A role of calcium in altered sodium ion transport of hypertensives?

Research on the etiology of essential hypertension has led to many reports of altered ion transport in cells from hypertensive patients and animal models. Abnormalities in sodium and calcium ion gradients and transport in vascular smooth muscle, neuronal tissue, cardiac muscle as well as erythrocytes have been extensively investigated. It is not clear whether these abnormalities are of primary or secondary nature. The current knowledge of sodium and calcium ion transport in essential hypertension is briefly reviewed here. Furthermore, evidence is presented which suggests a role of calcium in the regulation of sodium transport activity.     

Complete dissociation of DOCA-salt hypertension and red cell ion transport alterations.

Our previous study revealed major ion transport alterations that resulted in a pronounced elevation of red cell Na+ content in DOCA-salt treated homozygous vasopressin-deficient (DI) Brattleboro rats in which only a moderate increase of systolic blood pressure occurred. In contrast, no changes of red cell Na+ content were observed in heterozygous vasopressin-secreting (non-DI) Brattleboro rats with a severe DOCA-salt hypertension. Using a chronic supplementation of DI rats with an antidiuretic agonist dDAVP (1-desamino-8-D-arginine vasopressin) we did not demonstrate any significant changes of red cell ion transport in dDAVP-treated DI rats with a fully developed DOCA-salt hypertension. The absence of ion transport alterations seems to be mainly due to dDAVP-induced correction of altered K+ metabolism seen in DOCA-salt treated DI animals. It can be concluded that DOCA-salt hypertension can develop even without red cell ion transport alterations which are usually caused by cell K+ depletion.     

Conformational Coupling: The Moving Parts of an Ion Pump

The Na,K-ATPase carries out the coupled functions of ATP hydrolysis and cation transport. These functions are performed by two distinct regions of the protein. ATP binding and hydrolysis is mediated by the large central cytoplasmic loop of about 430 amino-acids. Transmembrane cation transport is accomplished via coordination of the Na and K ions by side-chains of the amino-acids of several of the transmembrane segments. The way in which these two protein domains interact lies at the heart of the molecular mechanism of active transport, or ion pumping. We summarize evidence obtained from protein chemistry studies of the purified renal Na,K-ATPase and from bacterially expressed polypeptides which characterize these separate functions and point to various movements which may occur as the protein transits through its reaction cycle. We then describe recent work using heterologous expression of renal Na,K-ATPase in baculovirus-infected insect cells which provides a suitable system to characterize such protein motions and which can be employed to test specific models arising from recently acquired high resolution structural information on related ion pumps.     

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.     

Relationships between membrane lipids and ion transport in red blood cells of Dahl rats

Distinct changes of membrane lipid content could contribute to the abnormalities of ion transport that take part in the development of salt hypertension in Dahl rats. The relationships between lipid content and particular ion transport systems were studied in red blood cells (RBC) of Dahl rats kept on low- and high-salt diets for 5 weeks since weaning. Dahl salt-sensitive (SS/Jr) rats on high-salt diet had increased blood pressure, levels of plasma triacylglycerols and total plasma cholesterol compared to salt-resistant (SR/Jr) rats. Furthermore, RBC of SS/Jr rats differed from SR/Jr ones by increased content of total membrane phospholipids, but membrane cholesterol was not changed significantly. SS/Jr rats had higher RBC intracellular Na+ (Na(i)+) content and enhanced bumetanide-sensitive Rb+ uptake. RBC membrane content of cholesterol and phospholipids correlated positively with RBC Na(i)+ content, with the activity of Na+-K+ pump and Na+-K+-2Cl- cotransport and also with Rb+ leak. The content of phosphatidylserines plus phosphatidylinositols was positively associated with RBC Na(i)+ content, with the activity of Na+-K+ pump and Na+-K+-2Cl- cotransport and with Rb+ leak. The content of sphingomyelins was positively related to Na+-K+-2Cl- cotransport activity and negatively to ouabain-sensitive Rb+-K+ exchange. We can conclude that observed relationships between ion transport and the membrane content of cholesterol and/or sphingomyelins, which are known to regulate membrane fluidity, might participate in the pathogenesis of salt hypertension in Dahl rats.     

Secondary active transport

Secondary active transport is defined as the transport of a solute in the direction of its increasing electrochemical potential coupled to the facilitated diffusion of a second solute (usually an ion) in the direction of its decreasing electrochemical potential. The coupling agents are membrane proteins (carriers), each of which catalyzes simultaneously the facilitated diffusion of the driving ion and the active transport of a given solute. The review starts with some considerations on the energetics followed by a presentation of the kinetics of secondary active transport. Examples of information which may be gained by such studies are discussed. In the second part, some examples of secondary transport are given; we also describe the characteristics of the corresponding carriers. The various transport systems presented are: the D-glucose/Na+ symport in brush-border membranes, the lactose/H+ symport in E. coli, the Na+/H+ antiport, the different transport systems in the inner mitochondrial membrane.     

Dysfunction of calcium handling by smooth muscle in hypertension

Dysfunction of ion handling, including binding and fluxes (passive and active transport) of physiologically important ions such as potassium, sodium, calcium, and magnesium, by vascular smooth muscle cell membranes has repeatedly been reported to be associated with the pathophysiology of hypertension. The specific purpose of this review is to summarize and evaluate the evidence for alterations of calcium ion (Ca2+) handling by vascular smooth muscle in various forms of hypertension in the animal model on the basis that regulation of cytoplasmic Ca2+ concentration is a complex and yet vitally important process for a normal function of vascular smooth muscle and that derangement of such a regulation may result in excessive retention of cytoplasmic Ca2+, contribute toward increase of total peripheral resistance, and ultimately lead to elevation of blood pressure. Emphasis is placed upon the consideration of the usefulness of the subcellular membrane fractionation technique in studies of binding and transport of Ca2+ by vascular and nonvascular smooth muscle membranes from genetic as well as experimental hypertensive rats. The limitations of the interpretation of data using such an approach are also considered. Decreased active transport of Ca2+ across isolated plasma membrane vesicles from large and small arteries occurs in several but not all forms of hypertension. This membrane abnormality also occurs in nonvascular smooth muscles and other tissues or cells not confined to the cardiovascular system in genetic hypertension, but not in experimental hypertension. A hypothesis of general membrane defects in spontaneous hypertension is proposed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structure of valinomycin by molecular dynamics studies.

Valinomycin is an important ionophore which exhibits a high conformational flexibility. The study of various conformations adopted by this molecule together with the study of flexibility in a given conformation can throw light on the ion transport by the ionophore across the membrane. Molecular dynamics (MD) studies are ideal to characterize the flexibility in different parts of the molecule and can also give an idea of various conformations adopted by the molecule at a given temperature. Hence MD studies at 100K have been carried out on the minimized crystal structure of the molecule to scan the possible conformations in the neighbourhood of the well known 'bracelet' like structure of uncomplexed Valinomycin, Properties, like the flexibility, average values, r.m.s. fluctuations of the various intramolecular hydrogen bonds are discussed. Energy minimization has been carried out on selected MD simulated points to analyze the characteristics of the unique conformation adopted by this molecule at this temperature.     

WNK kinases and the kidney

In the kidney, the renal tubule plays a major role in maintaining fluid and electrolyte balance. This balance is achieved by an interplay between various hormones and nerves that signal changes throughout the body and transfer these signals to transport proteins. Increased or reduced activity of these transporters helps to restore homeostasis, but can also contribute to disease (e.g. sodium retention in hypertension). In recent years, it has become clear that the signal transfer to transporters is largely mediated by kinases. Among these, WNK kinases (With No lysine=K) stand out, because they regulate the major sodium and potassium transporters in the distal nephron. Moreover, mutations in genes encoding WNK kinases result in an inherited form of salt-sensitive hypertension with hyperkalemia, illustrating their important role in sodium, potassium, and blood pressure regulation. More recently, WNK kinases were found to play a role in acquired forms of hypertension as well. Together, the evolving insight in the kinase regulation of ion transport is providing new insights in the longstanding question how salt and blood pressure are related. Here, we review the current models of how WNK kinases regulate the various transport proteins and which roles they play in health and disease.     

Increased Renal Iron Accumulation in Hypertensive Nephropathy of Salt-Loaded Hypertensive Rats

Although iron is reported to be associated with the pathogenesis of chronic kidney disease, it is unknown whether iron participates in the pathophysiology of nephrosclerosis. Here, we investigate whether iron is involved in the development of hypertensive nephropathy and the effects of iron restriction on nephrosclerosis in salt- loaded stroke-prone spontaneously hypertensive rats (SHRSP). SHRSP were given either a normal or high-salt diet for 8 weeks. Another subset of SHRSP were fed a high-salt with iron-restricted diet. SHRSP given a high-salt diet developed severe hypertension and nephrosclerosis. As a result, survival rate was decreased after 8 weeks diet. Importantly, massive iron accumulation and increased iron content were observed in the kidneys of salt-loaded SHRSP, along with increased superoxide production, urinary 8-Hydroxy-2′-deoxyguanosine excretion, and urinary iron excretion; however, these changes were markedly attenuated by iron restriction. Of interest, expression of cellular iron transport proteins, transferrin receptor 1 and divalent metal transporter 1, was increased in the tubules of salt-loaded SHRSP. Notably, iron restriction attenuated the development of severe hypertension and nephrosclerosis, thereby improving survival rate in salt-loaded SHRSP. Taken together, these results suggest a novel mechanism by which iron plays a role in the development of hypertensive nephropathy and establish the effects of iron restriction on salt-induced nephrosclerosis.     

Red cell sodium in DOCA-salt hypertension: a Brattleboro study.

The alteration of red cell Na+ content (Na+i), its causes and the possible relationship to the development of DOCA-salt hypertension were studied in Brattleboro rats. A pronounced hypertension developed in heterozygous (non-DI) animals that synthesize vasopressin (VP) although no substantial Na+i elevation was observed in their erythrocytes. In contrast, Na+i rose progressively in red cells of homozygous VP-deficient (DI) rats in which only marginal increase of systolic blood pressure was found after six weeks of DOCA-salt regimen. DOCA-salt treatment of non-DI rats did not cause major alterations in ouabain-resistant (OR) net Na+ uptake or ouabain-sensitive (OS) net Na+ extrusion but moderately increased furosemide-sensitive (FS) Rb+ uptake. The same treatment of DI rats doubled Na+i by an increased OR net Na+ uptake (due to a major elevation in both Na(+)-K+ cotransport and Na+ leak). Consequently, OS net Na+ extrusion was augmented in red cells of these animals. This was accompanied by an about threefold elevated FS Rb+ uptake. It can be concluded that a) the alterations of OR and/or OS Na+ or K+ transport observed in erythrocytes of Brattleboro DI rats are not essential for the development of severe DOCA-salt hypertension, b) red cell ion transport abnormalities revealed in DOCA-salt treated DI rats might be rather ascribed to cell potassium depletion, and c) increased inward Na(+)-K+ cotransport and Na+ leak causes red cell Na+i elevation that stimulates Na(+)-K+ pump activity.     

Evidence for a circulating sodium transport inhibitor in essential hypertension.

The active sodium transport of white cells and red cells obtained from patients with essential hypertension was impaired. Incubating white cells from normotensive subjects in serum obtained from patients with essential hypertension caused an impairment in sodium transport in the white cells of normotensive subjects similar to that found in the white cells of hypertensive patients. The impairment in sodium transport was due to a fall in the ouabain-sensitive component of the total sodium efflux rate constant. These results show that the serum of patients with essential hypertension contains a substance which influences sodium transport and that it has ouabain-like activity. They also suggest that it is this substance which causes the impairment in sodium transport in the leucocytes of patients with essential hypertension. These findings support the hypothesis that the rise in blood pressure in patients with essential hypertension is due to an increased concentration of a circulating sodium transport inhibitor which is continuously correcting a tendency for sodium retention by the kidney.     

Semiconductor theory of ion transport in thin lipid membranes: I. Potential and field distributions

In the theory as presented in this paper and the following one, we shall attempt to apply the semiconductor principles and methods to the study of ion transport in thin lipid membranes. Detailed formulations are given on the potential energy barriers at the interfaces, voltage drops in the polar and non-polar regions, and potential and field distributions in the diffuse double layer and within a charged membrane. These results will be used mainly as the boundary conditions for the solution of ion flow as to be given in the following paper. The analysis clearly indicates that the ion transport is interface-limited and is profoundly influenced by the presence of surface charges. An explanation of Na⁺ extrusion in nerve membrane is given based on the field distribution analysis. The theory also suggests that the “membrane potential” depends mainly on surface charges but not necessarily on ion permeation through the membrane.     

Speculations on the evolution of ion transport mechanisms.

Primate cells evolved a plasma membrane to restrict the loss of important molecules. The osmotic problems that then arose were solved in one of several ways. Of major importance was the evolution of specific ion pumps, to actively extrude those salts whose inward diffusion would have led to swelling and lysis. In addition, these pumps allowed the cell to store energy in the form of ion gradients across the membrane. Thus, even in the earliest stages, the evolution of ion transport systems coincided with the development of mechanisms which catalyzes the energy transformations. It is postulated that an "ATP"-driven proton pump was one of the first ion transport systems. Such a proton pump would extrude hydrogen ions from the cell, establishing both a transmembrane pH gradient (alkaline inside) and a membrane potential (negative inside). This difference in electrochemical potential for protons (the proton-motive force) could then drive a variety of essential membrane functions, such as the active transport of ions and nutrients. A second major advance was the evolution of an ion transport system that converted light energy into a form which could be used by the cell. The modern model for this is the "purple membrane" of Halobacterium halobium, which catalyzes the extrusion of protons after the capture of light. The protonmotive force generated by such a light-driven proton pump could then power net synthesis of ATP by a reversal of the ATP-driven proton pump. A third important evolutionary step associated with ion transport was the development of a system to harness energy released by biological oxidations. Again, the solution of this problem was to conserve energy as a protonmotive force by coupling the activity of a respiratory chain to the extrusion of protons. Finally, with the development of animal cells a more careful regulation of internal and external pH was required. Thus, an ATP-driven Na+-K+ pump replaced the proton-translocating ATPase as the major ion pump found in plasma membranes.     

Detection of DNA molecules in a lipid nanotube channel in the low ion strength conditions

Investigation of the transport phenomena in the nanoscopic channels/pores with the diameter smaller than 100 nm is of utmost importance for various biological, medical, and technical applications. Presently, the main line of development of nanofluidics is creation of biosensors capable of detecting single molecules and manipulating them. Detection of molecules is based on the measurement of electric current through a channel of appropriate size: when the molecule enters the channel, which diameter is comparable with the molecule size, the ion current reduces. In order to improve transport properties of such channels, their walls are often coated with a lipid bilayer, which behaves as two-dimensional liquid and thus is capable of supporting transport phenomena. In the present work, we utilized this property of lipid membranes for the development of a method for detecting and controlling transport of single-stranded DNA through channels formed by membrane cylinders with the luminal radii of 5–7 nm. We have demonstrated that in the conditions of small ion strength, the appearance of a DNA molecule inside such channel is accompanied by an increase of its ion conductivity and can be controlled by the polarity of the applied voltage. The amplitude of the ion current increase allows evaluating the amount of DNA molecules inside the channels. It was also demonstrated that upon adsorption of DNA molecules on the lipid bilayer surface, the membrane cylinder behaves as a voltage-sensitive selective ion channel.     

Involvement of ion channels and active transport in osmoregulation and signaling of higher plant cells

The transport of inorganic and organic ions across the plasma membrane and organelle membranes of higher plants by ion channels, electrogenic pumps and co-transporters is essential to vital processes such as osmoregulation, growth, development, signal transduction and the storage of solutes. Recent studies have led to the identification of specialized transport proteins in the plasma membrane and vacuolar membrane of higher plant cells. Here we have integrated the functional aspects of these membrane proteins into a model which proposes a novel basis for ion transport processes involved in the regulation of gas exchange in leaves.     

Assembly and maintenance of nodes of ranvier rely on distinct sources of proteins and targeting mechanisms

We have investigated the source(s) and targeting of components to PNS nodes of Ranvier. We show adhesion molecules are freely diffusible within the axon membrane and accumulate at forming nodes from local sources, whereas ion channels and cytoskeletal components are largely immobile and require transport to the node. We further characterize targeting of NF186, an adhesion molecule that pioneers node formation. NF186 redistributes to nascent nodes from a mobile, surface pool. Its initial accumulation and clearance from the internode require extracellular interactions, whereas targeting to mature nodes, i.e., those flanked by paranodal junctions, requires intracellular interactions. After incorporation into the node, NF186 is immobile, stable, and promotes node integrity. Thus, nodes assemble from two sources: adhesion molecules, which initiate assembly, accumulate by diffusion trapping via interactions with Schwann cells, whereas ion channels and cytoskeletal components accumulate via subsequent transport. In mature nodes, components turnover slowly and are replenished via transport. VIDEO ABSTRACT:     

Effects of cytosolic Ca2+ on membrane voltage and conductance of cultured mesangial cells from stroke-prone spontaneously hypertensive rats and WKY rats

Mesangial cells (MC) are considered to play an important role in the development of hypertension. The purpose of this study was to characterize the effects of cytosolic Ca2+ on membrane voltage and conductance of MC using stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY). We applied the patch-clamp technique in the whole-cell configuration to measure membrane potential (Vm) and ion currents. There was no significant difference in resting Vm values between MC from WKY and SHRSP. The cytosolic Ca2+ increase induced membrane depolarization and the increase of Cl- currents in MC from WKY but not in MC from SHRSP. On the other hand, the Ca2+ increase induced membrane hyperpolarization and the increase of K+ currents in MC from SHRSP but not in MC from WKY. Such differences between MC from two rat strains may play an important role in the alterations in renal hemodynamics observed in hypertension.     

Relationships among endogenous digitalis-like factors in essential hypertension

Elements of a hypothesis that relate endogenous digitalis-like factors to both natriuretic hormone and hypertension are briefly reviewed. The stimulus for secretion of these factors appears to involve a tendency toward a state of extracellular fluid volume expansion as a consequence of an inherited or an acquired defect in renal function. Several studies implicate the brain and, in particular, the hypothalamus in the control of the secretion. The digitalis-like factors are thought to act by partial inhibition of active sodium transport, thereby promoting increased intracellular levels of Na+ and Ca2+ in a variety of cell types. In the kidney, inhibition of sodium transport leads to a compensatory natriuresis to correct the tendency for volume overload. In smooth muscle, the inhibition of sodium transport will indirectly increase intracellular calcium levels. The increased availability of Ca2+ will elevate muscle tone and increase peripheral vascular resistance. Also presented are criteria that may be used to characterize digitalis-like activity in samples and extracts obtained from purification procedures. Finally, we review our measurements of the 6-h integrated plasma levels of digitalis-like factors and other hormones for normotensive subjects and patients with essential hypertension. The data indicate the presence of two classes of digitalis-like factors with potentially different roles in electrolyte metabolism and hypertension.     

Dual mechanisms of ion absorption in relation to long distance transport in plants

The characteristics of ion transport to the shoots of young corn seedlings were studied with respect to the nature of the isotherm through a wide concentration range, the competitive influence of closely related ions upon the transport of a given ion, and the influence of the counter-ion. Both with respect to ³⁶Cl and ⁸⁶Rb transport, the characteristics of the process in every way resemble uptake by non-vacuolate root tips wherein the plasma membrane is the only membrane involved in absorption, and where system 1 — of the 2 systems which can be shown to participate in absorption by vacuolate tissue — is the only system operative. Net ion uptake by the roots per se was shown to display both the high affinity (system 1) and low affinity (system 2) mechanisms. It is concluded that the symplastic theory of ion movement to the xylem is valid, and that the contention that system 1 operates at the plasma membrane while system 2 functions at the tonoplast is strengthened.