Transport of potassium inChara australis: I. A symport with sodium

Summary Voltage-clamp and tracer techniques, applied simultaneously or separately to individual cells, have been used to show that K⁺-starved internodal cells ofChara australis can develop an electrogenic transport system, which requires and transports K⁺ with high affinity (K _1/2 about 30 m) and Na⁺ with lower affinity (K _1/2 about 500 m). The most likely mechanism is symport of K⁺ with Na⁺, with a stoichiometric ratio of 11. In simultaneous measurements of quantities of charge and of ions entering individual cells, the quantity of K⁺ was consistently half the quantity of electric charge, while that of Na⁺ was consistently somewhat lower than that. Possible reasons for this discrepancy are discussed. The electrogenic symport of K⁺ with Na⁺ has not previously been reported for any cell. Its functional significance inChara is apparently the active uptake of K⁺ at the expense of the electrochemical potential difference for Na⁺. This new symport reveals the unexpected presence inChara of a Na⁺-linked chemiosmotic circuit alongside the known H⁺-linked circuit.     

UPTAKE OF GUANINE BY THE DIATOM,PHAEODACTYLUM TRICORNUTUM1

Nitrate-cultured cells of Phaeodactylum tricornutum Bohlin lack the ability to take up guanine but can do so after a period of nitrogen deprivation, i.e. photosynthesis in nitrogen-free medium. Maximum rate of uptake occurred after 24 h of nitrogen deprivation. The development of ability to take up guanine required CO₂ fixation and was prevented by cycloheximide, ammonium or nitrate. The guanine taken up accummulated in the cells almost entirely as a compound which is probably methylated hypoxanthine. Guanine uptake was dependent upon metabolism and exhibited Michaelis-Menten like kinetics with a half-saturation value of 0.48 ± 0.05 μM guanine and a maximum uptake rate for guanine of ca. 200 nmol · 10^?8 cells · h^?1. Rate of uptake increased hyperbolically with Na⁺ concentration, with 8.25 mM Na⁺ supporting half-maximal rate, and it was inhibited by K⁺ ions.     

Nitrate Uptake by the Halotolerant Cyanobacterium <Emphasis Type="Italic">Aphanothece halophytica</Emphasis> Grown Under Non-Stress and Salt-Stress Conditions

We have compared the characteristics of nitrate uptake by Aphanothece halophytica grown under non-stress and salt-stress conditions. Both cell types showed essentially similar patterns of nitrate uptake toward ammonium, nitrite, and DL-glyceraldehyde. Although the affinities of nitrate to non-stress cells and salt-stress cells were not significantly different, i.e., K_s = 416 and 450 µM, respectively, the V_max value for non-stress cells was about twofold of that for salt-stress cells (9.1 vs 5.3 µmol min^–1 mg^–1 Chl). Nitrate uptake by A. halophytica was found to be dependent on Na⁺. Ammonium inhibited nitrate uptake, and the presence of methionine sulfoximine could not release the inhibition by ammonium. Nitrite appeared to competitively inhibit nitrate uptake with a K_i value of 84 µM. Both chloride and phosphate anions did not affect nitrate uptake. DL-Glyceraldehyde, an inhibitor of CO₂ fixation, caused a reduction in the uptake of nitrate.Received: 22 October 2002 / Accepted: 6 December 2002     

Characterization of the Lactose Transport System in the Strain Bifidobacterium bifidum DSM 20082

Lactose was fermented but not assimilated by the strain Bifidobacterium bifidum DSM 20082. The sugar uptake was measured with lactose ¹⁴C. K _m and V _max values were respectively 2.6 mM and 12.11 nmol/min/mg of cell protein. The lactose transport system and the β-D-galactosidase were stimulated when the cells were grown with lactose, but isopropyl-β-D-thiogalactopyranoside had no effect. Lactose uptake was inhibited by compounds which interfered with proton and metal ionophore. Na⁺, Li⁺, or K⁺ did not affect incorporation of lactose. Furthermore, the lactose uptake decreased when an inhibitor of ATP synthesis was used. From the results of this study, the strain contained an active lactose transport system, probably a proton symport as described for Escherichia coli but with a different regulation system.     

L-alanine transport in isolated cells of interscapular brown adipose tissue in rat

The pattern of L-alanine uptake in isolated cells of interscapular brown adipose tissue has been determined. The uptake can be divided into the diffusion component (Kd=0.55 min^–1) and a saturable Na⁺-dependent transport (K _M =0.87 mM andV _max=155 nmol/min/10⁶ cells). The saturable component can be subdivided into MeAIB-sensitive (K _M =1.63 mM andV _max=162 nmol/min/10⁶ cells) and MeAIB-insensitive (K _M =3.2 mM andV _max=39.5 nmol/min/10⁶ cells). This kinetic pattern could indicate the presence of transport system (s) that resemble the commonly described transport systems for alanine uptake in several tissues.Abbreviations MeAIB Methyl-aminoisobutyric acid - AIB Aminoisobutyric acid     

Uridine uptake by isolated intestinal epithelial cells of guinea pig

Uptake of uridine was studied in isolated intestinal epithelial cells of guinea pig. Uptake was not severely influenced by metabolism. Free uridine was accumulated within cells 13-fold. Uptake was saturable with an apparent K_m value of 46 μM and a V_max of 0.9 nmol/mg protein per min. Uracil inhibited uptake only slightly; adenosine, guanosine and cytosine inhibited strongly. Antimycin A and ouabain inhibited almost 90%. If the extracellular Na⁺ concentration was decreased to 5 mM, the rate of uptake decreased 6.5-fold. The stimulatory effect of Na⁺ was related to the transmembraneous Na⁺-gradient. Cells from jejunum transported about 30% faster than cells from ileum. In conclusion, isolated enterocytes of guinea pig posses an active transport system for uridine.     

Uptake of the components of phenylalanylphenylalanine and maltose by intestinal epithelium

The observed rate of phenylalanine absorption into rat intestinal rings with 0.5 or 5.0 mM phenylalanine is greater than that for absorption of phenylalanine from 0.25 or 2.5 mM Phe-Phe, respectively. With the amino acid phenylalanine, V for absorption is the same whether Na⁺ is present (149 mM) or absent, but the concentration at which the half-maximal transport rate occurred (K_t) is greater in the absence of Na⁺. For Phe-Phe, the V decreases in the absence of Na⁺ whilst K_t is not influenced by the Na⁺ concentration. The different effect of Na⁺ on Phe and Phe-Phe transport indicates that the absorptive mechanism for Phe-Phe is different from that for phenylalanine. Absorption of a mixture of [U-¹⁴C]Phe-Phe and Phe-[G-³H]Phe showed identical rates of uptake of the carboxyl and amino terminal amino acids.Studies of transport of radioactive maltose showed that the rates of uptake of the reducing and non-reducing glucosyl moieties are identical. Radioactive maltose absorption is not inhibited by glucose oxidase.These results provide evidence that in intestinal epithelium, hydrolysis of Phe-Phe and maltose does not occur on the cell surface with release of the hydrolyzed products to the medium. Rather, hydrolysis and release of the reaction products occur at a point on the cytosol side of a diffusion barrier located in the brush border membrane.     

Transport of potassium inChara australis: II. Kinetics of a symport with sodium

Summary An electrogenic K⁺–Na⁺ symport with a high affinity for K⁺ has been found inChara (Smith & Walker, 1989). Under voltage-clamp conditions, the symport shows up as a change in membrane current upon adding either K⁺ or Na⁺ to the bathing medium in the presence of the other. Estimation of kinetic parameters for this transport has been difficult when using intact cells, since K⁺–Na⁺ current changes show a rapid falling off with time at K⁺ concentrations above 50 m. Cytoplasm-enriched cell fragments are used to overcome this difficulty since they do not show the rapid falling off of current change seen with intact cells. Current-voltage curves for the membrane in the absence or presence of either K⁺ or Na⁺ are obtained, yielding difference current-voltage curves which isolate the symport currents from other transport processes. The kinetic parameters describing this transport are found to be voltage dependent, withK _m for K⁺ ranging from 30 down to 2 m as membrane potential varies from –140 to –400 mV, andK _m for Na⁺ ranging between 470 and 700 m over a membrane potential range of –140 to –310 mV.Two different models for this transport system have been investigated. One of these involves the simultaneous transport of both the driver and substrate ions across the membrane, while the other allows for the possibility of the two ions being transported consecutively in two distinct reaction steps. The experimental results are shown to be consistent with either of these cotransport models, but they do suggest that binding of K⁺ occurs before that of Na⁺, and that movement of charge across the membrane (the voltage-dependent step) occurs when the transport protein has neither K⁺ nor Na⁺ bound to it.     

Requirement of Glycogenolysis for Uptake of Increased Extracellular K+ in Astrocytes: Potential Implications for K+ Homeostasis and Glycogen Usage in Brain

The importance of astrocytic K⁺ uptake for extracellular K⁺ ([K⁺]_e) clearance during neuronal stimulation or pathophysiological conditions is increasingly acknowledged. It occurs by preferential stimulation of the astrocytic Na⁺,K⁺-ATPase, which has higher K_m and V_max values than its neuronal counterpart, at more highly increased [K⁺]_e with additional support of the cotransporter NKCC1. Triggered by a recent DiNuzzo et al. paper, we used administration of the glycogenolysis inhibitor DAB to primary cultures of mouse astrocytes to determine whether K⁺ uptake required K⁺-stimulated glycogenolysis. KCl was increased by either 5 mM (stimulating only the Na⁺,K⁺-ATPase) or 10 mM (stimulating both transporters) in glucose-containing saline media prepared to become iso-osmotic after the addition. DAB completely inhibited both uptakes, the Na⁺,K⁺-ATPase-mediated by preventing Na⁺ uptake for stimulation of its intracellular Na⁺-activated site, and the NKCC1-mediated uptake by inhibition of depolarization- and L-channel-mediated Ca²⁺ uptake. Drugs inhibiting the signaling pathways involved in either of these processes also abolished K⁺ uptake. Assuming similar in vivo characteristics, partly supported by literature data, K⁺-stimulated astrocytic K⁺ uptake must discontinue after normalization of extracellular K⁺. This will allow Kir1.4-mediated release and reuptake by the less powerful neuronal Na⁺,K⁺-ATPase.     

Transport Pathways for Therapeutic Concentrations of Lithium in Rat Liver

Although both amiloride- and phloretin-sensitive Na⁺/Li⁺ exchange activities have been reported in mammalian red blood cells, it is still unclear whether or not the two are mediated by the same pathway. Also, little is known about the relative contribution of these transport mechanisms to the entry of therapeutic concentrations of Li⁺ (0.2–2 mm) into cells other than erythrocytes. Here, we describe characteristics of these transport systems in rat isolated hepatocytes in suspension. Uptake of Li⁺ by hepatocytes, preloaded with Na⁺ and incubated in the presence of ouabain and bumetanide, comprised three components. (a) An amiloride-sensitive component, with apparent K _m 1.2 mm Li⁺, V _max 40 μmol · (kg dry wt · min)⁻¹, showed increased activity at low intracellular pH. The relationship of this component to the concentration of intracellular H⁺ was curvilinear suggesting a modifier role of [H⁺] _i . This system persisted in Na⁺-depleted cells, although with apparent K _m 3.8 mm. (b) A phloretin-sensitive component, with K _m 1.2 mm, V _max 21 μmol · (kg · min)⁻¹, was unaffected by pH but was inactive in Na⁺-depleted cells. Phloretin inhibited Li⁺ uptake and Na⁺ efflux in parallel. (c) A residual uptake increased linearly with the external Li⁺ concentration and represented an increasing proportion of the total uptake. The results strongly suggest that the amiloride-sensitive and the phloretin-sensitive Li⁺ uptake in rat liver are mediated by two separate pathways which can be distinguished by their sensitivity to inhibitors and intracellular [H⁺]. Received: 8 April 1999/Revised: 19 July 1999     

Properties and nature of (Na+ + Mg2+)-dependent adenosine triphosphatase in the gills of the eel, angvilla anguilla (L.)

The specific activity of (Na⁺ + Mg²⁺)-dependent ATPase is three times greater in the microsomes of sea-water eels than in freshwater eels; the specific activity is one quarter of that of (Na⁺ + K⁺ + Mg²⁺)-dependent ATPase in both cases.(Na⁺ + Mg²⁺)-dependent ATPase is optimally active in a medium containing 8 mM NaCl, 4 mM MgCI₂, 4 mM ATP, pH 8.8 and at 30 °C; the enzyme is inhibited by ouabain, by NaCl concentrations > 100 mM and by treatment with urea.It is concluded that the (Na⁺ + Mg²⁺)-dependent ATPase activity of gills arises from the presence of a (Na⁺ + K⁺ + Mg²⁺)-dependent ATPase.     

Electrophysiology of the marine diatom Coscinodiscus wailesii. II. Potassium currents

The dominating mechanism of K⁺ uniport through the plasmalemma of Coscinodiscus wailesii has been studied in some detail as part of a general study of ionic relations in marine diatoms. Electrical measurements with double-barrelled glass-microelectrodes have been made in intact cells (diameter 100 m) bathed in artificial sea-water in which [K⁺] has been changed from 3 mM to 100 mM. Using a modified Goldman equation, these results provide an estimate of [K⁺]i of about 400 mM and a selectivity for K⁺ over Na⁺ and CI^-, which could spontaneously vary by orders of magnitude and reach values of about 1000. Voltage-clamp experiments have been carried out in these states of high K⁺ selectivity using bipolar staircase command voltages over a range from -180 to +60 mV. The resulting steady-state current-voltage relationships have inward rectifying sigmoid characteristics with a negative saturation current around -30 nA, and a slope conductance of the order or 1 S at free running voltages <-60 mV. Temporal responses of the clamp currents upon rectangular voltage steps were basically rectangular, i.e. they did not show the familiar relaxation kinetics of voltage-induced activation/inactivation. The sigmoid steady-state current-voltage relationships could not be described by a usual model of constant-field currents through a voltage-gated pore, where the positive current of an inward rectifier would not saturate but vanish. Alternatively, the observed steady-state inward rectifying current-voltage relationship and its changes upon changes in [K⁺]o, are well described by a three-state reaction cycle for catalysis of K⁺ translocation with a steady activity.     

Sodium transport in Na+-rich Chlorella cells

Summary The rate of Na⁺/Na⁺ exchange as measured with ²⁴Na⁺ in Na⁺-rich cells of Chlorella pyrenoidosa is governed by a single rate constant and saturates with increasing external Na⁺ concentration. The K _mvalue for this process is 0.8 mM Na⁺ and the maximum rate of exchange in illuminated cells is about 5 pmoles cm^-2 sec^-1. These values contrast with a K _mof 0.18 mM K⁺ and maximum rate of about 17 pmoles K⁺·cm^-2·sec^-1 for net K⁺ influx. Although the Na⁺/Na⁺ exchange was only slightly sensitive to light it was inhibited by the uncouplers CCCP and DNP and by the energy transfer inhibitor DCCD. This inhibition of the rate of Na⁺/Na⁺ exchange was not accompanied by a loss of internal Na⁺. Both the effect of external K⁺ on ²⁴Na⁺ influx into Na⁺-rich cells and the inhibition of net K⁺ uptake by the presence of external Na⁺ indicates that Na⁺/Na⁺ and K⁺/Na⁺ exchanges share the same carrier and that the external site of this carrier has a three to four times higher affinity for K⁺ over Na⁺.     

K+-conductance and electrogenic Na+/K+ transport of cultured bovine pigmented ciliary epithelium

Summary Using intracellular microelectrode technique, we investigated the changes in membrane voltage (V) of cultured bovine pigmented ciliary epithelial cells induced by different extracellular solutions. (1)V in 213 cells under steady-state conditions averaged –46.1±0.6 mV (sem). (2) Increasing extracellular K⁺ concentration ([K⁺] _o ) depolarizedV. Addition of Ba²⁺ could diminish this response. (3) Depolarization on doubling [K⁺] _o was increased at higher [K⁺] _o (or low voltage). (4) Removing extracellular Ca²⁺ decreasedV and reduced theV amplitude on increasing [K⁺] _o . (5)V was pH sensitive. Extra-and intracellular acidification depolarizedV; alkalinization induced a hyperpolarization.V responses to high [K⁺] _o were reduced at acidic extracellular pH. (6) Removing K _o ⁺ depolarized, K _o ⁺ readdition after K⁺ depletion transiently hyperpolarizedV. These responses were insensitive to Ba²⁺ but were abolished in the presence of ouabain or in Na⁺-free medium. (7) Na⁺ readdition after Na⁺ depletion transiently hyperpolarizedV. This reaction was markedly reduced in the presence of ouabain or in K⁺-free solution but unchanged by Ba²⁺. It is concluded that in cultured bovine pigmented ciliary epithelial cells K⁺ conductance depends on Ca²⁺, pH and [K⁺] _o (or voltage). An electrogenic Na⁺/K⁺-transport is present, which is stimulated during recovery from K⁺ or Na⁺ depletion. This transport is inhibited by ouabain and in K⁺-or Na⁺-free medium.     

Validation of the use of the lipophilic thiocyanate anion for the determination of membrane potential in Ehrlich ascites tumor cells

Summary The utility of the lipophilic anion thiocyanate (SCN⁺) as a probe for the indirect estimation of the cell membrane potential (V _m ) in Ehrlich ascites tumor cells has been evaluated by comparison to direct electrophysiological measurements. SCN accumulation is consisten with first-order uptake into a single kinetically-identifiable cellular compartement, achieving steadystate distribution in 20–30 min at 22°C. The steady-state distribution ratio ([SCN^–] _c /[SCN^–] _e ) in physiological saline is 0.44±0.02. Treatment of the cells with proparanolol (0.13 mM), an activator of Ca²⁺ dependent K⁺ channels, reduces the steady-state distribution ratio to 0.19±0.02. Conversely, treatmetn with BACl₂ (10 mM), an antagonist of the pathway, increases the SCN^– distribution ratio to 0.62±0.01. The equilibrium potentials (V _SCN ) calculated under these conditions are virtually identical to direct electrophysiological measurements of theV _m made under the same conditions. The effect of varing extracellular [K⁺]([K⁺] _e ) in the presence of constant [Na⁺] _e =100 mM has also been tested. In control cells, elevation of [K⁺] _e from 6 to 60 mM reducesV _SCN from –20.6±1.0 to –13.2±1.2 mV. Again, microelectrode measurements give excellent quantitative agreement. Propranolol increases the sensitivity of the cells to varying [K⁺] _e , so that a 10-fold elevation reducesV _SCN by approximately 31 mV. BaCl₂ greatly reduces this reponse: a 10-fold elevation in [K⁺] _e yielding only a 4-mV rediction inV _SCN . It is concluded that the membrane potential of Ehrlich cells can be estimated accurately from SCN^– distribution measurements.     

Ethanol exerts differential effects on high affinity choline uptake in neuron-enriched cultures from 8-day-old chick embryo cerebral hemispheres

Neuronal-enriched cultures were prepared from 8-day-old chick embryo cerebral hemispheres and exposed to ethanol (50 mM) from day 4 to 8 in culture. At day 8, both control and ethanol-treated cultures were processed for [³H]choline uptake in situ. Uptake was performed on cultures containing either Na⁺-plus or Na⁺-free (Li⁺) HEPES buffer. Total choline uptake as well as Na⁺-dependent and Na⁺-independent choline uptake were calculated. The K_m and V_max were calculated using the Lineweaver-Burke analysis. Our analysis of the data revealed that ethanol-treated cultures exhibited two values for V_max, one similar to that found in control cultures and one significantly lower than controls. No differences were observed in K_m values between control and ethanol-treated cultures. We interpret the low V_max to represent a population of cholinergic neurons which have been arrested at an immature stage as a result of ethanol insult.     

Transport of hemicellulose monomers in the xylose-fermenting yeastCandida shehatae

Summary Cells ofCandida shehatae repressed by growth in glucose- or D-xylose-medium produced a facilitated diffusion system that transported glucose (K _s±2 mM,V _max±2.3 mmoles g⁻¹ h⁻¹),d-xylose (K _s±125 mM,V _max±22.5 mmoles g⁻¹ h⁻¹) and D-mannose, but neither D-galactose norl-arabinose. Cells derepressed by starvation formed several sugar-proton symports. One proton symport accumulated 3-0-methylglucose about 400-fold and transported glucose (K _s±0.12 mM,V _max ± 3.2 mmoles g⁻¹ h⁻¹) andd-mannose, a second proton symport transportedd-xylose (K _s± 1.0 mM,V _max 1.4 mmoles g⁻¹ h⁻¹) andd-galactose, whilel-arabinose apparently used a third proton symport. The stoicheiometry was one proton for each molecule of glucose or D-xylose transported. Substrates of one sugar proton symport inhibited non-competitively the transport of substrates of the other symports. Starvation, while inducing the sugar-proton symports, silenced the facilitated diffusion system with respect to glucose transport but not with respect to the transport of D-xylose, facilitated diffusion functioning simultaneously with thed-xylose-proton symport.     

Identification of a third isoform of Na+, K+-ATPase activity in rat brain synaptosomes

³H-ouabain binding and ouabain-inhibitable ⁸⁶Rb⁺ (K⁺) uptake were investigated as a means to identify a third isoform of Na⁺, K⁺-ATPase in crude synaptosome preparations. The specific binding of low concentrations (10 nM and 1 uM) of ³H-ouabain, in crude synaptosome preparations, was markedly inhibited by K⁺ (0.5–5 mM). Accordingly, ⁸⁶Rb⁺ (K⁺) uptake, in the presence of 5 mM K⁺ was not sensitive to inhibition by low concentrations (10⁻¹¹–10⁻⁷ M) of ouabain. Higher concentrations (10⁻⁶–10^−2.6 M) of ouabain resulted in a biphasic inhibition of K⁺ uptake, which distinguished the activities of the presumed alpha 2 and alpha 1 isozymes of Na⁺, K⁺-ATPase. Reduction of K⁺ (1.25 mM and 0.5 mM) in the incubation, resulted in the observation of a third component of ouabain- sensitive K⁺ uptake. This Na⁺, K⁺-ATPase activity, which was defined, pharmacologically, as very sensitive (VS) to ouabain, exhibited IC₅₀s of 3.6 nM and 92 nM at 1.25 mM K⁺ and 0.5 mM K⁺, respectively. Inhibition of ouabain binding and VS-dependent K⁺ uptake, at a high, physiological cocentration (5 mM) of K⁺, suggests that VS may be an inactive isoform of brain Na⁺, K⁺-ATPase under resting conditions.     

Effect of reduced pH in absence of HCO3− on anomalous and normal potential responses in bullfrog antrum

Effect of changing [K⁺], [Na⁺] and [Cl^?] in nutrient solution on potential difference (PD) and resistance was studied in bullfrog antrum with and without nutrient HCO₃^? but with 95% O₂/5% CO₂ in both cases. In both cases, changing from 4 to 40 mM K⁺ gave about the same initial PD maximum (anomalous response) which was followed by a decrease below control level. Latter effect was much less with zero than with 25 mM HCO₃^?. Changing from 102 to 8 mM Na⁺ gave initial normal PD response about the same in both cases. However, 10 min later the change in PD with zero HCO₃^? was insignificant but with 25 mM HCO₃^? the PD decreased (anomalous response of electrogenic NaCl symport). PD maxima due to K⁺ and Na⁺ were largely related to ($\text{Na}{}^{\mathrm{+}}\text{+}\text{K}{}^{\mathrm{+}}\text{)-}\text{ATPase}$ pump. Changes in nutrient Cl^? from 81 to 8.1 mM gave only a decrease in PD (normal response). Initial PD increases are explained by relative increases in resistance of simple conductance pathways and of parallel pathways of ($\text{Na}{}^{\mathrm{+}}\text{+}\text{K}{}^{\mathrm{+}}\text{)-}\text{ATPase}$ pump and Na⁺/Cl^? symport. Removal of HCO₃^? and concurrent reduction of pH modify resistance of these pathways.     

Sodium-potassium exchange in sea urchin egg. I. Kinetic and biochemical characterization at fertilization

Biochemical and kinetic characteristics of the Na⁺-K⁺ exchange were studied in Paracentrotus lividus eggs. Measurement of the ⁸⁶Rb uptake shows that ouabain-sensitive ⁸⁶Rb uptake is dramatically stimulated within the first minute following fertilization. The Na⁺-K⁺ pump-mediated K⁺ entry presents a maximal rate at 8 min postfertilization and then decreases to reach a plateau within 30 min. We assess that the steep rise in cell K⁺ occurring at fertilization (J.P. Girard, P. Payan, C. Sardet, Exp. Cell. Res. 142:215–221, 1982) does not originate from a net entry of external K⁺. Measured 30 min postfertilization, the half-maximal activation by K⁺ of the ouabain-sensitive Na⁺-K⁺ exchange is 5–6 mM and the ouabain lC₅₀ is 5.10^?5 M. Egg cortices from unfertilized and fertilized eggs show comparable Na⁺-K⁺ ATPase activity with a 50% ouabain-sensitive fraction. V_m and K_m for Na⁺ and K⁺ of the enzyme are of the same order of magnitude in cortices of unfertilized and fertilized eggs. Cortical Na⁺-K⁺ ATPase from unfertilized eggs shows a ten fold increase of activity between pH 6.7 and pH 7.7. The results strongly suggest that the plasma membrane of unfertilized eggs contains a preexisting Na⁺-K⁺ transporting system which is obligatorily stimulated at fertilization.