localization of the alpha subunit of Na+, K+-ATPase is regulated with isoform-specific mechanisms and the physiological importance of DRM in the signal transduction of the endogenous ouabain-like steroid hormone in neurons.

A simple method for rapid separation of endothelial and smooth muscle mRNA reveals Na/K+ -ATPase alpha-subunit distribution in rat arteries

BACKGROUND/AIMS

The endothelium has been recognized as a key component in the regulation of blood vessels. We designed a simple procedure to separate endothelial and smooth muscle RNA from rat aorta and mesenteric artery and used this method to establish the distribution of Na(+)/K(+)-ATPase alpha-subunit isoforms (NaKalpha1, NaKalpha2 and NaKalpha3) within the arterial wall.

METHODS

Rat aorta was perfused with Tripure, a reagent for RNA isolation, yielding 3 successive RNA fractions (E1-E3) and the remaining tissular RNA (Ao[E-]). A similar procedure was applied to the mesenteric artery. Gene expression was studied by semiquantitative reverse-transcription polymerase chain reaction.

RESULTS

Compared to unperfused aorta (Ao[E+]), typical endothelial mRNAs were enriched 3- to 5-fold in E1-E3 but almost absent in Ao[E-], whereas smooth muscle mRNAs were low in E1-E3 but similarly expressed in Ao[E-] and Ao[E+]. NaKalpha1 was uniformly expressed in all fractions, NaKalpha2 closely followed the expression pattern of smooth muscle markers and NaKalpha3 expression was weak and attributable to blood contamination. Comparable results were obtained with the mesenteric artery.

CONCLUSION

We conclude that, in aorta and mesenteric artery, Tripure perfusion allows for a rapid and reliable separation of endothelial mRNA from smooth muscle mRNA, and that endothelium only expresses NaKalpha1, whereas smooth muscle expresses NaKalpha1 and NaKalpha2, but not NaKalpha3.

Na+-K+-ATPase is involved in the sustained ACh-induced hyperpolarization of endothelial cells from rat aorta

BACKGROUND AND PURPOSE

Inhibition of Na(+)-K(+)-ATPase is known to attenuate endothelium-dependent relaxation in many arteries. The purpose of this study was to evaluate the role of Na(+)-K(+)-ATPase in the regulation of endothelial membrane potential at rest and during stimulation by ACh.

EXPERIMENTAL APPROACH

Membrane potential was recorded from the endothelium of rat aorta using the perforated patch-clamp technique.

KEY RESULTS

Superfusion with K(+)-free solution produced a depolarization of about 11 mV from the resting value of -42.9+/-0.9 mV. Reintroduction of 4.7 mM K(+) transiently hyperpolarized endothelial cells to -52.4+/-1.8 mV and the membrane potential recovered within 10 min. Ouabain 500 microM depolarized endothelium by about 11 mV and inhibited the hyperpolarization induced by K(+) reintroduction into the K(+)-free solution. However, 500 nM ouabain did not affect the resting membrane potential or the hyperpolarization induced by K(+) reintroduction. Pre-exposure to ouabain 500 microM, but not 500 nM, attenuated the sustained component of hyperpolarization to ACh without affecting the amplitude of the transient peak hyperpolarization. In K(+)-free solution, the amplitude of peak hyperpolarization to ACh was increased, while the sustained component of hyperpolarization was attenuated.

CONCLUSIONS AND IMPLICATIONS

These results indicate that electrogenic Na(+)-K(+)-ATPase partially contributes to the sustained hyperpolarization of endothelial cells from rat aorta in response to ACh. They also suggest that the alpha1, but not alpha2 or alpha3 isoforms, is involved in ACh-mediated hyperpolarization.

Na+, K+-ATPase: An Indispensable Ion Pumping-Signaling Mechanism Across Mammalian Cell Membranes

Na(+), K(+)-adenosine triphosphatase is a ubiquitous enzyme present in higher eukaryotes responsible for the maintenance of ionic gradients across the plasma membrane. It creates appropriate conditions for critical cellular processes such as secondary transport of solutes and water, for pH regulation, and also for creating an electrical potential that gives singular qualities to excitable cells. It also served as a platform for a higher level of cellular complexity because many important signaling networks appear to be downstream events of the pump's function. Renal physiology and pathology are affected significantly by its presence, and it seems that both molecular and pharmacologic manipulations of its action can create different venues to deal with diverse disease states.

Local subplasma membrane Ca2+ signals detected by a tethered Ca2+ sensor

Accumulating evidence indicates that plasma membrane (PM) microdomains and the subjacent "junctional" sarcoplasmic/endoplasmic reticulum (jS/ER) constitute specialized Ca(2+) signaling complexes in many cell types. We examined the possibility that some Ca(2+) signals arising in the junctional space between the PM and jS/ER may represent cross-talk between the PM and jS/ER. The Ca(2+) sensor protein, GCaMP2, was targeted to different PM domains by constructing genes for fusion proteins with either the alpha1 or alpha2 isoform of the Na(+) pump catalytic (alpha) subunit. These fusion proteins were expressed in primary cultured mouse brain astrocytes and arterial smooth muscle cells. Immunocytochemistry demonstrated that alpha2(f)GCaMP2, like native Na(+) pumps with alpha2-subunits, sorted to PM domains that colocalized with subjacent S/ER; alpha1(f)GCaMP2, like Na(+) pumps with alpha1-subunits, was more uniformly distributed. The GCaMP2 moieties in both constructs were tethered just beneath the PM. Both constructs detected global Ca(2+) signals evoked by serotonin (in arterial smooth muscle cells) and ATP, and by store-operated Ca(2+) channel-mediated Ca(2+) entry after S/ER unloading with cyclopiazonic acid (in Ca(2+)-free medium). When cytosolic Ca(2+) diffusion was markedly restricted with EGTA, however, only alpha2(f)GCaMP2 detected the local, store-operated Ca(2+) channel-mediated Ca(2+) entry signal. Thus, alpha1 Na(+) pumps are apparently excluded from the PM microdomains occupied by alpha2 Na(2+) pumps. The jS/ER and adjacent PM may communicate by Ca(2+) signals that are confined to the tiny junctional space between the two membranes. Similar methods may be useful for studying localized Ca(2+) signals in other subPM microdomains and signals associated with other organelles.

Subunit composition and role of Na+,K+-ATPases in ventricular myocytes

Na+,K+-ATPases are composed of one alpha and one beta subunit; four alpha and three beta isoforms have been found to date. We elucidated which alpha and beta subunits were present in the ventricular myocytes of rat and guinea-pig and what roles the Na+,K(+)-ATPase isozymes play in cardiac contraction. The presence of the alpha1, alpha2, and alpha3 subunits and the beta1 and beta2 subunits in rat and guinea-pig hearts were confirmed at the protein or mRNA level. Immunocytochemistry showed a patchy presence of alpha1 in the transverse tubules and surface sarcolemma, whereas alpha2 was distributed continuously in the transverse tubules alone. The alpha3 isoform was expressed prominently in the guinea-pig intercalated disc and slightly in the rat. On the other hand, the beta1 isoform was located in the transverse tubules and surface sarcolemma, whereas the beta2 was mainly located in the intercalated disc. The immunocytochemistry and immunoprecipitation findings indicated that the alpha1 and alpha2 form heterodimers with beta1 and the alpha3 with beta2 in ventricular myocytes. The application of low concentrations of ouabain enhanced the amplitudes of twitch without a change in resting tension in rat and guinea-pig ventricular stripts, whereas that of high concentrations resulted in a decrease in twitch with an increase in the resting tension. We thus conclude that the alpha2beta1 and alpha3beta2 isozymes are selectively located in the transverse tubules and intercalated disc of the ventricular myocytes, respectively, and the alpha2beta1 is involved in the regulation of the Ca2+ contents in the SR.

Alpha3Na+/K+-ATPase is a neuronal receptor for agrin

Agrin, through its interaction with the receptor tyrosine kinase MuSK, mediates accumulation of acetylcholine receptors (AChR) at the developing neuromuscular junction. Agrin has also been implicated in several functions in brain. However, the mechanism by which agrin exerts its effects in neural tissue is unknown. Here we present biochemical evidence that agrin binds to the alpha3 subunit of the Na+/K+-ATPase (NKA) in CNS neurons. Colocalization with agrin binding sites at synapses supports the hypothesis that the alpha3NKA is a neuronal agrin receptor. Agrin inhibition of alpha3NKA activity results in membrane depolarization and increased action potential frequency in cortical neurons in culture and acute slice. An agrin fragment that acts as a competitive antagonist depresses action potential frequency, showing that endogenous agrin regulates native alpha3NKA function. These data demonstrate that, through its interaction with the alpha3NKA, agrin regulates activity-dependent processes in neurons, providing a molecular framework for agrin action in the CNS.

Role of FXYD proteins in ion transport

The FXYD proteins are a family of seven homologous single transmembrane segment proteins (FXYD1-7), expressed in a tissue-specific fashion. The FXYD proteins modulate the function of Na,K-ATPase, thus adapting kinetic properties of active Na+ and K+ transport to the specific needs of different cells. Six FXYD proteins are known to interact with Na,K-ATPase and affect its kinetic properties in specific ways. Although effects of FXYD proteins on parameters such as K(1/2)Na+, K(1/2)K+, K(m)ATP, and V(max) are modest, usually twofold, these effects may have important long-term consequences for homeostasis of cation balance. In this review we summarize basic features of FXYD proteins and present recent evidence for functional effects, structure-function relations and structural interactions with Na,K-ATPase. We then discuss possible physiological roles, based on in vitro observations and newly available knockout mice models. Finally, we also consider evidence that FXYD proteins affect functioning of other ion transport systems.

An N-terminal Sequence Targets and Tethers Na+ Pump α2 Subunits to Specialized Plasma Membrane Microdomains

Sodium pumps (alphabeta dimers) with the alpha1 isoform of the catalytic (alpha) subunit are expressed in all cells. Additionally, most cells express Na+ pumps with a second alpha isoform. For example, astrocytes and arterial myocytes also express Na+ pumps with the alpha2 isoform. The alpha2 pumps localize to plasma membrane (PM) microdomains overlying "junctional" sarco-/endoplasmic reticulum (S/ER), but the alpha1 pumps are more uniformly distributed. To study alpha2 targeting, we expressed alpha1/alpha2 and alpha2/alpha1 chimeras and 1-90 and 1-120 amino acid N-terminal peptides in primary cultured mouse astrocytes. Immunocytochemistry revealed that alpha2/alpha1 (but not alpha1/alpha2) chimeras markedly reduced native alpha2 (i.e. were "dominant negatives"). N-terminal (1-120 and 1-90 amino acids) alpha2 (and alpha3), but not alpha1 peptides also targeted to the PM-S/ER junctions and were dominant negative for native alpha2 in astrocytes and arterial myocytes. Thus alpha2 and alpha3 have the same targeting sequence. Ca2+ (fura-2) signals in astrocytes expressing the 1-90 alpha2 peptide were comparable to signals in cells from alpha2 null mutants (i.e. functionally dominant negative): 1 microM ATP-evoked Ca2+ transients were augmented, and 100 nM ouabain-induced amplification was abolished. Amino acid substitutions in the 1-120 alpha1 and alpha2 constructs, and in full-length alpha1, revealed that Leu-27 and Ala-35 are essential for targeting/tethering the constructs to PM-S/ER junctions.

The influence of changes in extracellular and intracellular sodium concentration on detrusor contractility

OBJECTIVE

To determine the role of Na+-Ca2+ exchange in the regulation of isolated detrusor smooth muscle contractility.

MATERIALS AND METHODS

Isolated guinea-pig detrusor strips were used to record isometric tension generated by; (a) electrical-field stimulation to elicit nerve-mediated responses; and (b) adding carbachol or superfusing with a high-K+ solution. The [Na+] gradient between extracellular and intracellular compartments was altered by: (i) reducing superfusate [Na+] in stages from 140.2 to 10.2 mm; (ii) addition of the cardiac glycoside strophanthidin (200 microm). RESULTS Reducing extracellular [Na+] reversibly reduced the magnitude of nerve-mediated contractions but increased the resting tension and magnitude of carbachol-induced contracture. The mean (sd) [Na+] required for a half-maximum effect on attenuating contractions, at 85.9 (6.2) mm, and developing contracture, at 59.1 (14.3) mm, were significantly different. The time constants of changes to nerve-mediated contractions and carbachol contracture were also significantly different, at 147 (5) vs 1207 (386) s, respectively. These differences suggest that separate mechanisms influence nerve-mediated contraction and contracture in low-Na+ solutions. Exposure to the cardiac glycoside strophanthidin produced a similar effect to low-Na+ solutions for carbachol contracture. Low-Na+ solutions had no significant effect on contractures induced by high extracellular [K+]. CONCLUSION Reducing the transmembrane [Na+] difference increases intracellular [Ca2+]. This increase is largely accommodated in intracellular stores, that can be released by exogenous carbachol. The results are consistent with the presence of a functional Na+-Ca2+ exchanger in the surface membrane. The lack of effect of low-Na+ solutions on contractures evoked by membrane depolarization is consistent with this conclusion. The reduction of the nerve-mediated contraction by low-Na+ solution might result from blockade of the nerve action potential.

Ouabain modulation of endothelial calcium signaling in descending vasa recta

Using fura 2-loaded vessels, we tested whether ouabain modulates endothelial cytoplasmic calcium concentration ([Ca(2+)](CYT)) in rat descending vasa recta (DVR). Over a broad range between 10(-10) and 10(-4) M, ouabain elicited biphasic peak and plateau [Ca(2+)](CYT) elevations. Blockade of voltage-gated Ca(2+) entry with nifedipine did not affect the response to ouabain mitigating against a role for myo-endothelial gap junctions. Reduction of extracellular Na(+) concentration ([Na(+)](o)) or Na(+)/Ca(2+) exchanger (NCX) inhibition with SEA-0400 (10(-6) M) elevated [Ca(2+)](CYT), supporting a role for NCX in the setting of basal [Ca(2+)](CYT). SEA-0400 abolished the [Ca(2+)](CYT) response to ouabain implicating NCX as a mediator. The transient peak phase of [Ca(2+)](CYT) elevation that followed either ouabain or reduction of [Na(+)](o) was abolished by 2-aminoethoxydiphenyl borate (5 x 10(-5) M). Cation channel blockade with La(3+) (10 muM) or SKF-96365 (10 muM) also attenuated the ouabain-induced [Ca(2+)](CYT) response. Ouabain pretreatment increased the [Ca(2+)](CYT) elevation elicited by bradykinin (10(-7) M). We conclude that inhibition of ouabain-sensitive Na(+)-K(+)-ATPase enhances DVR endothelial Ca(2+) store loading and modulates [Ca(2+)](CYT) signaling through mechanisms that involve NCX, Ca(2+) release, and cation channel activation.

How does salt retention raise blood pressure?

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

Physiological role of the α1- and α2-isoforms of the Na+-K+-ATPase and biological significance of their cardiac glycoside binding site

An interesting feature of Na+-K+-ATPase is that it contains four isoforms of the catalytic α-subunit, each with a tissue-specific distribution. Our laboratory has used gene targeting to define the functional role of the α1- and α2-isoforms. While knockout mice demonstrated the importance of the α1- and α2-isoforms for survival, the knockin mice, in which each isoform can be individually inhibited by ouabain and its function determined, demonstrated that both isoforms are regulators of cardiac muscle contractility. Another intriguing aspect of the Na+-K+-ATPase is that it contains a binding site for cardiac glycosides, such as digoxin. Conservation of this site suggests that it may have an in vivo role and that a natural ligand must exist to interact with this site. In fact, cardiac glycoside-like compounds have been observed in mammals. Our recent study demonstrates that the cardiac glycoside binding site of the Na+-K+-ATPase plays a role in the regulation of blood pressure and that it mediates both ouabain-induced and ACTH-induced hypertension in mice. Whereas chronic administration of ouabain or ACTH caused hypertension in wild-type mice, it had no effect on blood pressure in mice with a ouabain-resistant α2-isoform of Na+-K+-ATPase. Interestingly, animals with the ouabain-sensitive α1-isoform and a ouabain-resistant α2-isoform develop ACTH-induced hypertension to a greater extent than wild-type animals. Taken together, these results demonstrate that the cardiac glycoside binding of the Na+-K+-ATPase has a physiological role and suggests a function for a naturally occurring ligand that is stimulated by administration of ACTH.

Salt-sensitive hypertension, Na+/Ca2+ exchanger, and vascular smooth muscle

Hypertension is the most common chronic disease and is the leading risk factor for death caused by stroke, myocardial infarction, and end-stage renal failure. The critical importance of excess salt intake in the pathogenesis of hypertension is widely recognized. However, the molecular mechanisms underlying salt-sensitive hypertension remain obscure. Recent studies using selective Na(+)/Ca(2+) exchanger (NCX) inhibitors and genetically engineered mice provide compelling evidence that salt-sensitive hypertension is triggered by Ca(2+) entry through NCX type 1 (NCX1) in arterial smooth muscle. Cardiotonic steroids, such as endogenous ouabain, which may contribute to the pathogenesis of salt-sensitive hypertension, seem to be necessary for NCX1-mediated hypertension. These findings have enabled us to explain how high salt intake leads to hypertension and further to describe the potential of vascular NCX1 as a new therapeutic or diagnostic target for salt-sensitive hypertension.

Functional differences of Na+/Ca2+ exchanger expression in Ca2+ transport system of smooth muscle of guinea pig stomach

The plasma membrane ATP-dependent Ca2+ pump and the Na+/Ca2+ exchanger (NCX) are the major means of Ca2+ extrusion in smooth muscle. However, little is known regarding distribution and function of the NCX in guinea pig gastric smooth muscle. The expression pattern and distribution of NCX isoforms suggest a role as a regulator of Ca2+ transport in cells. Na+ pump inhibition and the consequent to removal of K+ caused gradual contraction in fundus. In contrast, the response was significantly less in antrum. Western blotting analysis revealed that NCX1 and NCX2 are the predominant NCX isoforms expressed in stomach, the former was expressed strongly in antrum, whereas the latter displayed greater expression in fundus. Isolated plasma membrane fractions derived from gastric fundus smooth muscle were also investigated to clarify the relationship between NCX protein expression and function. Na+-dependent Ca2+ uptake increased directly with Ca2+ concentration. Ca2+ uptake in Na+-loaded vesicles was markedly elevated in comparison with K+-loaded vesicles. Additionally, Ca2+ uptake by the Na+- or K+-loaded vesicles was substantially higher in the presence of A23187 than in its absence. The result can be explained based on the assumption that Na+ gradients facilitate downhill movement of Ca2+. Na+-dependent Ca2+ uptake was abolished by the monovalent cationic ionophore, monensin. NaCl enhanced Ca2+ efflux from vesicles, and this efflux was significantly inhibited by gramicidin. Results documented evidence that NCX2 isoform functionally contributes to Ca2+ extrusion and maintenance of contraction-relaxation cycle in gastric fundus smooth muscle.

Ankyrin-B coordinates the Na/K ATPase, Na/Ca exchanger, and InsP3 receptor in a cardiac T-tubule/SR microdomain

We report identification of an ankyrin-B-based macromolecular complex of Na/K ATPase (alpha 1 and alpha 2 isoforms), Na/Ca exchanger 1, and InsP3 receptor that is localized in cardiomyocyte T-tubules in discrete microdomains distinct from classic dihydropyridine receptor/ryanodine receptor "dyads." E1425G mutation of ankyrin-B, which causes human cardiac arrhythmia, also blocks binding of ankyrin-B to all three components of the complex. The ankyrin-B complex is markedly reduced in adult ankyrin-B(+/-) cardiomyocytes, which may explain elevated [Ca2+]i transients in these cells. Thus, loss of the ankyrin-B complex provides a molecular basis for cardiac arrhythmia in humans and mice. T-tubule-associated ankyrin-B, Na/Ca exchanger, and Na/K ATPase are not present in skeletal muscle, where ankyrin-B is expressed at 10-fold lower levels than in heart. Ankyrin-B also is not abundantly expressed in smooth muscle. We propose that the ankyrin-B-based complex is a specialized adaptation of cardiomyocytes with a role for cytosolic Ca2+ modulation.

Digitalis: new actions for an old drug

The mechanisms by which digitalis causes its therapeutic and toxic actions have been studied for nearly a half century, revealing a great deal about cardiac cell regulation of intracellular ions via the Na-K-ATPase (NKA) and how it is altered by cardiac glycosides. However, recent observations suggest that digitalis may have additional effects on cardiac cell function in both the short and long term that include intracellular effects, interactions with specific NKA isoforms in different cellular locations, effects on intracellular (including nuclear) signaling, and long-term regulation of intracellular ionic balances through circulating ouabain-like compounds. The purpose of this review is to examine the current status of a number of the newest and most interesting developments in the study of digitalis with a particular focus on cardiac function, although we will also discuss some of the new advances in other relevant cardiovascular effects. This new information has important implications for both our understanding of ionic regulation in normal and diseased hearts as well as for potential avenues for the development of future therapeutic interventions for the treatment of heart failure.