Kinetic mechanism of ATP action in Na(+)-K(+)-Cl- cotransport of HeLa cells determined by Rb+ influx studies

Functional differences between human Cx37 polymorphic hemichannels

A polymorphism in the human Cx37 gene (C1019T), resulting in a non-conservative amino acid change in the regulatory C-terminus of the Cx37 protein (P319S), has been proposed as a prognostic marker for atherosclerosis. We have recently demonstrated that Cx37 hemichannels control the initiation of atherosclerotic plaque development by regulating ATP-dependent monocyte adhesion in atherosclerosis-susceptible apolipoprotein E-deficient mice. In this study, we have measured the electrical properties of Cx37 hemichannels (HCs) and gap junction channels (GJCs) with voltage-clamp methods. To this end, we have transfected hCx37-P319, hCx37-S319 or empty pIRES-eGFP vector cDNA into communication-deficient HeLa cells. In clones expressing similar levels of Cx37, exposure of single cells to low-Ca(2+) solution induced a voltage-sensitive HC current. The analysis yielded a bell-shaped function g(hc)=f(V(m)) (g(hc): normalized conductance at steady state; V(m): membrane potential) with a maximum around V(m)=-30 mV. The peak g(hc) of Cx37-P319 was 3-fold larger than that of Cx37-S319 HCs. Experiments on cell pairs revealed that Cx37-P319 GJCs exhibited a 1.5-fold larger unitary conductance than Cx37-S319 GJCs. Hence, the larger peak g(hc) of the former may reflect a larger conductance of their HCs. Using the same clones, we found that Cx37-P319 cells released more ATP and were less adhesive than Cx37-S319 cells. The reduction in adhesiveness of Cx37-expressing cells was prevented by extracellular apyrase. We conclude that the differences in biophysical properties between polymorphic HCs may be responsible for inequality in ATP release between Cx37-P319 and Cx37-S319 cells, which results in differential cell adhesion.

SPAK and OSR1, key kinases involved in the regulation of chloride transport

Reversible phosphorylation by protein kinases is probably one of the most important examples of post-translational modification of ion transport proteins. Ste20-related proline alanine-rich kinase (SPAK) and oxidative stress response kinase (OSR1) are two serine/threonine kinases belonging to the germinal centre-like kinase subfamily VI. Genetic analysis suggests that OSR1 evolved first, with SPAK arising following a gene duplication in vertebrate evolution. SPAK and OSR1 are two recently discovered kinases which have been linked to several key cellular processes, including cell differentiation, cell transformation and proliferation, cytoskeleton rearrangement, and most recently, regulation of ion transporters. Na-K-2Cl cotransporter activity is regulated by phosphorylation. Pharmacological evidence has identified several kinases and phosphatases which alter cotransporter function, however, no direct linkage between these enzymes and the cotransporter has been demonstrated. This article will review some of the physical and physiological properties of SPAK and OSR1, and present new evidence of a direct interaction between the Na-K-Cl cotransporter and the stress kinases.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Regulation of Na-K-2Cl cotransport by phosphorylation and protein-protein interactions

The Na-K-2Cl cotransporter plays important roles in cell ion homeostasis and volume control and is particularly important in mediating the movement of ions and thus water across epithelia. In addition to being affected by the concentration of the transported ions, cotransport is affected by cell volume, hormones, growth factors, oxygen tension, and intracellular ionized Mg(2+) concentration. These probably influence transport through three main routes acting in parallel: cotransporter phosphorylation, protein-protein interactions and cell Cl(-) concentration. Many effects are mediated, at least in part, by changes in protein phosphorylation, and are disrupted by kinase and phosphatase inhibitors, and manoeuvres that reduce cell ATP content. In some cases, phosphorylation of the cotransporter itself on serine and threonine (but not tyrosine) is associated with changes in transport rate, in others, phosphorylation of associated proteins has more influence. Analysis of the stimulation of cotransport by calyculin A, arsenite and deoxygenation suggests that the cotransporter is phosphorylated by several kinases and dephosphorylated by several phosphatases. These kinases and phosphatases may themselves be regulated by phosphorylation of residues including tyrosine, with Src kinases possibly playing an important role. Protein-protein interactions also influence cotransport activity. Cotransporter molecules bind to each other to form high molecular weight complexes, they also bind to other members of the cation-chloride cotransport family, to a variety of cytoskeletal proteins, and to enzymes that are part of regulatory cascades. Many of these interactions affect transport and may override the effects of cotransporter phosphorylation. Cell Cl(-) may also directly affect the way the cotransporter functions independently of its role as substrate.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Improving contrast and tracking of tags in cardiac magnetic resonance images

Cardiac tagging permits non-invasive study of myocardial motion with high accuracy. Unfortunately, tagging contrast is impaired at later heart phases due to longitudinal relaxation. Histogram modification is presented as a method for improving contrast in later, faded images of a tagging series by altering these images such that their intensity histograms approximate the shape of the first, unfaded image of the series. This technique greatly improves the contrast and facilitates automatic detection of tags. Furthermore, a method is described for automatically tracking tag positions in short-axis images of the left ventricle modulated with a tagging grid. The method differs from previously reported methods in that, in one single filtering process in the Fourier domain, both the grid crossings as well as the grid centers are detected, and thus increased sampling resolution is obtained. The method was validated by applying a mathematical model of left ventricular motion to tagged images of the thigh muscle. The average discrepancy between theoretically predicted and automatically detected tag positions was 0.04 +/- 0.17 mm (mean +/- SD).

Reduction of the clutter component in Doppler ultrasound signals based on singular value decomposition: a simulation study

In pulsed Doppler ultrasound systems, the ultrasound radiofrequency (RF) signals received can be employed to estimate noninvasively the time-dependent blood velocity distribution within and artery. The RF signals are composed of signals originating from clutter (e.g., vessel walls) and scatterers (e.g., red blood cells). The clutter, which is induced by stationary or slowly-moving structure interfaces, must be suppressed to get reliable estimates of the mean blood flow velocities. In conventional pulsed Doppler systems, this is achieved with a static temporal high-pass filter. The static cut-off frequency and the roll-off of these filters cause the culture not always to be optimally suppressed. This paper introduces a clutter removal filter that is based on Singular Value Decomposition (SVD). Unlike conventional high-pass filters, which take into account only the information of the temporal direction, the SVD filter makes use of the information of the temporal and spatial directions. The advantage of this approach is that it does not matter where the clutter is located in the RF signal. The performance of the SVD filter is examined with computer-generated Doppler RF signals. The results are compared with those of standard linear regression (SLR) filter. The performance of the SVD filter is good, especially if a large temporal window (i.e., approximately 100 RF signals) is applied, which improves the performance for low blood flow velocities, A major disadvantage of the SVD filter is its computational complexity, which increases considerably for larger temporal windows.

Cytoplasmic ATP-dependent regulation of ion transporters and channels: mechanisms and messengers

Many ion transporters and channels appear to be regulated by ATP-dependent mechanisms when studied in planar bilayers, excised membrane patches, or with whole-cell patch clamp. Protein kinases are obvious candidates to mediate ATP effects, but other mechanisms are also implicated. They include lipid kinases with the generation of phosphatidylinositol phosphates as second messengers, allosteric effects of ATP binding, changes of actin cytoskeleton, and ATP-dependent phospholipases. Phosphatidylinositol-4,5-bisphosphate (PIP2) is a possible membrane-delimited messenger that activates cardiac sodium-calcium exchange, KATP potassium channels, and other inward rectifier potassium channels. Regulation of PIP2 by phospholipase C, lipid phosphatases, and lipid kinases would thus tie surface membrane transport to phosphatidylinositol signaling. Sodium-hydrogen exchange is activated by ATP through a phosphorylation-independent mechanism, whereas ion cotransporters are activated by several protein kinase mechanisms. Ion transport in epithelium may be particularly sensitive to changes of cytoskeleton that are regulated by ATP-dependent cell signaling mechanisms.

The ATP and Mg2+ dependence of Na(+)-K(+)-2Cl- cotransport reflects a requirement for protein phosphorylation: studies using calyculin A

Na(+)-K(+)-2Cl- cotransport activity has previously been shown to depend on both intracellular ATP and Mg2+, but the mechanisms remain unknown. Cotransport in avian erythrocytes can be stimulated by a variety of agents including cAMP and permeant serine/threonine phosphatase inhibitors and is inhibited by prior depletion of either ATP with antimycin A, or mg2+ by incubation in A23187 plus EDTA. However, when cells were first stimulated with cAMP rather than calyculin A then subjected to either depletion strategy, a differential effect was found. The phosphatase-inhibitor-treated cells were resistant to subsequent ATP or Mg2+ depletion while cAMP-treated cells were sensitive to both treatments. Parallel examination of protein phosphorylation confirmed that ATP or Mg2+ depletion leads to dephosphorylation of membrane proteins in cAMP-treated but not in calyculin-A-treated cells. These results suggest that, for cotransport, ATP and Mg2+ are required primarily to maintain the system in a phosphorylated state rather than as direct modulators. The relative effectiveness of okadaic acid (EC50 approximately 630 nM) and calcyulin A (EC50 approximately 8 nM) in stimulating the cotransporter indicate that a type-1 protein phosphatase is probably responsible for dephosphorylating the system. Cells stimulated by hypertonicity were also resistant to ATP or Mg2+ depletion suggesting that the mechanism of shrinkage-induced cotransport stimulation might also involve protein phosphatase modulation.

Relation between regional electrical activation time and subepicardial fiber strain in the canine left ventricle

To determine the relation between regional electrical activation time and fiber strain, epicardial electrical activation and deformation were measured in six open-chest dogs at the left ventricular anterior free wall after 15 min of right atrial, left ventricular free wall, left ventricular apex, or right ventricular outflow tract pacing, when end-diastolic pressure was normal or elevated (volume-loading). Regional electrical activation was measured using a 192-electrode brush. Regional subepicardial fiber strain (ef) was measured simultaneously in 16 regions, using optical markers which were attached to the epicardial surface and recorded on video. When relating regional ef during the ejection phase to regional activation time, the best correlation was found when a hemodynamic time reference rather than an electrophysiological one is used. Using the moment of the maximum rate of change of left ventricular pressure as the time reference for electrical activation, regional electrical activation time (t(ea)) and the degree of ef during the ejection phase could be fitted by a linear regression equation ef = a t(ea) + b, in which a = -3.46 +/- 0.73 s-1 an b = -0.28 +/- 0.05. For electrical activation times ranging from -40 to -80 ms, fiber strain was estimated with an accuracy of +/- 0.026 (+/- SE) with this relation. During right atrial pacing, t(ea) and ef were on the average -48 ms and -0.10 respectively. On further investigation, the relation between ef and t(ea) appeared to be influenced by end-diastolic pressure. For normal (1.1 kPa) and elevated end-diastolic pressure (1.8 kPa), the slope of the linear regression line was -3.96 and -2.86 s-1, respectively. Three conclusions may be drawn. Firstly, the time interval between the moment of regional electrical activation and the moment of the maximum rate of change of left ventricular pressure is an index of regional fiber strain. Secondly, it can be concluded from the above equations that electrical asynchrony of more than 30 ms causes non-uniformities in the degree of ef of the order of mean ef during pacing from the right atrium. Finally, differences in fiber strain during asynchronous electrical activation are less pronounced at larger filling pressures.

Effects of seven months' exposure to a static 0.2 T magnetic field on growth and glycolytic activity of human gingival fibroblasts

Human gingival fibroblasts in confluent cultures were continuously exposed to a static 0.2 T magnetic field for 6 or 8 months. Culture flasks were not changed during the exposure, but culture medium was renewed. After dilution and mixing of the cultures surviving intact, field-exposed and sham-exposed cultures received further field- or sham-exposure on Sm-Co blocks. Rate of cell proliferation, histogram of the nuclear DNA content, rates of lactate production and glucose consumption and the ATP content were determined and cell morphology was investigated by both light- and electron-microscopy. Results show no marked differences between exposed and control cells.

Rubidium transport in cultured monkey retinal pigment epithelium

Rb+ influx was used to assess Na-K-Cl cotransport and Na,K-ATPase activities in cultured monkey retinal pigment epithelium. Bumetanide-sensitive (Na-K-Cl cotransport-mediated) Rb+ influx exceeds ouabain-sensitive (Na,K-ATPase-mediated) Rb+ influx, with these two transporters accounting for approximately 95% of total Rb+ uptake. Half-maximal inhibition of Rb+ influx by bumetanide is attained at 75 nM bumetanide. The bumetanide-sensitive Rb+ influx depends on both extracellular Na+ and Cl-, and is activated by extracellular Rb+ with a relatively high affinity. Na-K-Cl cotransport activity is stimulated (2.5-fold) by increased extracellular osmolarity. Elevated cAMP content and glycolytic inhibition both depress cotransport activity. Cyanide application, however, had very little effect on Na-K-Cl cotransport activity. Monkey retinal pigment epithelial cells, maintained in culture, provide a system in which the activity and regulation of cation transport mechanisms can be examined.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

Mapping the sequence of contraction of the canine left ventricle

A method has been developed to map the sequence of contraction as measured at the epicardial surface of the anterior free wall of the canine left ventricle during sinus rhythm and electrical stimulation of the ventricle. In an area of 35 x 45 mm, 40-60 white markers were attached to the epicardial surface. The motion of the markers was recorded on video and analysed off-line by computer. In an array of 35 regions, regional surface deformation and epicardial fibre strain were calculated from the motion of the markers. Between all adjacent regions, the differences in timing of contraction were determined by cross-correlation of the related fibre strain signals. A map of the time sequence of contraction has been calculated so that the sum of the squares of the deviations between time intervals of the map and the measurements was minimised. If individual correlation coefficients were found to be less than 0.85, the related time difference was discarded from the analysis. If more than 25% of the time differences were discarded because of this reason, the whole map was obtained by determining time of the negative peak of the second time derivative in the early phase of contraction. The accuracy in time marking was sufficient (+/- 7 ms), as compared to the time differences over the epicardial surface, which were found to be on the average between 10 and 80 ms in case of sinus rhythm and electrical stimulation of the right ventricular outflow tract, respectively.

Vasopressin alters the mechanism of apical Cl- entry from Na+:Cl- to Na+:K+:2Cl- cotransport in mouse medullary thick ascending limb

Experiments were performed using in vitro perfused medullary thick ascending limbs of Henle (MTAL) and in suspensions of MTAL tubules isolated from mouse kidney to evaluate the effects of arginine vasopressin (AVP) on the K+ dependence of the apical, furosemide-sensitive Na+:Cl- cotransporter and on transport-related oxygen consumption (QO2). In isolated perfused MTAL segments, the rate of cell swelling induced by removing K+ from, and adding one mM ouabain to, the basolateral solution [ouabain(zero-K+)] provided an index to apical cotransporter activity and was used to evaluate the ionic requirements of the apical cotransporter in the presence and absence of AVP. In the absence of AVP cotransporter activity required Na+ and Cl-, but not K+, while the presence of AVP the apical cotransporter required all three ions. 86Rb+ uptake into MTAL tubules in suspension was significant only after exposure of tubules to AVP. Moreover, 22Na+ uptake was unaffected by extracellular K+ in the absence of AVP while after AVP exposure 22Na+ uptake was strictly K(+)-dependent. The AVP-induced coupling of K+ to the Na+:Cl- cotransporter resulted in a doubling in the rate of NaCl absorption without a parallel increase in the rate of cellular 22Na+ uptake or transport-related oxygen consumption. These results indicate that arginine vasopressin alters the mode of a loop diuretic-sensitive transporter from Na+: Cl- cotransport to Na+: K+: 2Cl- cotransport in the mouse MTAL with the latter providing a distinct metabolic advantage for sodium transport. A model for AVP action on NaCl absorption by the MTAL is presented and the physiological significance of the coupling of K+ to the apical Na+: Cl- cotransporter in the MTAL and of the enhanced metabolic efficiency are discussed.