Activity coefficients of intracellular Na+ and K+ during development of frog oocytes

Ion conduction and selectivity in acid-sensing ion channel 1

The selectivity of acid-sensing ion channels to cations depends on interactions with binding sites both within the pore and in the outer vestibule.

The ability of acid-sensing ion channels (ASICs) to discriminate among cations was assessed based on changes in conductance and reversal potential with ion substitution. Human ASIC1a was expressed in Xenopus laevis oocytes, and acid-induced currents were measured using two-electrode voltage clamp. Replacement of extracellular Na⁺ with Li⁺, K⁺, Rb⁺, or Cs⁺ altered inward conductance and shifted the reversal potentials consistent with a selectivity sequence of Li ∼ Na > K > Rb > Cs. Permeability decreased more rapidly than conductance as a function of atomic size, with P_K/P_Na = 0.1 and G_K/G_Na = 0.7 and P_Rb/P_Na = 0.03 and G_Rb/G_Na = 0.3. Stimulation of Cl⁻ currents when Na⁺ was replaced with Ca²⁺, Sr²⁺, or Ba²⁺ indicated a finite permeability to divalent cations. Inward conductance increased with extracellular Na⁺ in a hyperbolic manner, consistent with an apparent affinity (K_m) for Na⁺ conduction of 25 mM. Nitrogen-containing cations, including NH₄⁺, NH₃OH⁺, and guanidinium, were also permeant. In addition to passing through the channels, guanidinium blocked Na⁺ currents, implying competition for a site within the pore. The role of negative charges in an external vestibule of the pore was evaluated using the point mutation D434N. The mutant channel had a decreased single-channel conductance, measured in excised outside-out patches, and a macroscopic slope conductance that increased with hyperpolarization. It had a weakened interaction with Na⁺ (K_m = 72 mM) and a selectivity that was shifted toward larger atomic sizes. We conclude that the selectivity of ASIC1 is based at least in part on interactions with binding sites both within and internal to the outer vestibule.

Capacity and plasticity of potassium channels and high-affinity transporters in roots of barley and Arabidopsis

The role of potassium (K(+)) transporters in high- and low-affinity K(+) uptake was examined in roots of intact barley (Hordeum vulgare) and Arabidopsis (Arabidopsis thaliana) plants by use of (42)K radiotracing, electrophysiology, pharmacology, and mutant analysis. Comparisons were made between results from barley and five genotypes of Arabidopsis, including single and double knockout mutants for the high-affinity transporter, AtHAK5, and the Shaker-type channel, AtAKT1. In Arabidopsis, steady-state K(+) influx at low external K(+) concentration ([K(+)]ext = 22.5 µm) was predominantly mediated by AtAKT1 when high-affinity transport was inhibited by ammonium, whereas in barley, by contrast, K(+) channels could not operate below 100 µm. Withdrawal of ammonium resulted in an immediate and dramatic stimulation of K(+) influx in barley, indicating a shift from active to passive K(+) uptake at low [K(+)]ext and yielding fluxes as high as 36 µmol g (root fresh weight)(-1) h(-1) at 5 mm [K(+)]ext, among the highest transporter-mediated K(+) fluxes hitherto reported. This ammonium-withdrawal effect was also established in all Arabidopsis lines (the wild types, atakt1, athak5, and athak5 atakt1) at low [K(+)]ext, revealing the concerted involvement of several transport systems. The ammonium-withdrawal effect coincided with a suppression of K(+) efflux and a significant hyperpolarization of the plasma membrane in all genotypes except athak5 atakt1, could be sustained over 24 h, and resulted in increased tissue K(+) accumulation. We discuss key differences and similarities in K(+) acquisition between two important model systems and reveal novel aspects of K(+) transport in planta.

Intracellular monosaccharide and amino acid concentrations and activities and the mechanisms of insulin action

Current amino acid and monosaccharide transport models are based on an assumption which equates the intracellular chemical activity of a solute with its concentration. This assumption was tested for alpha-aminoisobutyric acid and 3-O-methylglucose in a giant cell, the amphibian oocyte, by using recently developed cryomicrodissection and internal reference phase techniques. We found the following. (i) alpha-Aminoisobutyric acid and 3-O-methylglucose activities were much greater in cytoplasm than was suggested by concentration data; i.e., activity coefficients were higher than in ordinary water solutions. This is attributable to the inaccessibility of considerable water as solvent (solute exclusion). (ii) Solute concentrations varied regionally as follows: nucleus > > animal cytoplasm > vegetal cytoplasm. Insulin increased the nucleus/cytoplasm concentration asymmetry, apparently by increasing cytoplasmic solute exclusion. (iii) Nuclear activity coefficients more closely resembled those of ordinary saline solutions so that nucleus/ extracellular concentration ratios reflected transmembrane activity gradients better than did cytoplasm (or whole cell)/extracellular ratios. (iv) Mediated passive alpha-aminoisobutyric acid and 3-O-methylglucose transport were constituent oocyte membrane properties. Membrane active transport was initiated with time (in the presence of substrate) and by insulin. (v) Increased temperature mimicked insulin in enhancing transmembrane alpha-aminoisobutyric acid activity gradients and increasing the nucleus/cytoplasm concentration asymmetry. These results indicated that concentration data are a misleading measure of cellular amino acid and monosaccharide activity; some consequences of this observation were explored. A model is proposed in which cell water has reduced solvent capacity or is compartmentalized (considered less likely) and is susceptible to physiological modulation. The model accounts for many observations in small cells, suggesting generality of the exclusion phenomenon and a previously unrecognized metabolic control mechanism.

Yolk platelets in Xenopus oocytes maintain an acidic internal pH which may be essential for sodium accumulation

Yolk platelets constitute an embryonic endocytic compartment that stores maternally synthesized nutrients. The pH of Xenopus yolk platelets, measured by photometry on whole oocytes which had endocytosed FITC-vitellogenin, was found to be acidic (around pH 5.6). Experiments on digitonin-permeabilized oocytes showed that acidification was due to the activity of an NEM- and bafilomycin A1-sensitive vacuolar proton-ATPase. Proton pumping required chloride, but was not influenced by potassium or sodium. Passive proton leakage was slow, probably due to the buffer capacity of the yolk, and was dependent on the presence of cytoplasmic monovalent cations. In particular, sodium could drive proton efflux through an amiloride-sensitive Na+/H+ exchanger. 8-Bromo-cyclic-AMP was found to increase acidification, suggesting that pH can be regulated by intracellular second messengers. The moderately acidic pH does not promote degradation of the yolk platelets, which in oocytes are stable for weeks, but it is likely to be required to maintain the integrity of these organelles. Furthermore, the pH gradient created by the proton pump, when coupled with the Na+/H+ exchanger, is probably responsible for the accumulation and storage of sodium into the yolk platelets during oogenesis.

Expression of epithelial Na channels in Xenopus oocytes

Epithelial Na channel activity was expressed in oocytes from Xenopus laevis after injection of mRNA from A6 cells, derived from Xenopus kidney. Poly A(+) RNA was extracted from confluent cell monolayers grown on either plastic or permeable supports. 1-50 ng RNA was injected into stage 5-6 oocytes. Na channel activity was assayed as amiloride-sensitive current (INa) under voltage-clamp conditions 1-3 d after injection. INa was not detectable in noninjected or water-injected oocytes. This amiloride-sensitive pathway induced by the mRNA had a number of characteristics in common with that in epithelial cells, including (a) high selectivity for Na over K, (b) high sensitivity to amiloride with an apparent K1 of approximately 100 nM, (c) saturation with respect to external Na with an apparent Km of approximately 10 mM, and (d) a time-dependent activation of current with hyperpolarization of the oocyte membrane. Expression of channel activity was temperature dependent, being slow at 19 degrees C but much more rapid at 25 degrees C. Fractionation of mRNA on a sucrose density gradient revealed that the species of RNA inducing channel activity had a sedimentation coefficient of approximately 17 S. Treatment of filter-grown cells with 300 nM aldosterone for 24 h increased Na transport in the A6 cells by up to fivefold but did not increase the ability of mRNA isolated from those cells to induce channel activity in oocytes. The apparent abundance of mRNA coding for channel activity was 10-fold less in cells grown on plastic than in those grown on filters, but was increased two- to threefold by aldosterone.

Determination of ionic permeability coefficients of the plasma membrane of Xenopus laevis oocytes under voltage clamp

1. A method of estimating absolute ionic permeability coefficients which does not depend on the use of impermeant substitutes is reported. 2. The method is based on a pump leak model of the Xenopus laevis oocyte membrane. The procedure consists of measuring, in the same experiment, the pump current and the currents generated under voltage clamp by the partial substitution of one or two ions at a time. For each experimental condition, the measured currents are substituted in a Goldman-Hodgkin-Katz type equation with two unknowns (the permeability coefficients). The set of equations thus generated enables the computation of all the ionic permeability coefficients. 3. The Xenopus oocyte membrane (stages IV and V, Dumont, 1972) has been found to be permeable to conventional ion substitutes such as N-methyl-D-glucamine (NMG), sulphate, isethionate and gluconate. 4. The values for sodium, potassium and chloride permeability coefficients obtained from sixty-eight pooled experiments were, respectively, 5.44, 17.41 and 1.49 x 10(-8) cm s-1. 5. The diffusional currents for sodium, potassium and chloride computed from the experiments referred to above were, respectively, -1.16, 0.69 and -0.038 microA cm-2. 6. A stoichiometry of the Na+-K+ pump exchange of 3/1.8 was computed. 7. The intracellular concentrations of sodium, potassium and chloride ions, as determined by ion-selective microelectrodes, were, respectively, 10.1 +/- 0.66 mM (n = 12), 109.5 +/- 3.3 mM (n = 13) and 37.7 +/- 1.18 mM (n = 19), corresponding to equilibrium potentials of 61, -95 and -28 mV. 8. Since chloride is not at equilibrium across the membrane, we propose that there is an inward uphill Cl- transport.

The use of Xenopus oocytes for the study of ion channels

Recently, in addition to the "traditional" research on meiotic reinitiation and fertilization mechanisms, the oocytes of the African frog Xenopus laevis have been exploited for the study of numerous aspects of ion channel function and regulation, such as the properties of several endogenous voltage-dependent channels and the involvement of second messengers in mediation of neurotransmitter-evoked membrane responses. In addition, injection of these cells with exogenous messenger RNA results in production and functional expression of foreign membranal proteins, including various voltage- and neurotransmitter-operated ion channels originating from brain, heart, and other excitable tissues. This method provides unique opportunities for the study of the structure, function, and regulation of these channels. A multidisciplinary approach is required, involving molecular biology, electrophysiology, biochemistry, pharmacology, and cytology.

Regulatory and energetic role of Na+ in amino acid uptake by fertilized sea urchin eggs

Relationships between the Na+ dependent amino acid uptake displayed by fertilized sea urchin eggs and the electrochemical gradient of Na+ was investigated. The time course of Na+ content and valine or alanine uptake was simultaneously monitored in Na+ loaded eggs [by fertilization in K+-free artificial sea water (OK-ASW), or by using monensin, antimycin, cyanide, or ciguatoxin]. Our results demonstrate that the uphill amino acid uptake follows the "Na+ gradient hypothesis." Subsequent fertilization of eggs Na+ depleted by ammonia for 40 min stimulates to a great extent the development of amino acid uptake as compared with controls eggs. By using simultaneous change of external and intracellular Na+ concentration, we studied the specific role of this ion. An increase in internal Na+ inhibits the uptake through trans inhibitory action while an increase in external Na+ stimulates the efficiency of the uptake system. In eggs fertilized since 30 min, hyperpolarization obtained in K+-free ASW stimulates amino acid uptake while depolarization (transfer from K+ free ASW to ASW) inhibits it. This potential-dependent effect developed after fertilization with a time course similar to that the establishment of K+ conductance described by R. A. Steinhardt, L. Lundin, and D. Mazia (1971, Proc. Natl. Acad. Sci. USA 68, 2426-2430). In conclusion, our results point out that slight modulations in the activity of the Na+ pump can widely affect the amino acid uptake, suggesting that activation of Na+/K+ ATPase has a key role in the stimulation of amino acid transport.

Neutral carrier sodium ion-selective microelectrode for extracellular studies

A Na+-selective microelectrode based on a synthetic neutral carrier (ETH 157) is described. The selectivities in respect to K+, Ca2+ and Mg2+ are adequate for extracellular measurements of Na+ activities. Microelectrodes with tip diameters of about 0.7 micron have an electrical resistance of 3 X 10(10) omega and a 90% response time of less than or equal to 3 s. The drift of the potential difference of the Na+-microelectrode cell assembly in aqueous extracellular electrolyte solutions is less than or equal to 0.2 m V/3 h.

A voltage-dependent K+ channel controlling the membrane potential in frog oocytes

Full-grown frog ovarian oocytes (Rana esculenta), were voltage clamped with a conventional two-microelectrode system. Depolarizations from a holding potential of--60 mV produced slowly developing outward currents. Two-step clamp experiments showed that, in Ringer's solution, this current has a reversal potential at about--84 mV. Substitution of either sodium or chloride with impermeant ions in the external solution did not alter significantly the activation of the current nor its reversal potential. Increasing the potassium ions concentration caused a shift on the reversal potential in the positive direction with a slope of about 48 mV per decade. The presence of TEA ions (50 mM) in the external solution partially reduced the current. It is concluded that the membrane of full-grown frog ovarian oocytes possesses voltage-dependent ionic channels permeated mainly by potassium. They appear to play an important role in the control of membrane potential.

Changes in membrane hydrogen and sodium conductances during progesterone-induced maturation of Ambystoma oocytes

A voltage-gated hydrogen ion-selective conductance has been previously described in the immature oocyte of the urodele amphibian Ambystoma. The present study was prompted by reports that changes in membrane voltage and internal pH, as well as in internal sodium ion concentration, occur during the hormone-induced maturation of oocytes from other amphibians. As activation of membrane currents might mediate changes in internal ion concentrations in addition to altering the membrane voltage, microelectrode recording techniques have been employed to examine changes in membrane conductances which occur during maturation of Ambystoma oocytes. It was observed that during the first 5 hr of maturation the magnitude of the hydrogen ion conductance gradually decreased, and that subsequently there was an increase in the amplitude of a voltage-dependent noninactivating sodium conductance. After 6 to 7 hr, after the loss of the hydrogen conductance and at about the time of germinal vesicle breakdown, the resting potential of the oocyte spontaneously shifted from approximately -10 mV to approximately +30 mV, where it remained until at least 24 hr after the initiation of maturation. This voltage transition was due to the appearance of mechanisms generating inward current in the oocyte membrane; part of this inward current was due to the tonic activation of the sodium conductance. Changes in internal pH and internal sodium ion concentration occurred during maturation, as judged from shifts in the reversal potentials of both hydrogen and sodium currents. A gradual decrease in internal hydrogen ion concentration was observed up until the time of disappearance of the hydrogen conductance (change in internal pH from about 7.15 in immature oocytes to about 7.40 by 3 hr after application of progesterone). This was followed, as sodium conductance increased, by an apparent rise in the internal sodium ion concentration (from about 6 mM to about 17 mM by 10 hr postprogesterone).

Chloride secretion by canine tracheal epithelium: IV. Basolateral membrane K permeability parallels secretion rate

We evaluated the K conductance properties of the basolateral membranes of the surface cells of canine tracheal epithelium using microelectrode techniques. Studies were conducted under basal conditions (indomethacin, 10(-6) M, mucosal solution) and after stimulation of electrogenic Cl secretion with epinephrine (10(-6) M, serosal solution). Elevated serosal solution [K] depolarized the electrical potential differences across the apical (psi a) and basolateral (psi b) membranes in both the presence and absence of epinephrine. Serosal barium (0.5 mM) also depolarized psi a and psi b and selectively increased basolateral membrane resistance threefold. We also used K-selective microelectrodes to determine cell K activity (acK) and the driving force for K transport across the limiting membranes under basal and stimulated conditions. Stimulation of Cl secretion was not associated with significant changes in psi b or acK so that the driving force for K exit from cell to serosal solution (ca. 20 mV) was not altered. There was close agreement between the basolateral membrane electromotive force (Eb) determined from prior studies (M.J. Welsh, P.L. Smith and R.A. Frizzell, J. Membrane Biol. 71:209-218, 1983) and the chemical potential difference for K across this barrier (EbK) in the presence and absence of epinephrine. These findings support the notion that the basolateral membrane is characterized by a high conductance to K under both secreting and nonsecreting conditions and indicate that the decrease in basolateral membrane resistance that accompanies stimulation of Cl secretion results from an increase in its K conductance. This obviates changes in acK that would otherwise accompany increased Na/K pump activity and, by hyperpolarizing psi a, establishes the electrical driving force for Cl secretion across the apical membrane.

Water, potassium, and sodium during amphibian oocyte development

Water, K+, and Na+ were measured in Rana pipiens oocytes during growth and prematurational development using low-temperature microdissection. Whole oocytes were analyzed during previtellogenic and vitellogenic growth. Ooplasm and germinal vesicle (nucleus) were analyzed at the onset and conclusion of vitellogenic growth. In previtellogenic oocytes (less than 40 micrograms), water, K+, and Na+ concentrations resembled those in somatic cells and were independent of cell size. With the onset of yolk deposition, water and K+ concentrations progressively decreased and Na+ progressively increased. These changes were restricted to ooplasm, the site of yolk deposition. In full-grown oocytes, vegetal ooplasm, with greater yolk density than animal ooplasm, contained less water and K+ and more Na+ than animal ooplasm. Collectively, the data indicate that yolk is poorer in water and K+ and richer in Na+ than yolk-free ooplasm (cytoplasm) or nucleoplasm. Yolk concentrations were estimated to be approximately 32%, water, approximately 69 meq K+/liter H2O, and approximately 94 meq Na+/liter H2O. Several nonyolk parameters, such as cation activities and nucleoplasmic binding, also appear to change during oogenesis.

Characterization of valine transport in sea urchin eggs

In unfertilized eggs, the mechanism of valine uptake can be summarized as follows. It is saturable over the external concentration of valine and insensitive to the presence of external sodium, depletion of cellular energy supplies and intracellular acidosis. The activation energy for the transport reaction (16.3 kcal/mol) is within the range of values reported for active transport of small molecules. In fertilized eggs, the total rate of valine uptake can be divided into two components: (i) a Na+-insensitive uptake which accounts for about 7% of total absorption as shown by studies in Na+-free medium seems to possess the same characteristics as in unfertilized eggs, (ii) a Na+-dependent transport of valine which constitutes the main entry is formed about 5 min after fertilization. It follows Michaelis-Menten kinetics characterized by 15-fold increase in Vmax with no change in Km. These two mechanisms have characteristics in common, such as their insensitivity to metabolic energy supply, their energy of activation and their ability to concentrate valine. The relationship between the establishment of the Na+-dependent valine uptake and the ionic events triggered by fertilization is discussed.

The distribution of small ions during the early development of Xenopus laevis and Ambystoma mexicanum embryos

The free ion concentrations of Na+, K+ and Cl- have been measured in the cells and intercellular spaces of developing amphibian embryos using ion-sensitive microelectrodes. Ca2+, H+ and buffering capacity have also been measured in the intercellular spaces. The free intracellular Na+ and K+ concentrations in the early cleavage stages remain approximately constant at 21 mM and 90 mM respectively. The free intracellular Cl- concentration, approximately 60 mM during the egg to 4-cell stage, falls during development to around 30 mM by the 128-cell stage. Embryos from different batches show a wide variation in intracellular free ion content. A strong correlation was observed between the ion content, particularly of K+, and the viability of the embryo. If the intracellular K+ concentration was less than 60 mM, embryos invariably died between late blastula and early gastrula stages; if greater than 80 mM the embryo generally survived. The intercellular free Na+, K+, Cl- and Ca2+ concentrations in Xenopus mid-blastula embryos are 91 mM, 4 mM, 59 mM and 1.5 mM respectively. The intercellular potential at late neurula stages is +3.5 mV when the embryo is bathed in 5% Ringer solution; changing to Ringer solution reduced the potential to -4 mV. Before gastrulation the intercellular Ca2+ concentration in Xenopus falls to approximately 0.5 mM, where it remains throughout gastrulation before returning to its mid-blastula value during neurulation. In Xenopus, the intercellular Na+ concentration falls to around 75 mM and the intercellular K+ concentration rises to around 17 mM at the end of gastrulation. These changes are not maintained and both Na+ and K+ concentrations return to their earlier values. In Ambystoma, the intercellular K+ concentration falls steadily from 7 mM at the mid-blastula stage to 2 mM at the end of neurulation. The Na+ concentration is constant at approximately 93 mM up to stage 14; between stages 14 and 16 the measurements show some scatter (minimum value 60 mM, maximum value 100 mM), and beyond stage 16 the Na+ concentration increases to 123 mM.

Microelectrode study of K+ accumulation by tight epithelia: I. Baseline values of split frog skin and toad urinary bladder

Toad bladder and split frog skin were impaled with fine-tipped single- and double-barrelled K+-selective microelectrodes. In order to circumvent membrane damage induced by impaling toad bladder, a null point method was developed, involving elevations of mucosal potassium concentration. The results suggest that intracellular potassium activity of short-circuited toad bladder is approximately 82 mM, twice as large as earlier estimates. Far more stable and rigorously defined intracellular measurements were recorded from short-circuited split frog skins. The intracellular positions of the micropipette and microelectrode tips were verified by transient hyperpolarizations of the membrane potential with mucosal amiloride or by transient depolarizations with serosal barium or strophanthidin. Simultaneous impalement of distant cells with separate micropipettes demonstrated that both the baseline membrane potentials and the responses to depolarizing agents were similar, further documenting that frog skin is a functional syncytium. Measurements with double-barrelled microelectrodes and simultaneous single-barrelled microelectrodes and reference micropipettes suggest that the intracellular potassium activity is about 104 mM, lower than previously reported. Taken together with measurements of intracellular potassium concentration, this datum suggests that potassium is uniformly distributed within the epithelial cells.

A transient calcium-dependent chloride current in the immature Xenopus oocyte

Ionic currents were studied in immature full-grown Xenopus oocytes using the two-micro-electrode voltage-clamp technique. Recordings of total membrane current showed a transient outward peak during depolarizations from the approximate resting voltage (-70 or -80 mV) to voltages more positive than -20 mV. The current-voltage relation for peak outward current was U-shaped, with a maximum at about 0 mV. Replacement of external Cl with methanesulphonate reversed this transient outward current to a transient inward current. Current relaxations recorded after the membrane potential was stepped to different voltages at the time of the peak showed a component that inverted at about -25 to -30 mV. This value was close to ECl as determined by measurement of the intracellular Cl ion concentration. The reversal potential for these current relaxations changed with the external Cl concentration as predicted by the Nernst relation. Replacement of external Ca with Mg, Sr or Ba, or addition of low concentrations of Ni in the presence of Ca, eliminated the transient outward current. Increasing the external Ca concentration increased the amplitude of the transient outward current without affecting the amplitude of the steady-state current. It was concluded that the outward peak in records of total membrane current represented the contribution of a transient outward current carried by Cl ions which was dependent on the entry of external Ca. It will be noted as ICl(Ca). Decay of ICl(Ca) could be described at the normal Ca concentration by a single exponential function whose time constant showed a shallow U-shaped voltage dependence. ICl(Ca) was maximally activatable by depolarizations from a holding potential of about -100 mV, but could not be activated by depolarizations from -40 mV. The amplitude of ICl(Ca) showed a large temperature dependence as compared to the steady-state current, suggesting complex control of its activation.

A role for action potentials in maturing Rana pipiens oocytes

Intracellular electrical recording and voltage-clamp techniques were used to investigate possible roles for the action potentials (ap's) of Rana pipiens oocytes. The peak of each spontaneous or evoked ap is at or near the equilibrium potential for Na; therefore, the internal Na+ activity (aiNa) can be calculated from the Nernst equation. During maturation from metaphase I to metaphase II, aiNa increases from about 5 to 23 mM. By using three methods to prevent ap's from firing (voltage clamping, increasing the mechanical damage, adding 5 mM CoCl2 to the Ringer's) it was shown that the increase in aiNa required the existence of ap's. External Co2+ appeared to prevent the ap's by blocking only the Na+ current. Calculations showed that the Na+ influx during an ap could account for the observed increase in aiNa if a gradient or nonuniform distribution of Na+ exists within the oocyte. Preventing the ap's also delayed the onset of shock activatibility, a criterion of maturity. I propose that ap's load the oocyte with Na+ which may regulate the rate of maturation either directly or indirectly.

Developmental regulation of Ca2+ and K+ currents during hormone-induced maturation of starfish oocytes

Changes in the electrical properties of starfish oocytes during hormone-induced maturation (the reinitiation of meiosis prior to fertilization) were studied by using the voltage-clamp technique. Three voltage-dependent ionic currents dominate the current-voltage relation of the immature oocyte: an inward Ca2+ current, a fast transient K+ current similar to the "A current" of molluscan neurons, and an inwardly rectifying K+ current. During in vitro maturation stimulated by the natural maturing hormone 1-methyladenine, gradual changes in the amplitudes of all three currents were seen: the Ca2+ currents became larger, and both K+ currents became smaller. The kinetics of the currents were not significantly altered during maturation. As a result of these changes, action potentials in the mature egg had lower thresholds, faster rates of rise, and larger overshoots than those of the immature oocyte. We also found that the total membrane capacitance decreased substantially during maturation, perhaps indicating a decrease in membrane surface area triggered by the hormone. The significance of these results is discussed in terms of the preparation of the immature oocyte for fertilization and the mechanisms of modification of ion channel properties during development.

Lipovitellin-phosvitin crystals with orthorhombic features: thin-section electron microscopy, gel electrophoresis, and microanalysis in teleost and amphibian yolk platelets and a comparison with other vertebrates

Yolk-platelet crystals in the teleosts Pelvicachromis pulcher and Noemacheilus barbatulus and the amphibians Xenopus laevis, Rana temporaria, R. esculenta, and Triturus sp. have been studied by electron diffraction and imaging using a standardized processing (glutaraldehyde-osmium tetroxide fixation, glutaraldehyde-urea embedding, thin-section staining), by X-ray microanalysis, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of their constituents. The crystal lattice is orthorhombic having--following standardized processing--in three amphibians a = 9.0 nm, b = 17.6 nm, c = 19.2 nm, and in the two teleosts a = 8.9 nm, b = 17.6 nm, c = 20.0 nm (averages). These values are very close to X-ray data from wet crystals (Xenopus laevis). Crystal images in the three axial projections point to the presence of space group P212121 (or an approximation of it since the lipovitellin dimers cannot be fully equivalent in some cases), to differences between the phosvitins of the two teleosts, and to a highly unusual stain exclusion from large crystal constituents interpreted as representing lipovitellin dimers. Microanalysis in ultrathin cryosections and other preparations revealed K and Cl to be the prominent ions in the crystals (and in the superficial layer of the platelet). Gel electrophoresis (including data of cyclostomes) showed considerable molecular variations despite a closely similar crystal architecture, emphasizing a physiological significance of the architecture, which may have remained conserved for nearly 400 million years according to paleontologic views.

The distribution of sodium and potassium in toad (Bufo bufo) oocytes estimated by a stereological method

As the toad oocyte grows, the intracellular Na concentration rises and the K concentration falls. By stereological analysis using a point counting method in a projection apparatus, it is shown that as the oocyte grows the volume density of the nucleus, endoplasmic reticulum and mitochondria falls, while that of the yolk platelets and cytoplasmic vesicles rises. A multiple regression analysis applied to the sodium and potassium concentrations and to the volume densities of the subcellular components indicates that sodium is most concentrated in cytoplasmic vesicles while potassium is most concentrated in a component composed of the cell cytoplasmic ground substance, the mitochondria and the cell nucleus.

Sodium channels induced by depolarization of the Xenopus laevis oocyte

An electrically gated Na+ channel can be made to appear in the membrane of the Xenopus laevis oocyte by simple depolarization. This membrane normally responds passively to imposed transmembrane currents with resting potentials around -60 mV, but when it is held depolarized to more than about +30 mV it becomes possible to obtain long-lasting regenerative depolarizations up to +80 mV; these depolarizations can last as long as 20 min. This potential is due to an "induction" of a Na+-dependent channel that is electrically gated open and closed. Its threshold for opening is about -20 mV and it is selective for Na+ over Cs+ and choline+ but is blocked by relatively small quantities of Li+. When a long voltage clamp step to a positive potential under ENa (+70 to +90 mV) is applied, an inward current is observed for many minutes, implying that this channel does not have an inactivation mechanism. The inward Na+ current is blocked by 0.50 mM tetrodotoxin. When the membrane is held at or near resting potential, the excitability will disappear with time, but it can be made to reappear by again depolarizing the membrane.

Structure of the active nucleolar chromatin of Xenopus laevis Oocytes

Active nucleolar chromatin of Xenopus laevis oocytes was prepared for electron microscopy by a step gradient method, which separates the chromatin from proteins and other constituents that might nonspecifically bind at low ionic strength. Between putative RNA polymerases and within the nontranscribed spacer region, the chromatin appears as smooth, thin filaments. For the first time, it is shown here that these filaments are indistinguishable from pure DNA absorbed to the same specimen, even when the ionic strength is raised up to 100 mM NaCl. Bulk rat liver chromatin, however, which was coprepared as a biochemically well-characterized standard with the active nucleolar chromatin, shows nucleosomes containing fibers, which condense into supranucleosomal structures with increasing ionic strength. Since the appearance and the behavior of active nucleolar chromatin at different ionic strengths and pHs resembles tht of pure DNA, but not of any known type of chromatin, it is suggested that, except for the transcription apparatus, very few macromolecular constituents are associated with ribosomal DNA during transcription. The observations described in this paper explain most of the published and partly conflicting results obtained by electron microscopy of nucleolar chromatin.

Elemental content in the nucleus and in yolk and yolk-free cytoplasm of developing Rana pipiens oocytes: an x-ray microanalysis study

Energy dispersive x-ray microanalysis measured the Na, K, Cl, P, Mg, S, and Ca contents (mM/kg dry weight) in the nucleus, yolk-free cytoplasm, and yolk platelets of Rana pipiens oocytes quick frozen in the ovary. The data revealed that significant content changes occur in frog oocytes during intraovarian growth. All elements but Ca changed in content in the nucleus and cytoplasm, while in the yolk platelet only Na content did not change. A nucleus to cytoplasm K gradient develops and increases in magnitude during oocyte growth. The data from this and previous reports lead to the hypothesis that intra-oocytic water and elements undergo changes in state during oocyte growth and that three subcellular Na compartments exist.

Intracellular monosaccharide and amino acid concentrations and activities and the mechanisms of insulin action

Current amino acid and monosaccharide transport models are based on an assumption which equates the intracellular chemical activity of a solute with its concentration. This assumption was tested for alpha-aminoisobutyric acid and 3-O-methylglucose in a giant cell, the amphibian oocyte, by using recently developed cryomicrodissection and internal reference phase techniques. We found the following. (i) alpha-Aminoisobutyric acid and 3-O-methylglucose activities were much greater in cytoplasm than was suggested by concentration data; i.e., activity coefficients were higher than in ordinary water solutions. This is attributable to the inaccessibility of considerable water as solvent (solute exclusion). (ii) Solute concentrations varied regionally as follows: nucleus > > animal cytoplasm > vegetal cytoplasm. Insulin increased the nucleus/cytoplasm concentration asymmetry, apparently by increasing cytoplasmic solute exclusion. (iii) Nuclear activity coefficients more closely resembled those of ordinary saline solutions so that nucleus/ extracellular concentration ratios reflected transmembrane activity gradients better than did cytoplasm (or whole cell)/extracellular ratios. (iv) Mediated passive alpha-aminoisobutyric acid and 3-O-methylglucose transport were constituent oocyte membrane properties. Membrane active transport was initiated with time (in the presence of substrate) and by insulin. (v) Increased temperature mimicked insulin in enhancing transmembrane alpha-aminoisobutyric acid activity gradients and increasing the nucleus/cytoplasm concentration asymmetry. These results indicated that concentration data are a misleading measure of cellular amino acid and monosaccharide activity; some consequences of this observation were explored. A model is proposed in which cell water has reduced solvent capacity or is compartmentalized (considered less likely) and is susceptible to physiological modulation. The model accounts for many observations in small cells, suggesting generality of the exclusion phenomenon and a previously unrecognized metabolic control mechanism.

Microprobe study of toad urinary bladder in absence of serosal K+

The bulk of the intracellular potassium in mucosal epithelial cells from toad urinary bladder has been previously reported to exchange very slowly with the serosal medium, with a half-time of some 9 hr. This observation, based on chemical analyses of mucosal cell scrapings, has been reexamined with stimultaneous diffractive and energy dispersive electron probe X-ray microanalysis. Fifty-three intracellular sites in hydrated sections and 286 sites in dehydrated sections were studied in bladders from eight toads under baseline conditions and after removal of serosal K+ for 83-133 min, with or without 10(-2) M ouabain. The baseline data confirm and extend previous examinations of the intracellular ionic composition, and provide the most direct measure of intracellular water thus far available for this tissue. Removal of serosal K+ reduced the intracellular K+ content by 20%, increased intracellular Na+ content threefold, and slightly reduced the intracellular Cl- and water contents, qualitatively consistent with published chemical analyses. The intracellular Na+ content of mucosal origin, measured by radioactive tracers and chemical analyses of cell scrapings, has been reported to be unchanged under these conditions. Simultaneous addition of ouabain and removal of external K+ produced a dramatic fall in intracellular K+ of more than 80% in a third of the cells and reduced the mean intracellular K+ content by 60%; 20% of the cells appeared to retain K+ more effectively than the bulk of the epithelial cell population. We conclude that: (i) the low rate of net exchange of intracellular K+ with the serosal bulk solution primarily reflects recycling of K+ across the basolateral membranes, (ii) radioactive tracer and chemical measurements of the intracellular Na+ pool of mucosal origin substantially understimate the total intracellular Na+ content under certain experimental conditions, and (iii) the epithelial cells display a functional heterogeneity of response to the effects of adding ouabain and withdrawing external K+.

Intracellular ionic activities and transmembrane electrochemical potential differences in gallbladder epithelium

Intracellular ion activities in Necturus gallbladder epithelium were measured with liquid ion-exchanger microelectrodes. Mean values for K, Cl and Na activities were 87, 35 and 22 mM, respectively. The intracellular activities of both K and Cl are above their respective equilibrium values, whereas the Na activity is far below. This indicates that K and Cl are transported uphill toward the cell interior, whereas Na is extruded against its electrochemical gradient. The epithelium transports NaCl from mucosa to serosa. From the data presented and the known Na and Cl conductances of the cell membranes, we conclude that neutral transport driven by the Na electrochemical potential difference can account for NaCl entry at the apical membrane. At the basolateral membrane, Na is actively transported. Because of the low Cl conductance of the membrane, only a small fraction of Cl transport can be explained by diffusion. These data suggest that Cl transport across the basolateral membrane is a coupled process which involves a neutral NaCl pump, downhill KCl transport, or a Cl-anion exchange system.

Effects of internal potassium and sodium on the anomalous rectification of the starfish egg as examined by internal perfusion

1. The effects of alterations of the intracellular ionic composition on the properties of anomalous (or inward) rectification of the egg membrane of the starfish, Mediaster aequalis, were studied by using an intracellular perfusion technique. The following results were obtained, analysing the membrane current with the voltage-clamp technique. 2. The inward rectification of the K conductance depends only on the membrane potential, V, when the K equilibrium potential, VK, is altered by changing the internal K+ concentration at a fixed external K+ concentration, while it depends on V-VK when VK is altered by changing the external K+ at a fixed internal K+ concentration. 3. From the above the conclusion is reached that the gating of the K channel of the inward rectification depends on V and external but not internal K+ concentration. 4. The conductance of the K channel at a given voltage is roughly proportional to the square root of [K+]i when the latter is altered at a fixed external K+ concentration. 5. Since the conductance is proportional to the square root of [K+]o when this is altered at a fixed internal K+ concentration, the final conclusion is that this conductance is proportional to the geometric mean of the external and internal K+ concentrations. 6. Intracellular Na+ ions are necessary for the activation of inward rectification; the K conductance increases sharply with internal Na+ concentration, reaching saturation at 200 mM. 7. A similar potentiating effect is found for Li+, although it is weaker. Rb+, Cs+ and organic cations such as arginine+ do not have this effect.