The relationship of sodium uptake,potassium rejection,and skin potential in isolated frog skin

1. Isolated surviving frog skin, when bathed with the same kind of diluted Ringer's solution on both sides, shows a negative correlation between net active salt uptake by the epithelium and spontaneous skin potential. Average values of 0.15 to 0.86 µeq. x hr.^–1 x cm.^–2 were measured and correlated with average skin potentials ranging from 107 to 25 mv. 2. Sodium uptake exceeded chloride uptake by about the same amount, irrespective of the height of the skin potential. 3. The same skins which exhibited a negative correlation between net uptake of sodium chloride and skin potential showed a positive correlation between net potassium rejection from the epithelium and skin potential, for voltages above 30 to 40 mv. In skins of voltages lower than this, potassium ions were taken up rather than rejected. Average values for rejection of +11.8 to –0.8 centi-µeq. x hr.^–1 x cm.^–2 were measured. 4. Net fluid uptake, associated with active uptake of sodium chloride, was small and occurred in the direction of the salt uptake. No dependence of net fluid uptake upon skin potential was observed. 5. Skins of winter frogs, pretreated with a commercial purified ACTH preparation, were less active than their respective controls with regard to uptake of sodium chloride. Rejection of potassium was the same in treated and untreated skins. Posterior pituitary factors, as possible contaminants, did not account for the effect of the ACTH preparation. 6. DOCA, DOC, and cortisone did not alter the normal correlation referred to under (1) and (3). 7. In interpreting the experimental results on theoretical grounds, it is suggested (a) that in normal skin, it is the variation in the electric conductance in skin of chloride ions which essentially, although not exclusively, determines the rate of net uptake of sodium chloride, (b) that a factor in the ACTH preparation used, possibly ACTH itself, may have lowered the electric conductance in skin of sodium ions either truly or apparently, (c) that potassium ions are treated by the skin primarily as passive ions. There is some indication that potassium ions are also actively taken up by the epithelium of skin.     

The competition transport of sodium ions and protons in the cytoplasmic access channel of the Na+,K+-ATPase

The changes in capacitance and conductance of lipid bilayer membranes have been studied with adsorbed membrane fragments containing Na⁺,K⁺-ATPase. These changes have been initiated by fast release of protons from a bound form (“caged H⁺”) induced by an UV flash. The changes of the capacitance in the presence of Na⁺,K⁺-ATPase were affected by the frequency of the applied voltage, pH and the concentration of sodium ions. Addition of sodium ions altered the changes of capacitance caused by a pH jump in the medium due to caged H⁺ photolysis, and the magnitude and sign of this effect depended on the initial pH. These results are explained by competitive binding of sodium ions and protons to the ion-binding sites of the Na⁺,K⁺-ATPase at its cytoplasmic side. The pH at which the sign of the sodium ion effect changed allows the evaluation of the pK of the proton binding site, which is about 7.6.     

An electrogenic Na+/K+ pump in the choroid plexus

Intracellular electrical potential and potassium activity was measured by means of microelectrodes in the epithelial cells of choroid plexus from bullfrogs (Rana catesbeiana). Ouabain applied from the ventricular side caused an abrupt depolarisation of 10 mV but only a gradual loss of potassium from the cells. Readministration of potassium to the ventricular solution of plexuses which were previously depleted of potassium, caused a hyperpolarisation of about 4 mV. These two experiments are consistent with the notion of an electrogenic Na⁺/K⁺ pump situated at the ventricular membrane and which pumps potassium into the cell and sodium into the ventricle. The numerical values obtained suggest that 3 sodium ions are pumped for 2 potassium ions. The permeability coefficient for potassium exit from the cell is calculated to be 1.24 · 10^?5 cm^?1 · s^?1 expressed per cm² of flat epithelium.     

The kinetics of sodium extrusion in striated muscle as functions of the external sodium and potassium ion concentrations

After a 20 min initial washout, the rate of loss of radioactively labeled sodium ions from sodium-enriched muscle cells is sensitive to the external sodium and potassium ion concentrations. In the absence of external potassium ions, the presence of external sodium ions increases the sodium efflux. In the presence of external potassium ions, the presence of external sodium ions decreases the sodium efflux. In the absence of external potassium ions about one-third of the Na⁺ efflux that depends upon the external sodium ion concentration can be abolished by 10^-5 M glycoside. The glycoside-insensitive but external sodium-dependent Na⁺ efflux is uninfluenced by external potassium ions. In the absence of both external sodium and potassium ions the sodium efflux is relatively insensitive to the presence of 10^-5 M glycoside. The maximal external sodium-dependent sodium efflux in the absence of external potassium ions is about 20% of the magnitude of the maximal potassium-dependent sodium efflux. The magnitude of the glycoside-sensitive sodium efflux in K-free Ringer solution is less than 10% of that observed when sodium efflux is maximally activated by potassium ions. The inhibition of the potassium-activated sodium efflux by external sodium ions is of the competitive type. Reducing the external sodium ion concentration displaces the plots of sodium extrusion rate vs. [K]_o to the left and upwards.     

Mechanism of the Interaction of Superoxide Ion and Ascorbate with Anthracycline Antibiotics

The interaction of superoxide ion and ascorbate anion with anthracycline antibiotics (adriamycin and aclacinimycin A) as well as with their Fe³⁺ complexes has been studied in aprotic and protic media. It was found that both superoxide and ascorbate reduce anthracyclines to deoxyaglycons via a one-electron transfer mechanism under all conditions studied. The reaction of ascorbate anion with adriamycin and aclacinomycin A in aqueous solution proceeded only in the presence of Fe³⁺ ions; it is supposed that an active catalytic species was Fe³⁺ adriamycin. It is also supposed that the reduction of anthracycline antibiotics by O,⁷ and ascorbate in cells may increase their anticancer effect.     

Cation specificity of 3H-sulpiride binding involves alteration in the number of striatal binding sites

Specific ³H-sulpiride binding to rat striatal membranes shows an absolute requirement for the presence of sodium ions in the incubation buffer. Potassium, rubidium and caesium ions were unable to initiate specific ³H-sulpiride binding in a sodium free buffer, and lithium ionscould only partially replace sodium ions. Specific ³H-spiperone binding was unaffected by variation of the cation content of the incubation buffer. The alteration in ³H-sulpiride binding caused by sodium and lithium ions was due predominantly to an increase in the number of available binding sites, rather than to altered receptor affinity. Sodium ions may be essential for the accessability of ³H-sulpiride to a single site labelled also by ³H-spiperone. However, the Ki value for sulpiride displacement of ³H-spiperone in the presence of sodium ions was 20 times greater than the K_D value for ³H-sulpiride binding. So, ³H-sulpiride may interact with a highly sodium dependent binding site distinct from that labelled by ³H-spiperone.     

Adriamycin-induced changes in the surface membrane of sarcoma 180 ascites cells

Adriamycin increases (a) the rate of agglutination of Sarcoma 180 cells by concanavalin A after brief exposure of 2–3 h and (b) membrane fluidity as measured by ESR within 30 min of exposure at concentrations of the anthracycline of 10^?7–10^?5 M. The effect of adriamycin on agglutination is not due to an increase in the number of surface receptors for concanavalin A, since the extent of binding of the lectin is not altered by adriamycin and no change occurs in the rate of occupancy of the concanavalin A binding sites by the lectin in cells treated with the antibiotic. The order parameter, a measurement of membrane fluidity, decreases in cells exposed to adriamycin and is dose-related. The results indicate that adriamycin can induce changes in the surface membrane of Sarcoma 180 cells within a brief period of exposure to a low but cytotoxic level of this agent.     

Fine structural changes associated with the onset of calcium,sodium and water transport by the chick chorioallantoic membrane

Summary An attempt is made to correlate structure and transport function in the embryonic chorioallantoic membrane. The fine structure of the endoderm and ectoderm in the membrane was examined with particular attention given to the morphological changes that occur when transport is established,in vivo. Two distinctive cells, a granule-rich cell and a mitochondria-rich cell, appear in the endoderm at the time allantoic fluid sodium, chloride and water reabsorption commences. These are indistinguishable from the cells described in toad bladder epithelium. It is suggested that the granule-rich cell is responsible for bulk water movement and the mitochondria-rich cell is specifically engaged in active sodium transport. In the ectoderm, two distinctive cell types are also found to be associated with the onset of active calcium transport. These are referred to as the capillary-covering cell and the villus-cavity cell. The preponderate capillary-covering cell is most likely responsible for transcellular calcium transport. It is postulated that the function of the villus-cavity cell is to secrete hydrogen ions which are necessary, along with carbonic anhydrase, to mobilize Ca⁺⁺ from the insoluble calcium carbonate of the eggshell.     

Parameters of gaseous ion effects on the mammalian trachea. A. Duration of effects. B. Minimal effective ion densities

A. Duration of Effects Groups of mice exposed to high densities of unipolar light air ions for 72 hours exhibited persistent alterations in the functional efficiency of their tracheas. These effects lasted at least 4 weeks, and in the case of animals treated with (+) ions included diminished ciliary activity, pale and contracted tracheal mucosa, and enhanced vulnerability to trauma. Following treatment with (-) ions, animals displayed increased ciliary activity with no other detectable changes. It required at least 60 minutes of exposure to ions to induce such "permanent" functional changes. B. Minimal Effective Ion Densities The minimal ion densities producing changes in ciliary activity within an arbitrary period of 30 minutes were determined with extirpated tracheal strips from rabbits and guinea pigs. The threshold value for (-) ions was approximately 2.5 x 10³ ions/cm.²/sec. and that for (+) ions was in the range between 1 x 10⁴ and 2.5 x 10⁵ ions/cm.²/sec.The minimal ion densities producing changes in ciliary activity within an arbitrary period of 30 minutes were determined with extirpated tracheal strips from rabbits and guinea pigs. The threshold value for (-) ions was approximately 2.5 x 10³ ions/cm.²/sec. and that for (+) ions was in the range between 1 x 10⁴ and 2.5 x 10⁵ ions/cm.²/sec. The evidence indicates that ion-induced functional changes in the ciliated epithelium of the pulmonary tree are the results of direct contact of ions with surface cells and do not involve participation of the central nervous system or circulation. So far as ciliary activity is concerned, the number of ions required to produce a change in rate is very small.     

Fluorescence measurement of intracellular sodium concentration in single Escherichia coli cells

The energy-transducing cytoplasmic membrane of bacteria contains pumps and antiports maintaining the membrane potential and ion gradients. We have developed a method for rapid, single-cell measurement of the internal sodium concentration ([Na⁺]_in) in Escherichia coli using the sodium ion fluorescence indicator, Sodium Green. The bacterial flagellar motor is a molecular machine that couples the transmembrane flow of ions, either protons (H⁺) or sodium ions (Na⁺), to flagellar rotation. We used an E. coli strain containing a chimeric flagellar motor with H⁺- and Na⁺-driven components that functions as a sodium motor. Changing external sodium concentration ([Na⁺]_ex) in the range 1–85 mM resulted in changes in [Na⁺]_in between 5–14 mM, indicating a partial homeostasis of internal sodium concentration. There were significant intercell variations in the relationship between [Na⁺]_in and [Na⁺]_ex, and the internal sodium concentration in cells not expressing chimeric flagellar motors was 2–3 times lower, indicating that the sodium flux through these motors is a significant fraction of the total sodium flux into the cell.     

Altered retinal metabolism in diabetes. II. Measurement of sodium-potassium ATPase and total sodium and potassium in individual retinal layers

Pathological changes in retinas of diabetics include specific morphological, biochemical, and functional abnormalities. As biochemical manifestations of the disease, increased sorbitol and decreased myo-inositol were found in retinas of experimentally diabetic animals. Similar alterations in polyol metabolism have been associated in nerves of diabetics with a reduction of Na+-K+-ATPase activity. To determine whether this association extends to the retinas of diabetic animals, we applied quantitative histochemical techniques to measure ATPase activities and the amounts of sodium and potassium in samples from nine individual layers of cryostat sections of rabbit retina. ATPase activities were determined fluorimetrically, and the ions were measured by atomic absorption with a carbon rod atomizer. The activity of Na+-K+-ATPase was reduced in the retinal pigmented epithelium (retinal pigment epithelium) and in selected layers of the neural retina, and total sodium in the retinal pigment epithelium layer was elevated in diabetes. The retinal pigment epithelium forms the outer component of the blood-retinal barrier and partly determines the composition of the retinal interstitial fluid. Changes in retinal pigment epithelium biochemistry and function might alter the intraretinal environment, predisposing neural retina or retinal blood vessels to disease. The morphologically and functionally well defined retinal pigment epithelium may provide a useful model for studying the pathogenesis of diabetic complications.     

Histochemical distribution of potassium-dependent p-nitrophenylphosphatase in the calf intestine

Summary Potassium-dependentp-nitrophenylphosphatase was demonstrated, using the lead citrate method of Mayaharaet al. (1980), in frozen sections of calf intestine fixed in formalin-calcium. The calcium chloride included in the fixative was shown to improve the localization of the reaction markedly. The phosphatase activity observed in the basolateral cell borders of the surface epithelium in the small intestine and colon was reduced by 10mm oubain and by substitution of sodium ions for potassium ions, confirming that the reaction was representative for the second step in the Na⁺/K⁺-ATPase complex. The intensity of the basolateral enzyme reaction was in the order: colon > duodenum, proximal jejunum > ileum > middle and distal jejunum. The crypts reacted weakly. A reaction in the brush border of the proximal jejunum and duodenum and a granular reaction in the supranuclear cytoplasm of the epithelial cells was not influenced by oubain. The staining pattern for the potassium-dependent phosphatase differed from that of alkaline phosphatase and Mg²⁺-dependent ATPase, which gave a reaction that was restricted to the brush border.     

Ionic Blockage of Sodium Channels in Nerve

Increasing the hydrogen ion concentration of the bathing medium reversibly depresses the sodium permeability of voltage-clamped frog nerves. The depression depends on membrane voltage: changing from pH 7 to pH 5 causes a 60% reduction in sodium permeability at +20 mV, but only a 20% reduction at +180 mV. This voltage-dependent block of sodium channels by hydrogen ions is explained by assuming that hydrogen ions enter the open sodium channel and bind there, preventing sodium ion passage. The voltage dependence arises because the binding site is assumed to lie far enough across the membrane for bound ions to be affected by part of the potential difference across the membrane. Equations are derived for the general case where the blocking ion enters the channel from either side of the membrane. For H⁺ ion blockage, a simpler model, in which H⁺ enters the channel only from the bathing medium, is found to be sufficient. The dissociation constant of H⁺ ions from the channel site, 3.9 x 10^-6 M (pK_a 5.4), is like that of a carboxylic acid. From the voltage dependence of the block, this acid site is about one-quarter of the way across the membrane potential from the outside. In addition to blocking as described by the model, hydrogen ions also shift the responses of sodium channel "gates" to voltage, probably by altering the surface potential of the nerve. Evidence for voltage-dependent blockage by calcium ions is also presented.     

Evidence for a morphological component in acid-base regulation during environmental hypercapnia in the brown bullhead (Ictalurus nebulosus)

Summary Exposure of adult brown bullheads Ictalurus nebulosus (120–450 g) to environmental hypercapnia (2% carbon dioxide in air) and subsequent recovery caused transient changes in whole body net sodium flux (J _net ^Na+ ) and net chloride flux (J _net ^Cl- ) resulting largely from changes in whole body sodium influx (J _in ^Na+ ) and chloride influx (J _in ^Cl- ). Scanning electron microscopy (SEM) revealed that the fractional area of chloride cells (CCs) on the interlamellar regions was reduced by 95% during environmental hypercapnia. During post-hypercapnic recovery, gill filament CC fractional area increased. The changes in J _in ^Cl- during and after environmental hypercapnia were closely associated with the changes in CC fractional area while the changes in J _in ^Na+ did not correspond to the changes in CC fractional area. Transmission electron microscopy (TEM) supported the SEM observations of CC surface area changes and demonstrated that these changes were caused by covering/uncovering by adjacent pavement cells (PVCs). Lamellar and filament PVC microvilli density increased during hypercapnia while there was a subsequent reduction in the post-hypercapnic period. These data suggest that an important mechanism of acid-base regulation during hypercapnic acidosis is modification of the chloride cell-associated Cl^-/HCO ₃ ^- exchange mechanism. We suggest that bullheads vary availability, and thus functional activity, of this transporter via reversible morphological alterations of the gill epithelium. The increase in density of PVC microvilli may be associated with sodium uptake and/or acidic equivalent excretion during acidosis.     

The influence of potassium- and sodium-free solutions on sodium efflux from squid giant axons

The sensitivity of sodium efflux to the removal of potassium ions from the external solution and the change in sodium efflux occurring when sodium ions are also removed were observed to be related. When Tris was used to replace external sodium ions, increases in sodium efflux were always observed whether the sensitivity of sodium efflux to external potassium ions was weak or strong. Greater percentage increases in sodium efflux occurred, however, the greater the sensitivity of sodium efflux to external potassium ions. When lithium ions were used to replace external sodium ions, increases in sodium efflux occurred if the sensitivity of efflux to external potassium ions was strong whereas decreases in sodium efflux took place if the sensitivity of efflux to external potassium ions was weak. Intermediate sensitivities of efflux to external potassium resulted in no change in efflux upon substitution of lithium ions for external sodium ions. In the presence of 10^-5 M ouabain, substitution of Tris for external sodium ions always resulted in a small decrease in sodium efflux. The data can be described in terms of a model which assumes the presence of efflux stimulation sites that are about 98% selective to potassium ions and about 2% selective to sodium or lithium ions.     

Energetics of sulfate transport in Desulfomicrobium baculatum

The freshwater sulfate reducer Desulfomicrobium baculatum accumulated ³⁵S-sulfate up to 120-fold by an energy-dependent transport system, as was concluded from inhibition of transport by tetrachlorosalicylanilide (TCS). Sulfate accumulation was completely reversible and depended on the presence of sodium ions. The sodium ion gradient ([Na⁺]_out/[Na⁺]_in) was eightfold and was built up by electrogenic Na⁺/H⁺ antiport. Together with a membrane potential of-145 m V, the sodium ion motive force was-199 m V, from which a symport stoichiometry of two sodium ions per sulfate was calculated. This is the first report of a freshwater sulfate reducer taking up sulfate electroneutrally in symport with sodium ions and not with protons.Abbreviations ETH 2120 N,N'-Dibenzyl-N,N'-diphenyl-1,2-phenylendioxydiacetamide - TCS Tetrachlorosalicylanilide     

Graded and All-or-None Electrogenesis in Arthropod Muscle : I. The effects of alkali-earth cations on the neuromuscular system of Romalea microptera

Graded electrically excited responsiveness of Romalea muscle fibers is converted to all-or-none activity by Ba⁺⁺, Sr⁺⁺, or Ca⁺⁺, the two former being much the more effective in this action. The change occurs with as little as 7 to 10 per cent of Na⁺ substituted by Ba⁺⁺. The spikes now produced have overshoots and may be extremely prolonged, lasting many seconds. During the spike the membrane resistance is lower than in the resting fiber, but the resting resistance and time constant are considerably increased by the alkali-earth ions. The excitability is also increased, spikes arising neurogenically from spontaneous repetitive discharges in the axon as well as myogenically from spontaneous activity in the muscle fibers. Repetitive responses frequently occur on intracellular stimulation with a brief pulse. The data indicate that the alkali-earth ions exert a complex of effects on the different action components of electrically excitable membrane. They may be described in terms of the ionic theory as follows: The resting K⁺ conductance is diminished. The sodium inactivation process is also diminished, and sodium activation may be increased. Together these changes can act to convert graded responsiveness to the all-or-none variety. The alkali-earth ions can also to some degree carry inward positive charge during activity, since spikes are produced when Na⁺ is fully replaced with the divalent ions.     

Molecular basis of ion permeability in a voltage‐gated sodium channel

Voltage‐gated sodium channels are essential for electrical signalling across cell membranes. They exhibit strong selectivities for sodium ions over other cations, enabling the finely tuned cascade of events associated with action potentials. This paper describes the ion permeability characteristics and the crystal structure of a prokaryotic sodium channel, showing for the first time the detailed locations of sodium ions in the selectivity filter of a sodium channel. Electrostatic calculations based on the structure are consistent with the relative cation permeability ratios (Na⁺ ≈ Li⁺ ≫ K⁺, Ca²⁺, Mg²⁺) measured for these channels. In an E178D selectivity filter mutant constructed to have altered ion selectivities, the sodium ion binding site nearest the extracellular side is missing. Unlike potassium ions in potassium channels, the sodium ions in these channels appear to be hydrated and are associated with side chains of the selectivity filter residues, rather than polypeptide backbones.     

The cationic composition and pH in the moulting fluid of Porcellio scaber (Crustacea, Isopoda) during calcium carbonate deposit formation and resorption

Before moulting, terrestrial isopods resorb calcium carbonate (CaCO₃) from the posterior cuticle and store it in sternal deposits. These consist mainly of amorphous calcium carbonate (ACC) spherules that develop within the ecdysial space between the anterior sternal epithelium and the old cuticle. Ions that occur in the moulting fluid, including those required for mineral deposition, are transported from the hemolymph into the ecdysial space by the anterior sternal epithelial cells. The cationic composition of the moulting fluid probably affects mineral deposition and may provide information on the ion-transport activity of the sternal epithelial cells. This study presents the concentrations of inorganic cations within the moulting fluid of the anterior sternites during the late premoult and intramoult stages. The most abundant cation is Na⁺ followed by Mg²⁺, Ca²⁺ and K⁺. The concentrations of these ions do not change significantly between the stages whereas the mean pH changed from 8.2 to 6.9 units between mineral deposition in late premoult, and resorption in intramoult, respectively. Measurements of the transepithelial potential show that there is little driving force for passive movements of calcium across the anterior sternal epithelium. The results suggest a possible role of magnesium ions in ACC formation, and a contribution of pH changes to CaCO₃ precipitation and dissolution.     

Potassium and sodium ions in a potassium channel studied by molecular dynamics simulations

We have performed simulations of both a single potassium ion and a single sodium ion within the pore of the bacterial potassium channel KcsA. For both ions there is a dehydration energy barrier at the cytoplasmic mouth suggesting that the crystal structure is a closed conformation of the channel. There is a potential energy barrier for a sodium ion in the selectivity filter that is not seen for potassium. Radial distribution functions for both ions with the carbonyl oxygens of the selectivity filter indicate that sodium may interact more tightly with the filter than does potassium. This suggests that the key to the ion selectivity of KcsA is the greater dehydration energy of Na⁺ ions, and helps to explain the block of KcsA by internal Na⁺ ions.