Sodium and rubidium as possible nutrients for sugar beet plants

This study concerned the degree to which Na or Rb could substitute for K in the growth of sugar beet plants when K in the culture solution was low (1 meq/liter) or high (12 meq/liter).

Sodium at high concentrations increased the growth of plants in a basal nutrient medium when either deficient in K or when adequately supplied with K alone. Redistribution of K from petioles to blades could not fully explain these results. Therefore, the essentiality of Na per se for growth of sugar beet plants may be inferred.

Rubidium increased the growth of plants significantly when supplied in small doses to a nutrient medium deficient or adequately supplied with K. The amount of K added and the mode of Rb addition to solution cultures should be carefully considered when studying the effect of Rb on growth. High Rb concentrations were toxic, especially to the growth of fibrous roots.

Sodium or Rb have been shown to enhance the growth of sugar beet plants under either low or high K conditions. Essentiality of either Na and/or Rb per se for growth of sugar beets may be inferred, but other criteria should be fulfilled also for conclusive proof.

     

Interactions of rubidium, sodium, and potassium on the nutrition of sugar beet plants

The effect of Rb on the growth and the development of sugar beet plants (Beta vulgaris, var. MS NB1 × NB4) depends on the Rb concentration, the K supply, and the relative abundance of Na. Rubidium added either to a low or high K solution with or without added Na increased leaf blade size greatly, possibly through an effect on phytohormones or through a “partitioning effect” on the distribution of carbohydrates, with top growth favored over storage roots.     

Sodium absorption by intact sugar beet plants

Sodium absorption by intact sugar beet plants (Beta vulgaris) was found to be mediated by at least two distinct mechanisms when uptake was studied over a wide range of Na and K concentrations. The first mechanism operates at low Na concentrations (<1 milliequivalent per liter); presence of K completely blocks this mechanism for Na. The second mechanism operates at high Na concentrations (>1 milliequivalent per liter), transporting Na as well as K; but apparently this mechanism is not active for Na absorption in young sugar beet plants up to the 10-leaf stage.     

Changes in k, rb, and na transport to shoots after anoxia

The effect of anoxia on subsequent uptake and transport of K, Rb, and Na was examined with seedlings of barley (Hordeum vulgare L.), corn (Zea mays L.), and tall fescue (Lolium × Festuca hybrid derivative) to further our understanding of xylem loading. Roots were incubated in solutions depleted of O₂ by flushing with N₂ gas. After 1 hour exposure, plants were returned to aerated solutions for 16 hours prior to measuring uptake and transport. For each species, anoxia pretreatment significantly enhanced Na transport to the shoot. The rate of Na accumulation into roots, however, was not affected. There was no enhancement of either K or Rb accumulation in shoots, indicating specificity for Na transport. A minimum exposure to anoxia of 30 minutes and a minimum of 12 hours elapsed time was necessary to achieve the maximum rate of Na transport to the shoot in barley seedlings. Accumulation of Na in the shoot of both the control and anoxia pretreated barley plants was inhibited by anoxia and by addition of the proline analog, l-azetidine-2-carboxylic acid, during the uptake period. Enhancement of Na transport was associated with a proportional increase in the rate of synthesis of a membrane bound protein with a molecular weight of 78,000 daltons.     

Na+-dependent amino acid transport is a major factor determining the rate of (Na+,K+)-ATPase mediated cation transport in intact HeLa cells

Little is known concerning the effects of Na+-coupled solute transport on (Na+,K+)-ATPase mediated cation pumping in the intact cell. We investigated the effect of amino acid transport and growth factor addition on the short term regulation of (Na+,K+)-ATPase cation transport in HeLa cells. The level of pump activity in the presence of amino acids or growth factors was compared to the level measured in phosphate buffered saline. These rates were further related to the maximal pump capacity, operationally defined as ouabain inhibitable 86Rb+ influx in the presence of 15 microM monensin. Of the growth factors tested, only insulin was found to moderately (22%) increase (Na+,K+)-ATPase cation transport. The major determinant of pump activity was found to be the transport of amino acids. Minimal essential medium (MEM) amino acids increased ouabain inhibitable 86Rb+ influx to a level close to that obtained with monensin, indicating that the (Na+,K+)-ATPase is operating near maximal capacity during amino acid transport. This situation may apply to tissue culture conditions and consequently measurements of (Na+,K+)-ATPase activity in buffer solutions alone may yield little information about cation pumping under culture conditions. This finding applies especially to cells having high rates of amino acid transport. Furthermore, rates of amino acid transport may be directly or indirectly involved in the long-term regulation of the number of (Na+,K+)-ATPase molecules in the plasma membrane.     

Effects of Alkali Metal Gations on the Potential across Toad and Bullfrog Urinary Bladder

Isolated urinary bladders of the bullfrog (R. catesbeiana) and the toad (B. marinus) were mounted in an Ussing chamber. Potential differences up to 114 mv were observed in bullfrog bladder when the mucosal surface was bathed in dilute Na₂SO₄ and the serosal surface in sulfate Ringer's. In experiments with bullfrogs, K was used to replace Na in the mucosal solution and Na was used for K in the serosal solutions. The selectivity was judged in terms of the relative effectiveness of the replacement cation in maintaining the bladder potential. In experiments with toads, K and Rb were equally poor replacements for Na at the mucosal border, while Rb was a good replacement for K at the serosal border. Li in the mucosal solution appeared to depress the potential in part irreversibly. At the serosal border, Li was a partially effective substitute for K, more so than was Na. However, both were poor replacements compared to Rb. The mucosal surface of the urinary bladder of both frog and toad appears to be Na-selective and the serosal surface appears to be K-selective, consistent with the Koefoed-Johnsen-Ussing model for frog skin.     

Ionic Permeability and Electrical Potential Differences in Necturus Kidney Cells

The cellular concentrations of Na, K, and Cl have been measured in kidney slices of the amphibian, Necturus maculosus. Permeability coefficients have been determined for Na, K, Cl, Rb, Cs, and choline, from studies both of the uptake of radioactive isotopes and the rate of cell swelling in anisotonic solutions. The results of both methods were found to agree well. Measurements were also made of electrical potential differences across the peritubular face of the kidney cells using bathing solutions in which the electrolyte composition and concentrations could be varied. The data obtained are consistent with a model cell in which the potential difference arises as a result of differences in Na permeability relative to K on the two faces of the cell. The intracellular Na concentration is considered to be regulated by a Na-K coupled pump located at the peritubular face of the cell.     

Monovalent ion effects on acetylcholine receptor from Torpedo californica

The effects of the five Group I monovalent ions, Li, Na, K, Rb, and Cs, on [³H]acetylcholine binding to Triton X-100 solubilized acetylcholine receptor from Torpedo californica electroplax were examined. Acetylcholine binding was not greatly affected by Li or Na, but was inhibited by the other ions in the order Cs > Rb > K. The inhibition by K appeared to occur by a mechanism identical to that for d-tubocurarine inhibition of acetylcholine binding.     

Sodium, an Obligate Growth Requirement for Predominant Rumen Bacteria

Sodium is an obligate growth requirement for most currently recognized predominant species of rumen bacteria. The isoosmotic deletion of Na(+) from a nutritionally adequate defined medium completely eliminated growth of most species. Growth yields and rates were both a function of Na(+) concentration for Na(+)-requiring species, and Na(+) could not be replaced by Rb(+), Li(+), or Cs(+) when these ions were substituted for Na(+) at a concentration equivalent to an Na(+) concentration that supported abundant growth. Li(+), Cs(+), or Rb(+) was toxic at an Na(+)-replacing concentration (15 mM) but not at a K(+)-replacing concentration (0.65 mM). K(+) was also an obligate growth requirement for rumen bacteria in media containing Na(+) and K(+) as major monovalent cations, but K(+) could be replaced, for most species, by Rb(+). The quantities of Na(+) that support rapid and abundant growth of Na(+)-requiring rumen bacteria show that these organisms are slight halophiles. A growth requirement for Na(+) appears more frequent among nonmarine bacteria than has been previously believed.     

Effect of extracellular sodium removal upon 86Rb outflow from pancreatic islet cells

The present study was undertaken to characterize the effect of extracellular Na+ removal on 86Rb outflow from perifused rat pancreatic islets. Complete Na+ omission inhibited 86Rb outflow whether the islets were perifused in the presence or in the absence of extracellular Ca2+. Ouabain (1 mM) did not reduce the inhibitory effect of Na+ deprivation, whilst diphenylhydantoin (72.9 microM) mimicked the Na+-removal-induced fall in 86Rb outflow. Glucose (16.7 mM) lost its capacity to inhibit 86Rb outflow when the perifusate was deprived of extracellular Na+. These results indicate that Na+ omission reproduces the inhibitory effect of glucose on 86Rb outflow. The reduction in 86Rb outflow recorded after Na+ deprivation could be mediated by an intracellular acidification and/or a decrease in the intracellular Na+ activity. It is tempting to speculate that the capacity of glucose to reduce the B-cell Na+ content may participate in the process by which the sugar decreases K+ permeability.     

Some total and partial reactions of Na+/K+-ATPase using ATP and acetyl phosphate as a substrate

Acetyl phosphate, as a substrate of (Na+ + K+)-ATPase, was further characterized by comparing its effects with those of ATP on some total and partial reactions carried out by the enzyme. In the absence of Mg2+ acetyl phosphate could not induce disocclusion (release) of Rb+ from E2(Rb); nor did it affect the acceleration of Rb+ release by non-limiting concentrations of ADP. In K+-free solutions and at pH 7.4 sodium ions were essential for ATP hydrolysis by (Na+ + K+)-ATPase; when acetyl phosphate was the substrate a hydrolysis (inhibited by ouabain) was observed in the presence and absence of Na+. In liposomes with (Na+ + K+)-ATPase incorporated and exposed to extravesicular (intracellular) Na+, acetyl phosphate could sustain a ouabain-sensitive Rb+ efflux; the levels of that flux were similar to those obtained with micromolar concentrations of ATP. When the liposomes were incubated in the absence of extravesicular Na+ a ouabain-sensitive Rb+ efflux could not be detected with either substrate. Native (Na+ + K+)-ATPase was phosphorylated at 0 degrees C in the presence of NaCl (50 mM for ATP and 10 mM for acetyl phosphate); after phosphorylation had been stopped by simultaneous addition of excess trans-1,2-diaminocyclohexane-N,N,N',N' tetraacetic acid and 1 M NaCl net synthesis of ATP by addition of ADP was obtained with both phosphoenzymes. The present results show that acetyl phosphate can fuel the overall cycle of cation translocation by (Na+ + K+)-ATPase acting only at the catalytic substrate site; this takes place via the formation of phosphorylated intermediates which can lead to ATP synthesis in a way which is indistinguishable from that obtained with ATP.     

Kinetic parameters and mechanism of active cation transport in HeLa cells as studied by Rb+ influx

On incubation of HeLa cells in chilled isotonic medium, intracellular Na+ (Nac+) increased and K+ (Kc+) decreased with time, reaching steady levels after 3 h. The steady levels varied in parallel with the extracellular cation concentrations ([Na+]e, [K+]e). The cell volumes and the protein and water contents, respectively, of cells kept for 3 h in chilled media of various [Na+]e and [K+]e were not significantly different. Ouabain-sensitive Rb+ influx took place at the initial rate for a certain period which depended on [Na+]c at the beginning of the assays. The existence of two external K+ loading sites per Na+/K+-pump was demonstrated. The affinities of the sites for Rb+ as a congener of K+ were almost the same. Na+e inhibited ouabain-sensitive Rb+ influx competitively, whereas K+ was not inhibitory. Kinetic parameters were determined: the K 1/2 for Rbe+ in the absence of Na+e was 0.16 mM and th Ki for Na+e was 36.8 mM; the K 1/2 for Na+c was 19.5 mM and the Ki for K+c seemed to be extremely large. The rate equation of the ouabain-sensitive Rb+ influx suggests that Na+ and K+ are exchanged alternately through the pump by a binary mechanism.     

Ion Absorption and Retention by Chlorella pyrenoidosa. III. Selective Accumulation of Rubidium, Potassium and Sodium

The selective preference of Chlorella pyrenoidosa for alkali metal cations was found to have the order Rb > K > > > Na. It was demonstrated that a cation of higher preference can replace in the cell cations of lower preference by an ion interchange process.

The replacement of Na from the cell by K or Rb occurred against high external Na concentrations up to 450 meq/1 Na.

It is suggested that the structural selectivity of the Chlorella cell may be amplified by a chromatographic type repetitive selection process, driven by metabolically dependent unsymmetric shape changes of cellular membranes.

     

Influence of calcium on sodium and potassium absorption by fresh and aged bean stem slices

The influence of Ca on the aging processes of bean stem (Phaseolus vulgaris) slices and on the absorption of K and Na by fresh and aged slices was investigated. In the presence of Ca, fresh tissue showed a preferential Na uptake. The preference for Na over K resulted from a differential depressive effect of Ca on absorption of these two ions. In aged tissue Na uptake was also depressed, but K absorption was accelerated, with a net result of a much greater absorption of K than Na.     

Characterization of Na(+)-K+ homeostasis of cultured human skin fibroblasts in the presence and absence of fetal bovine serum.

Previously, we demonstrated that removal of fetal bovine serum (FBS) from the medium of human skin fibroblasts resulted in an accelerated 86Rb+ washout, decreased cellular K+, and increased Na+ contents. In the present study we examined the mechanism underlying these changes. The efflux rate constant for 86Rb+, and the cellular contents of Na+ and K+ were measured. Verapamil (K1/2 = 15 microM) and chlorpromazine (K1/2 = 1 microM) reduced by approximately 70% the increased 86Rb+ washout evoked by FBS removal. The effect of the two drugs was additive at low, but not high, concentrations. Verapamil and chlorpromazine also attenuated the decrease in cellular K+ content and prevented the increase in cellular Na+ content associated with FBS depletion. Bumetanide (50 microM) was only partially effective in offsetting the enhanced 86Rb+ efflux and was completely without any effect on the cellular Na+ and K+ changes induced by FBS removal. In the presence of FBS, A-23187 produced a slight and transient increase of the 86Rb+ washout. The protein kinase C activator phorbol 12-myristate 13-acetate enhanced the 86Rb+ efflux in FBS-containing medium for a prolonged period but this increase was only a fraction of that caused by serum removal. Cellular Na+ and K+ contents were not changed by the phorbol ester. We conclude that FBS removal raises the cellular Na+ content, and enhances 86Rb+ efflux, through a calmodulin-dependent pathway activated by calcium influx.     

Insulin affects the sodium affinity of the rat adipocyte (Na+,K+)-ATPase

The K0.5 for intracellular sodium of the two forms of (Na+,K+)-ATPase which exist in rat adipocytes (Lytton, J., Lin, J. C., and Guidotti, G. (1985) J. Biol. Chem. 260, 1177-1184) has been determined by incubating the cells in the absence of potassium in buffers of varying sodium concentration; these conditions shut off the Na+ pump and allow sodium to equilibrate into the cell. The activity of Na+,K+)-ATPase was then monitored with 86Rb+/K+ pumping which was initiated by adding isotope and KCl to 5 mM, followed by a 3-min uptake period. Atomic absorption and 22Na+ tracer equilibration were used to determine the actual intracellular [Na+] under the different conditions. The K0.5 values thus obtained were 17 mM for alpha and 52 mM for alpha(+). Insulin treatment of rat adipocytes had no effect on the intracellular [Na+] nor on the Vmax of 86Rb+/K+ pumping, but did produce a shift in the sodium ion K0.5 values to 14 mM for alpha (p less than 0.025 versus control) and 33 mM for alpha(+) (p less than 0.005 versus control). This change in affinity can explain the selective stimulation of alpha(+) by insulin under normal incubation conditions. Measurement of the K0.5 for sodium ion of (Na+,K+)-ATPase in membranes isolated from adipocytes revealed only a single component of activation with a low K0.5 of 3.5 or 12 mM in the presence of 10 or 100 mM KCl, respectively. Insulin treatment of the isolated membranes or of the cells prior to membrane separation had no effect on these values.     

Ion specificity of cardiac sarcolemmal Na+/H+ antiporter

In bovine cardiac sarcolemmal vesicles, an outward H+ gradient stimulated the initial rate of amiloride-sensitive uptake of 22Na+, 42K+, or 86Rb+. Release of H+ from the vesicles was stimulated by extravesicular Na+, K+, Rb+, or Li+ but not by choline or N-methylglucamine. Uptakes of Na+ and Rb+ were half-saturated at 3 mM Na+ and 3 mM Rb+, but the maximal velocity of Na+ uptake was 1.5 times that of Rb+ uptake. Na+ uptake was inhibited by extravesicular K+, Rb+, or Li+, and Rb+ uptake was inhibited by extravesicular Na+ or Li+. Amiloride-sensitive uptake of Na+ or Rb+ increased with increase in extravesicular pH and decrease in intravesicular pH. In the absence of pH gradient, there were stimulations of Na+ uptake by intravesicular Na+ and K+ and of Rb+ uptake by intravesicular Rb+ and Na+. Similarly, there were trans stimulations of Na+ and Rb+ efflux by extravesicular alkali cations. The data suggest the existence of a nonselective antiporter catalyzing either alkali cation/H+ exchange or alkali cation/alkali cation exchange. Since increasing Na+ caused complete inhibition of Rb+/H+ exchange, but saturating K+ caused partial inhibitions of Na+/H+ exchange and Na+/Na+ exchange, the presence of a Na(+)-selective antiporter is also indicated. Although both antiporters may be involved in pH homeostasis, a role of the nonselective antiporter may be in the control of Na+/K+ exchange across the cardiac sarcolemma.     

CSF-1 stimulates Na+K+-ATPase mediated 86Rb+ uptake in mouse bone marrow-derived macrophages

86Rb+ was used as an isotopic tracer for the measurement of K+-uptake into quiescent murine bone marrow-derived macrophages. 86Rb+ uptake was inhibited by ouabain indicating a Na+K+-ATPase is being measured. In support of this finding, increased sensitivity to ouabain inhibition was seen when the K+ content of the medium was reduced. A purified colony stimulating factor (CSF-1) was shown to stimulate the ouabain-sensitive 86Rb+ uptake in a dose-dependent manner. Such colony stimulating factor stimulation of 86Rb+ (K+) influx was rapid, with a maximal effect seen 10 minutes after growth factor addition followed by a gradual decrease. Thus increased Na+K+-ATPase activity was an early response of macrophages to the colony stimulating factor.     

Symmetry and asymmetry of permeation through toxin-modified Na+ channels.

Single Na+ channels from rat skeletal muscle were inserted into planar lipid bilayers in the presence of either 200 nM batrachotoxin (BTX) or 50 microM veratridine (VT). These toxins, in addition to their ability to shift inactivation of voltage-gated Na+ channels, may be used as probes of ion conduction in these channels. Channels modified by either of the toxins have qualitatively similar selectivity for the alkali cations (Na+ approximately Li+ greater than K+ greater than Rb+ greater than Cs+). Biionic reversal potentials, for example, were concentration independent for all ions studied. Na+/K+ and Na+/Rb+ reversal potentials, however, were dependent on the orientation of the ionic species with respect to the intra- or extracellular face of the channel, whereas Na+/Li+ biionic reversal potentials were not orientation dependent. A simple, four-barrier, three-well, single-ion occupancy model was used to generate current-voltage relationships similar to those observed in symmetrical solutions of Na, K, or Li ions. The barrier profiles for Na and Li ions were symmetric, whereas that for K ions was asymmetric. This suggests the barrier to ion permeation for K ions may be different than that for Na and Li ions. With this model, these hypothetical energy barrier profiles could predict the orientation-dependent reversal potentials observed for Na+/K+ and Na+/Rb+. The energy barrier profiles, however, were not capable of describing biionic Na/Li ion permeation. Together these results support the hypothesis that Na ions have a different rate determining step for ion permeation than that of K and Rb ions.     

Cat Heart Muscle in Vitro : II. The steady state resting potential in quiescent papillary muscles

The steady state transmembrane resting potential difference (V_m) has been measured in quiescent papillary muscles. V_m was determined as a function of the external K concentration in Cl and SO₄ solutions and compared with the K equilibrium potential. Other measurements were made after replacement of external Na by choline, K by Rb and Cs, and Cl by SO₄, CH₃SO₄, and NO₃. Effects on V_m of albumin, temperature, and variation in internal K concentration are described.