Active Transport Of Potassium Research Articles

Resolution of the potassium pool size controversy in insect midgut

The isolated midgut of Lepidopteran larvae actively transports potassium from the hemolymph side to the lumen side when chamber-mountedin vitro. Active potassium transport is not affected by ouabain and is not dependent on sodium. A long controversy has existed over the fraction of the midgut cells that participate in active transport of potassium. One set of investigators demonstrated that only a small fraction of the tissue potassium was involved in the pool of potassium involved in active transport whereas a separate set of workers found that virtually all of the exchangeable potassium was thus involved. The results presented in this paper show that the insect's diet affects whether some or all the cells are in the pool, known formally as a transport pool. Leaf-reared insects are characterized by a small pool whereas diet-reared insects are characterized by a large pool. These results are shown to correlate with the pool size results of previous investigators.

Thermodynamic analysis of active sodium and potassium transport in the frog corneal epithelium.

The formalism of linear nonequilibrium thermodynamics for a three-flow system was applied to the isolated frog corneal epithelium to study the coupling between metabolism and the Na-K transport system across this layer. There is little or no net ion transport across the isolated frog corneal epithelium bathed in Na2SO4 Ringer. Addition of amphotericin B to the tear side solution increases apical membrane permeability, which results in a net Na transport (from tear to stroma) and a net K transport in the opposite direction. Corneas were mounted in a modified Ussing chamber that permitted the simultaneous measurements of electrical parameters and O2 consumption by means of Clark-type oxygen electrodes. The overall degree of coupling, q, of the Na-K transport system to metabolism was calculated from measuring the suprabasal O2 consumption rate at "static head" and "level flow" conditions and by a second independent technique. Measurements of electrical conductance used in conjunction with other previously measured parameters allowed the calculation of the affinity, A, of the metabolic reaction driving transport, all phenomenological coefficients, and the electromotive forces of sodium (ENa) and potassium transport (EK). Values of q determined by the two techniques agreed (q = 0.80 and 0.84, respectively). This indicates incomplete coupling and a variable stoichiometric relationship among O2 consumption rate, net Na transport, and net K transport. The value calculated for A was 70.5 kcal.mol-1, for ENa 142.5 mV, and for EK -34.9 mV.

Amino acid exchange mechanism in the basolateral membrane of the midgut epithelium from the larva ofHyalophora cecropia

It is concluded that the amino acid, α-aminoisobutyric acid, is taken up from the blood into the midgut epithelium by an exchange mechanism for the following reasons: 1. The amino acid uptake becomes saturated with increasing amino acid concentration on the blood side; 2. The amino acid in the tissue cannot be washed out again to the blood side unless an amino acid is present in the blood side bathing solution; 3. The uptake mechanism is independent of metabolism. This amino acid exchange mechanism is independent of the active transport of potassium from blood side to lumen side, the exchange of potassium between blood and midgut tissue, and the transport of the amino acid, α-aminoisobutyric acid, across the midgut epithelium from lumen to blood.

Sodium pump in skeletal muscle: central nervous system-induced suppression by alpha-adrenoreceptors.

The central nervous system regulates active sodium and potassium transport in rat skeletal muscle during hypokalemia. This regulation is achieved by the apparent release of catecholamines onto muscle following nerve activity. This effect can be prevented by treatment with agents that block the α-adrenoreceptor.

Characteristics of active sodium and potassium transport in erythrocytes in children with different stages of symptomatic uremia.

Erythrocytes from 15 uremic children aged from 7 months to 16 years were analyzed for adenosine triphosphatase (total ATPase and ouabain sensitive ATPase i.e. Na+, K+-ATPase), sodium and potassium ions and ATP concentration, in some cases before and after therapeutic measures had been undertaken. No correlation was found between the levels of Na+, K+-ATPase and serum creatinine and all uremic children had Na+, K+-ATPase levels within the range for normal children. The children with rapidly progressive uremia had higher activities of Na+, K+-ATPase at the corresponding serum creatinine concentration than those with slowly progressive uremia. Longitudinally the Na+, K+-ATPase activities fell and the erythrocyte Na+-K+ ratio increased in slowly progressive uremia. Introduction of a low-protein, high-energy diet giving accelerated growth did not change the Na+, K+-ATPase activities, the concentrations of erythrocyte sodium and potassium ions or ATP. Hemodialysis gave a slight increase of Na+, K+-ATPase and of the erythrocyte Na+-K+ ratio, whereas renal transplantation resulted in a remarkable increase of Na+, K+-ATPase activity and decrease of Na+-K+ ratio. A distinct feature of uremic children with hypertension was a low erythrocyte Na+-K+ ratio and a high Na+, K+-ATPase level.

The role of active sodium and potassium transport in hyponatremic states in infancy and childhood.

Erythrocytes from 14 infants and children with serum sodium concentrations of 125 mmol/l or less were analyzed for total and Na+, K+-ATPase, sodium and potassium content and ATP concentrations to evaluate the role of active Na+-K+ transport in hyponatremia. In six out of nine patients, with a duration of hyponatremia of more than 48 hours low Na+, K+-ATPase activities and high erythrocyte sodium concentrations and Na+-K+ ratios were found indicating that a decreased Na+, K+-ATPase activity leading to an increased intracellular accumulation of sodium ions may have reduced the extracellular sodium concentration. Three subjects with large sodium losses had high Na+, K+-ATPase activities and normal erythrocyte sodium concentrations. In five cases of hyponatremia with a duration of less than 24 hours the variables indicating active Na+-K+ transport were normal. The therapeutic implications with sodium substitution and the use of drugs affecting active Na+-K+ transport are discussed.

Characteristics of active sodium and potassium transport in erythrocytes of healthy infants and children.

Erythrocytes were analysed for adenosine triphosphatase (total and sodium-potassium activated) activities, concentrations of sodium and potassium ions and ATP in normal individuals from birth to adulthood age. A decrease of Na+, K+-ATPase activity and an increase of the sodium concentration was seen in infants aged 1-3 and 4-8 months. A decrease of Na+, K+-ATPase was seen during puberty. A correlation was seen between the Na+, K+-ATPase activity and the Na+-K+ ratio. Different influences which might lead to the observed changes in Na+, K+-ATPase activity and erythrocyte sodium concentration are discussed.

Comparison of Potassium Transport in Three Structurally Distinct Regions of the Insect Midgut

ABSTRACT Active potassium transport across the isolated midgut of the Tobacco Hornworm larva, Manduca sexta, was studied by measuring the short circuit current (Isc) and unidirectional 42-potassium fluxes. The midgut is composed of structurally distinct anterior, middle and posterior regions, all of which are shown to transport potassium, so that by comparing and contrasting their structural and functional properties new information on the mechanism of midgut potassium transport was obtained. It has previously been shown that the potassium pump is located on the apical membrane of the goblet cell. In the anterior and middle regions of the midgut the goblet cell has a large cavity and mitochondria are closely associated with the apical membrane while in the posterior midgut the goblet cavity is much smaller, and mitochondria are not associated with the apical membrane. However, the apical membrane particles which have been implicated in active potassium transport in a number of other insect epithelia are present in all three regions. This observation suggests that the particles are a structural requirement for active transport, and that close association between mitochondria and the transporting membrane is not essential. Comparison of the kinetic influx pool size and the differences in the Isc decay profiles between the three midgut regions suggest that part of the influx pool is a transported pool located in the goblet cavity. A new model to explain the driving force for potassium transport in the midgut is proposed, in which the rate of potassium transport controls the entrance of potassium into the cell, rather than the opposite, currently accepted view.

Demonstration of active potassium transport in the mammalian colon.

The mechanism responsible for K transport in the mammalian colon is controversial. Experiments were performed to determine whether K secretion involves active as well as passive driving forces in controls and in animals with a marked increase in K secretion. In these experiments a steady-state solution was established in proximal and distal colon of both control rats and animals fed a K-enriched diet during in vivo luminal perfusion, to compare the observed luminal [K] with predicted equilibrium [K] when net water and electrolyte movement approached zero. Transmural potential difference was measured simultaneously. A difference between the predicted equilibrium and observed luminal [K] under this condition indicates active transport. In controls the observed [K] of 20 mmol/liter in proximal colon markedly exceeded the predicted value of 6.2 +/- 0.3, mean +/- SE, indicating active secretion. In contrast, the observed [K] in distal colon of 5 mmol/liter was less than predicted (10.0 +/- 1.0), suggesting active absorption. In K-loaded animals active K secretion was demonstrable and increase above control in both segments of colon. In proximal colon the observed [K] rose to 40 mmol/liter, compared to a predicted value of 7.2 +/- 0.3, whereas in distal colon the observed [K] was 50 mmol/liter vs. a predicted value of 7.0 +/- 0.8. These studies suggest active K secretion in proximal, but not in distal colon of control animals. Further, these data suggest that the increase in the capacity for K secretion that occurs in response to chronic K loading involves stimulation of an active mechanism in both proximal and distal colon.

Influence of streptolysin S from Streptococcus pyogenes on some functions of lymphocyte membrane

Cytotoxic effect of streptolysin S on murine lymph node lymphocytes was estimated using 51Cr and 86Rb release assays. Streptolysin S concentration of 25 HU/ml produced slight cytotoxic effects in both tests. Influence of streptolysin S on 86Rb uptake reflecting active potassium transport through lymphocyte membrane was examined. A decrease of active 86Rb transport was observed using a streptolysin S concentration not affecting cell viability. No change in cAMP and cGMP levels in lymphocytes exposed to streptolysin S were noted; however, reactivity to isoprenaline, acetylcholine and ConA of streptolysin S-treated lymphocytes was significantly higher. Streptolysin S in subcytotoxic concentrations may alter lymphocyte membrane properties

Transport of sodium and potassium across the blood-brain barrier.

Active transport of sodium and potassium across the blood-brain barrier (BBB) has been suspected, but until quite recently, there has been no direct evidence that the blood vessels of the central nervous system (CNS) are even capable of this kind of transport. Evidence from in vivo experiments indicated that special mechanisms at the blood-brain barrier regulated the concentration of potassium in the brain extracellular fluid (ECF) (1). Interpretation of these experiments, however, is complicated by the difficulty in assigning specific transport properties to the walls of the blood vessels when the effects of choroid plexus and glia cannot be excluded (2).

Mechanism of inhibition of active potassium transport in isolated midgut of Manduca sexta by Bacillus thuringiensis endotoxin.

After incubation at pH 10 or higher, Bacillus thuringiensis spores and endotoxin, at concentrations above 0.1 IU/ml, affected transport parameters in the isolated midgut of Manduca sexta larvae. (Toxic activity was lost during roughly 1 week at pH 11.) About 60% of the short-circuit current was inhibited, and the remainder was reversibly inhibited by anoxia. Electrical resistance was reduced by about 55% and oxygen uptake stimulated by about 30%. Influx of potassium from blood-side to lumen-side ('active' flux) was unaffected but flux in the reverse direction was nearly tripled. These results suggest that hydrolysis of the toxin yields an inhibitor of potassium transport, presumably a polypeptide. It is argued that inhibition is not primarily by uncoupling of oxidative phosphorylation, but instead by interference with an active depression of the efflux of potassium from lumen-side to blood-side.

Potassium modulation of taurine transport across the frog retinal pigment epithelium.

Net taurine transport across the frog retinal pigment epithelium-choroid was measured as a function of extracellular potassium concentration, [K+]o. The net rate of retina-to-choroid transport increased monotonically as [K+]o increased from 0.2 mM to 2 mM on the apical (neural retinal) side of the tissue. No further increase was observed when [k+]o was elevated to 5 mM. The [K+]o changes that modulate taurine transport approximate the light-induced [K+]o changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium. The taurine-potassium interaction was studied by using rubidium as a substitute for potassium and measuring active rubidium transport as a function of extracellular taurine concentration. An increase in apical taurine concentration, from 0.2 mM to 2 mM, produced a threefold increase in active rubidium transport, retina to choroid. Net taurine transport can also be altered by relatively large, 55 mM, changes in [Na+]o. Apical ouabain, 10(-4) M, inhibited active taurine, rubidium, and potassium transport; in the case of taurine, this inhibition is most likely due to a decrease in the sodium electrochemical gradient. In sum, these results suggest that the apical membrane contains a taurine, sodium co-transport mechanism whose rate is modulated, indirectly, through the sodium pump. This pump has previously been shown to be electrogenic and located on the apical membrane, and its rate is modulated, indirectly, by the taurine co-transport mechanism.

Inhibition of K +-sensitive p-nitrophenylphosphatase activity on rhesus-positive red cells after incubation with IgG-anti-rhesus-D

The incubation of ghosts derived from human Rhesus-positive red cells with IgG-anti-Rhesus-D inhibited the K +-sensitive p-nitrophenylphosphatase activity. This enzyme has a partial function in the (Na + + K +)-ATPase system related to the phosphorylation step, which is important for active potassium transport through the red cell membrane. The specificity of the impairment by the antigen-antibody reaction in the Rhesus-D system was proved by the following controls. Ghosts obtained from Rhesus-negative red cells incubated by IgG-anti-Rhesus-D and those of Rhesus-positive red cells treated with non-immune serum did not show any reduction of the K +- p-nitrophenylphosphatase activity. The ghost preparation with lanthanum carried out after hypotonic hemolysis of the washed red cells in 2 mM LaCl 3 at pH 6 was the most suitable procedure to explore this topic in comparison to other techniques for preparing ghosts of red cells.

Properties of a goat anti-L antibody Further evidence for heterogeneity of the L antigen

The preparation and properties of an antibody (anti-L) against low potassium type (LK) goat red cells raised in a high potassium type (HK) goat are described. This reagent stimulated active potassium transport, but showed only weak serological activity against low potassium type (LK) sheep and goat red cells. The results are discussed in relation to the hypothesis that anti-L anti-body has two specificities — a sodium pump-stimulating activity (anti-Lp) and a serological activity (anti-L ly.

Refinements in the short-circuit technique and its application to active potassium transport across the cecropia midgut.

1. The conventional, two-electrode method for measuring potential difference across an epithelium is subject to error due to potential gradients caused by current flow in the bathing medium. Mathematical analysis shows that the error in measuring short-circuit current is proportional to the resistivity of the bathing medium and to the separation of the two recording electrodes. It is particularly serious for the insect larval midgut, where the resistivity of the medium is high, and that of the tissue is low. 2. A system has been devised, which uses a third recording electrode to monitor directly the potential gradient in the bathing medium. By suitable electrical connexions, the gradient can be automatically compensated, leaving a residual error which depends on the thickness of the tissue, but not on the electrode separation. Because the thicknesses of most epithelia are smaller than the smallest practical electrode spacing, this error is smaller than that inherent in a two-electrode system. 3. Since voltage-gradients are automatically compensated, it is possible to obtain continuous readings of potential and current. A 'voltage-clamp' circuit is described, which allows the time-course of the short-circuit current to be studied. 4.The three-electrode system has been used to study the larval midgut of Hyalophora cecropia. The average results from five experiments were: initial potential difference (open-circuit): 98+/-11 mV (S.E.M.); short-circuit current at time 60 min: 498+/-160 microA cm=2; 'steady-state' resistance at 60 min: 150+/-26 omega cm2. The current is equivalent to a net potassium transport of 18.6 mu-equiv cm-2 h-1. 5. The electrical parameters of the midgut change rapidly with time. The potential difference decays with a half-time of about 158 min, the resistance increases with a half-time of about 16 min, and the short-circuit current decays as the sum of two exponential terms, with half-times of about 16 and 158 min respectively. In addition, potential and short-circuit current show transient responses to step changes. 6. The properties of the midgut are compared with those of other transporting epithelia, and their dependence on the degree of folding of the preparation is discussed. Their time-dependence is discussed in the context of changes in potassium content of the tissue, and the implications for measurements depending on the assumption of a steady state are outlined.

Identification of active cell in potassium transporting epithelium.

Epithelial ion transport regulates the environment of cells and modulates the environment inside them, making possible the evolution of eukaryotic organisms. Analysis of the transport process requires that the route taken by ions as they are actively transported across epithelia be known. The route can be determined by locating ion pumps electrically and by measuring transport pool sizes kinetically provided that the epithelial structure is simple enough to enable the results to be interpreted in cellular terms. The route by which potassium is actively transported across the lepidopteran midgut is demonstrated here by a combination of electrical and kinetic analyses. Two sorts of cells, goblet and columnar cells, are distinguished by their electrical properties. The goblet cell is identified as the site of active potassium transport, the pump being localized in its apical membrane. The goblet cells are electrically coupled with columnar cells only when midguts that have been isolated from diet-reared larvae are short circuited. A corollary to this result is that the size of the pool of potassium which is involved in transport should be small when the goblet cells are not coupled with columnar cells and should become large when they are coupled. This corollary has been confirmed by direct measurement of the transport pools using isotope tracer kinetic analysis. This is the first time that the cell type responsible for active ion transport in a polymorphic tissue has been identified directly by physiological means.

Studies on the crystalline lens. XXVII. Kinetic and bioelectric measurements of galactose cataracts in rats

The pathogenesis of galactose cataract in rat lenses was investigated by measuring changes in bioelectric potential, hydration, ion distribution and kinetics of sodium and potassium movement in cataractous lenses. During initial and vacuolar stages of cataract formation, the potential decreased in parallel with the decrease in concentration of potassium. The latter results primarily from dilution consequent to an increase in hydration. Throughout these stages, concentrations of sodium and chloride in cataractous lenses increased gradually with time on the galactose ration up to the thirteenth day. The concentration of sodium and chloride rose abruptly by the fifteenth day at which time the concentration of potassium fell abruptly. These changes were parallel to the sudden appearance of nuclear cataract and were accompanied by almost total loss of potential reflecting the disappearance of the electrolyte gradient. The rate of efflux of 86Rb (as a substitute for 42K) in cultured lenses of rats maintained on a galactose ration for 3 days was found to increase while the rate of active transport appeared to decrease compared with that of normal lenses. These findings are incompatible with the constant level of potassium found in lenses in situ. That these results might not apply to lenses in vivo because of injury associated with their removal from the globe is discussed in light of experimental evidence showing that the bioelectric potential is reduced by half following excision of the lens from the globe. However, these observations do not preclude the possibility that the rate of active transport of potassium in cataractous lenses in situ had not, in fact, increased. Further studies involving saturation kinetics of potassium transport showed that in rabbit lenses the rate of active transport increases with increasing concentration of intracellular sodium. These observations may provide a rational basis for any possible increased activity in the cation pump in sodium-rich cataractous rat lenses.

Steady-state kinetic study of magnesium and ATP effects on ligand affinity and catalytic activity of sheep kidney sodium, potassium-adenosinetriphosphatase

A non-linear least squares computer analysis of steady-state kinetic data of ATP hydrolysis catalyzed by sheep kidney (Na +, K +-ATPase (ATP phosphohydrolase, EC 3.6.1.3) gives kinetic parameters of various enzyme intermediates. Chemical models that fit the whole group of experimental data allow an overview of the effects of varying concentrations of Mg 2+ and ATP on the ligand affinity and catalytic activity of the enzyme, and also make possible the following rationale in relation to the active transport of sodium and potassium across a cell membrane: In the presence of 60 mM of sodium and the absence of ATP, potassium binds favorably to the enzyme. This effective binding of potassium is consistent with extracellular binding of potassium prior to transport into a cell, because the extracellular medium consists of high sodium, low potassium and no ATP. Binding of potassium is decreased by free ATP and Mg · ATP complex. This “weakening” of potassium binding may be related to the low affinity sites of the enzyme for potassium inside a cell where ATP and Mg 2+ are present. In the presence of 10 mM potassium, binding of sodium is insignificant, but is stimulated by Mg 2+, ATP, and Mg · ATP. This is consistent with the intracellular event of sodium binding prior to transport out of the cell. The breakdown of the Mg · ATP-containing enzyme complexes may cause release of potassium into cell and put the enzyme in a form that does not bind sodium favorably in the absence of ATP; hence, sodium is released to extracellular medium which contains no ATP.

Membrangebundene Herzglykosidrezeptoren der Herzmuskelzelle

Cell membranes contain special binding proteins for hormones and drugs. These binding sites ("receptors") located on the outside surface are linked to or are part of an enzyme facing the inner side of the membrane and are transducing and probably amplifying the information carried by the pharmacological agent to the cell. As the first step of their action cardiac glycosides reversibly bind with high affinity to specific receptors in cardiac cells and by this inhibit the (Na+ + K+)-ATPase, which is the enzyme system responsible for the active transmembraneous transport of sodium and potassium. It is thought that the inhibition of this active cation transport precedes the positive inotropic effect. Cardio-inactive glycosides have but low affinity to this receptor and thus do inhibit the (Na+ + K+)-activated ATPase only at very high concentrations. The characterization of the cardiac glycoside-receptor interaction in the heart reveals several factors that influence the affinity of the binding sites for the glycosides and thereby determine the sensitivity to this widely used group of potent drugs.