Absence Of External Potassium Research Articles

External calcium sensitivity of low sodium contractures in the control and hypertrophied right ventricle of the ferret.

The existence of possible differences of calcium (Ca2+) fluxes through the sarcolemmal sodium-calcium (Na+/Ca2+) exchanger during hypertrophy has been tested by comparing the characteristics of the contracture--as an indicator of the intracellular Ca2+ concentration--induced by partial or total withdrawal of external sodium (Na+), in the absence of external potassium, in the right ventricular trabeculae of adult ferret hearts. Pressure-overload was induced by pulmonary artery clipping and led to an increase of the right ventricular weight of 60%. At an external Ca2+ concentration ([Ca2+]o) of 3 mM, the dependence of the contractures on extracellular sodium concentration ([Na+]o), the rate of tension development, the time course of spontaneous relaxation and the time course for the repriming of the contracture were unchanged by hypertrophy. However, the relationship between [Ca2+]o and contracture amplitude at various [Na+]o showed that the apparent affinity of the contracture for [Ca2+]o was decreased in hypertrophied preparations. Thus, in 0 mM [Na+]o, half-maximal contracture was induced at a [Ca2+]o of 0.012 +/- 0.016 mM and 0.171 +/- 0.021 mM in control (n = 11) and hypertrophy (n = 12) respectively (P less than 0.001). Although these data may be indicative of a decreased Ca2+ influx through the Na+/Ca2+ exchanger, it cannot be excluded that intracellular buffering mechanism may also be involved in this differential response to [Na+]o withdrawal.

Role of calcium in secretory and metabolic effects of substrates in the gastric mucosa

The role of extracellular Ca2+ in the effects and oxidation of metabolic substrates was investigated in the isolated toad gastric mucosa. In the presence of lipoate, the stimulating effect of 10 mM glucose on spontaneous acid secretion was significantly reduced by 76% in Ca2+-free solutions. The inhibition was overcome by addition of 5 mM Ca2+. The increment in respiration induced by glucose was also blocked in the absence of external Ca2+. The effect of 10 mM pyruvate on acid secretion was inhibited by 37% in Ca2+-free solutions. The secretory responses induced by 10 mM butyrate and 10 mM octanoate were not significantly affected by Ca2+-free solutions. The rates of oxidation of [14C]-glucose and [14C]pyruvate were significantly reduced by incubating in Ca2+-free solutions containing 0.1 mM of EGTA. When O2 uptake and glucose oxidation were measured simultaneously in the same preparation, the increment in the rate of glucose oxidation accounted for by 43% of the total increase of respiration observed in the presence of Ca2+. The rates of oxidation of [14C]butyrate and [14C]acetate were not significantly affected by Ca2+-free solutions. The rate of oxidation of [14C]glucose exhibited saturation kinetics versus concentration and was lower in the absence of external Ca2+ under a range of glucose concentrations. Similar results were observed when the experiments were performed in the absence of external potassium to block the acid secretory process. Ca2+ stimulated the rate of glucose oxidation in a dose-dependent manner. The kinetics of 45Ca2+ efflux and 45Ca2+ uptake were not significantly affected by glucose and butyrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Frusemide-sensitive sodium and potassium transport by human leucocytes.

Frusemide-sensitive sodium and potassium transport by normal human leucocytes has been studied in vitro by both isotopic and net flux techniques. In physiological media the leucocyte exhibits a frusemide-sensitive influx of sodium and potassium of equal magnitude compatible with a 1:1 co-transport system. Cells exposed to zero external sodium and potassium (osmolality maintained with choline) demonstrated a frusemide-sensitive sodium and potassium efflux. Frusemide-sensitive potassium influx was dependent on the presence of external sodium but frusemide-sensitive sodium influx persisted unchanged in the absence of external potassium. Frusemide-sensitive potassium influx was dependent on external chloride but frusemide-sensitive sodium influx was chloride-independent. These last two observations make it likely that the frusemide-sensitive pathway is capable of operating in modes other than sodium-potassium co-transport.

Cation dependence of hypotaurine uptake in mouse brain slices.

Mouse brain slices take up hypotaurine (2-aminoethanesulphinic acid) from medium by means of two concentrative low- and high-affinity transport systems. [35S]Hypotaurine uptake by the slices was significantly reduced in the absence of external potassium, calcium, or magnesium ions. An excess of potassium ions also inhibited hypotaurine uptake by one-half. Uptake was almost completely abolished on removal of sodium ions. The Km constants for both low- and high-affinity transport components increased in a low-sodium medium, suggesting that sodium ions are required when hypotaurine is attached to its possible carrier sites in plasma membranes. Sodium ions also mimicked allosteric effectors of hypotaurine transport, showing positive cooperativity. More than two sodium ions may be involved in the transport of one hypotaurine molecule across the cell membrane. The calculated activation energies of transport were fairly similar in normal and sodium-deficient media and thus sodium ions may not participate in the activation mechanisms of the transport. With respect to cation dependence, hypotaurine transport in brain slices exhibits features characteristic of neurotransmitter amino acids.

Electric current generated by squid giant axon sodium pump: external K and internal ADP effects.

The operation of the sodium pump of giant axons of the squid, Loligo pealei, has been studied simultaneously in two independent ways: 1) by measuring sodium efflux with 22Na, and 2) by calculating the transmembrane current generated by the pump from measurements of membrane resistance and digitalis-sensitive membrane potential. In normal, untreated axons, the effect of increasing the external potassium concentration on both sodium efflux and pump current is similar, which suggests that Na:K pump stoichiometry remains relatively constant in the range of 0-20 mM external K. The data are compatible with a 3:2 Na:K ratio. In axons whose intracellular ADP level has been elevated by injection of L-arginine, a large, electrically silent, cardiotonic steroid-sensitive sodium efflux takes place in the absence of external potassium; this suggests that pump-mediated Na:Na exchange is 1:1 or electroneutral. Finally, elevation of external potassium levels causes the appearance, in high-ADP axons, of electrogenic pumping, with little effect on sodium efflux; hence, in contrast to what is seen in normal (low-ADP) axons, the charge translocated, per sodium ion extruded, increases sharply with increasing extracellular potassium levels.

Transport in C 4 mesophyll chloroplasts. Characterization of the pyruvate carrier

1. Evidence is presented for high rates of carrier-mediated uptake of pyruvate into the stroma of intact mesophyll chloroplasts of the C 4 plant Digitaria sanguinalis, but not the chloroplasts of the C 3 plant Spinacea oleracea. Uptake of pyruvate in the dark with the C 4 mesophyll chloroplasts was followed using two techniques: uptake of [ 14C]pyruvate as determined by silicon oil centrifugal filtration and uptake as indicated by absorbance changes at 535 nm (shrinkage/swelling) after addition of 0.1 M pyruvate salts. 2. Uptake of the pyruvate anion by an electrogenic carrier is suggested to be the major mode of transport. Chloroplast swelling was observed in potassium pyruvate plus valinomycin and uptake of [ 14C]pyruvate was inhibited by membrane-permeant anions. Valinomycin reduced uptake in the absence of external potassium and the inhibition could be reversed by addition of external potassium. 3. Uptake of pyruvic acid (or a pyruvate −/OH − antiport) is ruled unlikely since [ 14C]pyruvate uptake was relatively independent of the pH gradient across the envelope and addition of pyruvate to chloroplasts did not result in an alkalization of the medium. The low rate of swelling observed in ammonium pyruvate may be due to non-mediated permeation of pyruvic acid, which is possible only at high pyruvate concentrations. 4. The concentration of pyruvate in the stroma increased with external concentration over the range tested (up to 40 mM) but the concentration ratio (internal/external) was always less than one. The steady-state concentration of [ 14C]pyruvate in the stroma was dependent on the ionic strength of the medium, with saturation at roughly I = 0.04 M, while accumulation of the membrane-permeant cation tetraphenylmethylphosphonium decreased with increasing ionic strength. This suggests that ionic strength modifies a membrane potential (inside negative) across the envelope and that pyruvate uptake responds to the magnitude and direction of that potential (−80 mV at low ionic strength). 5. Chloride and inorganic phosphate were potent inhibitors of [ 14C]pyruvate uptake. Of the sulfhydryl reagents tested, N-ethylmaleimide was not inhibitory while mersalyl completely blocked [ 14C]pyruvate uptake and swelling in potassium pyruvate plus valinomycin. Pyruvate uptake, as measured by valinomycin induced swelling in potassium pyruvate, was highly temperature sensitive, with an energy of activation of 39 kcal/mol above 9 °C. 6. Phenylpyruvate, α-ketoisovalerate, α-ketoisocaproate, α-cyano-4-hydroxycinnamic acid and α-cyanocinnamic acid inhibited [ 14C]pyruvate but not [ 14C]-acetate uptake in the dark and also reduced pyruvate metabolism by the chloroplasts in the light.

Ouabain-insensitive salt and water movements in duck red cells. II. Norepinephrine stimulation of sodium plus potassium cotransport.

Catecholamines induce net salt and water movements in duck red cells incubated in isotonic solutions. The rate of this response is approximately three times greater than a comparable effect observed in 400 mosmol hypertonic solutions in the absence of hormone (W.F. Schmidt and T. J. McManus. 1977 a.J. Gen. Physiol. 70:59-79. Otherwise, these two systems share a great many similarities. In both cases, net water and salt movements have a marked dependence on external cation concentrations, are sensitive to furosemide and insensitive to ouabain, and allow the substitution of rubidium for external potassium. In the presence of ouabain, but the absence of external potassium (or rubidium), a furosemide-sensitive net extrusion of sodium against a large electrochemical gradient can be demonstrated. When norepinephrine-treated cells are incubated with ouabain and sufficient external sodium, the furosemide-sensitive, unidirectional influxes of both sodium and rubidium are half- maximally saturated at similar rubidium concentrations; with saturating external rubidium, the same fluxes are half-maximal at comparable levels of external sodium. In the absence of sodium, a catecholamine-stimulated, furosemide-sensitive influx of rubidium persists. In the absence of rubidium, a similar but smaller component of sodium influx can be seen. We interpret these results in terms of a cotransport model for sodium plus potassium which is activated by hypertonicity or norepinephrine. When either ion is absent from the incubation medium, the system promotes an exchange-diffusion type of movement of the co-ion into the cells. In the absence of external potassium, net movement of potassium out of the cell leads to a coupled extrusion of sodium against its electrochemical gradient.

Influence of sodium pumping on smooth muscle contraction.

Isolated capsular muscle from cat spleen did not contract on exposure to a potassium-free bathing fluid. After exposure to conditions favouring downhill sodium and potassium movement (i.e. stimulation by noradrenaline in the absence of external potassium), subsequent reactivation of the sodium pump inhibited responses to noradrenaline. Under these special conditions, but not normally, the sodium pump may be behaving in an electrogenic manner.

Effect of pentylenetetrazol on the leech Retzius cell

Pentylenetetrazol, 1–10 m m, caused reversible hyperpolarization, cessation of spontaneous firing and fall in input resistance of the Retzius cells of leech segmental ganglia. Transient depolarization and increased firing also occurred in the early portion of the response in some cells. The hyperpolarization and input resistance changes occurred in the presence of 20-m m magnesium sulfate, but the early excitatory phenomena never did. Substitution of propionate for chloride in the bathing fluid reduced the hyperpolarization and input resistance change in proportion to the reduction of chloride. In the absence of all external chloride, and with a potassium chloride rather than a potassium acetate electrode in the cell, the drug caused depolarization and increased firing. The typical drug response occurred in the absence of external potassium and the presence of ouabain, 0.1 m m. These data demonstrate that the principal effect of pentylenetetrazol on this particular neuron is a direct, selective, and reversible increase in the chloride permeability of some substantial portion of its membrane. Increased electrogenic ion pumping does not appear to be a significant factor in the response. It is noted that the overall effect of increased chloride permeability in a population of neurons is theoretically quite complex, owing to the fact that the chloride equilibrium potential is negative to the firing level in some cells and positive to it in others.

Regulation of sodium and potassium in muscle fibers of the ventricle of Union, a fresh-water lamellibrach

1. 1. Union ventricle tissue loses potassium and gains sodium in the absence of external potassium. The muscle regions K + and extrudes Na + when returned to potassium-containing medium. Ouabain and 2,4-dinitrophenol, when placed in the pottasium containing medium, inhibit the net movements of both Na + and K +. 2. 2. These results suggest that sodium, and possibly potassium, are actively transported. The stoichiometry of potassium and sodium fluxes suggests coupled transport. 3. 3. The rate of unidirectional sodium efflux was reduced by the removal of external potassium, which is consistent with coupled transport of K + and Na +.

The influx of orthophosphate into squid giant axons.

1. The influx of (32)P, applied externally as orthophosphate, into the axoplasm of squid giant axons has been studied.2. An average orthophosphate influx of 20.9 f-mole/cm(2).sec is obtained if the (32)P found in the axoplasm is assumed to be indicative of orthophosphate which has crossed the axolemma.3. The influx does not show very much dependence on external orthophosphate concentration in the range 0.02-0.5 mM.4. The influx is reduced by cyanide, 2,4-dinitrophenol, ouabain and by the absence of external potassium.5. The (32)P appears to arrive in the axoplasm as orthophosphate.6. It is concluded that there is an inward movement of orthophosphate into the axons which is mediated by an active transport process and that this may have some connexion with the active transport of sodium and potassium.

The components of the sodium efflux in frog muscle.

1. In normal Ringer solution containing 2.5 mM-K only 37% of the efflux of labelled sodium from a freshly dissected frog muscle is blocked by treatment with ouabain; in sodium-loaded muscles the ouabain-sensitive fraction of the efflux increases to 75%.2. Under all conditions, the ouabain-insensitive component of the sodium efflux is markedly reduced if the sodium in the external medium is replaced by lithium; at least in sodium-loaded muscles, the ouabain-sensitive component is increased in lithium Ringer.3. Only the ouabain-sensitive component of the efflux is affected by the external potassium concentration.4. In the presence of 2.5 mM-K the sodium influx in a freshly dissected muscle is not significantly altered by ouabain, but in a K-free medium the influx and the efflux are both reduced by nearly 20%.5. The sodium efflux can therefore be regarded as consisting of (1) a sodium-potassium coupled component that is blocked by ouabain and involves a sodium-sodium exchange in the absence of external potassium, and (2) a potassium-insensitive component that is unaffected by ouabain and tends to reach saturation at relatively lower internal sodium concentrations.6. The evidence is considered for attributing component (1) to an efflux of sodium from the sarcoplasm proper, and component (2) to an efflux from the sarcoplasmic reticulum. Although such an interpretation is consistent with many of the observations, a definite identification of the possible sodium compartments in frog muscle cannot yet be made.

POTASSIUM MOVEMENTS IN RAT UTERUS STUDIED IN VITRO: II. EFFECTS OF METABOLIC INHIBITORS, OUABAIN, AND ALTERED POTASSIUM CONCENTRATIONS

1. Sodium fluoride (10−2 M), 2,4-dinitrophenol (10−4 M), and iodoacetate (10−4 M) caused a slight decrease in potassium uptake by the uterus and fluoride and dinitrophenol caused a larger and immediate increase in efflux, resulting in a net loss of potassium. There was apparently a delayed increase in efflux caused by iodoacetate. The effects of inhibitors on efflux were not prevented by the absence of external potassium. The effects of fluoride suggested that it produced inhomogeneity in uterine potassium and analysis of the longitudinal muscle layer separately from the remainder of the uterus suggested that efflux was speeded more in myometrium than in endometrium. This was attributed to the prolonged contracture induced by fluoride. The depolarization required to explain the increases in efflux produced by fluoride and DNP was sufficient to explain the decreases in influx. It was postulated that these inhibitors act by causing depolarization which might be the result of inhibition of an electrogenic sodium pump. Iodoacetate 10−3 M caused a 50% reduction in potassium influx and probably a large immediate increase in efflux, but no evidence was obtained that this concentration caused contraction.2. Ouabain in concentrations as high as 10−5 M had only minor effects on potassium inward and outward movements and on reaccumulation of potassium and extrusion of sodium during recovery from exposure to the cold. The resistance of rat uteri to cardiac glycosides derives either from insensitivity in rat tissues or from a unique feature of sodium transport in the rat uterus.3. When KCl was added to the Ringer fluid, there was no net gain of cellular potassium relative to dry weight. Osmotic balance was achieved mainly by water loss from cells, but uncertainty as to the extracellular fluid volume prevented a definite conclusion. When KCl was omitted from the Ringer fluid, there was a 50% decrease in efflux, suggesting that a part of the potassium movement was "exchange diffusion". The assumption of exchange diffusion also would aid in explaining the observed flux ratio near unity in view of the values reported for membrane potentials of uterine cells. Owing to the lack of data regarding intracellular activity of potassium and the incompleteness of data on membrane potentials of uterine cells, it was not possible to prove whether active influx of potassium was present or absent in addition to that entering passively either by free diffusion or exchang diffusion.

POTASSIUM MOVEMENTS IN RAT UTERUS STUDIED IN VITRO: II. EFFECTS OF METABOLIC INHIBITORS, OUABAIN, AND ALTERED POTASSIUM CONCENTRATIONS

1. Sodium fluoride (10−2 M), 2,4-dinitrophenol (10−4 M), and iodoacetate (10−4 M) caused a slight decrease in potassium uptake by the uterus and fluoride and dinitrophenol caused a larger and immediate increase in efflux, resulting in a net loss of potassium. There was apparently a delayed increase in efflux caused by iodoacetate. The effects of inhibitors on efflux were not prevented by the absence of external potassium. The effects of fluoride suggested that it produced inhomogeneity in uterine potassium and analysis of the longitudinal muscle layer separately from the remainder of the uterus suggested that efflux was speeded more in myometrium than in endometrium. This was attributed to the prolonged contracture induced by fluoride. The depolarization required to explain the increases in efflux produced by fluoride and DNP was sufficient to explain the decreases in influx. It was postulated that these inhibitors act by causing depolarization which might be the result of inhibition of an electrogenic sodium pump. Iodoacetate 10−3 M caused a 50% reduction in potassium influx and probably a large immediate increase in efflux, but no evidence was obtained that this concentration caused contraction.2. Ouabain in concentrations as high as 10−5 M had only minor effects on potassium inward and outward movements and on reaccumulation of potassium and extrusion of sodium during recovery from exposure to the cold. The resistance of rat uteri to cardiac glycosides derives either from insensitivity in rat tissues or from a unique feature of sodium transport in the rat uterus.3. When KCl was added to the Ringer fluid, there was no net gain of cellular potassium relative to dry weight. Osmotic balance was achieved mainly by water loss from cells, but uncertainty as to the extracellular fluid volume prevented a definite conclusion. When KCl was omitted from the Ringer fluid, there was a 50% decrease in efflux, suggesting that a part of the potassium movement was "exchange diffusion". The assumption of exchange diffusion also would aid in explaining the observed flux ratio near unity in view of the values reported for membrane potentials of uterine cells. Owing to the lack of data regarding intracellular activity of potassium and the incompleteness of data on membrane potentials of uterine cells, it was not possible to prove whether active influx of potassium was present or absent in addition to that entering passively either by free diffusion or exchang diffusion.