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S B Horowitz

Publications and source records attributed to S B Horowitz.

34 records · Page 2Linked to original sources
PubMed1980

Potassium exchange in the whole cell, cytoplasm, and nucleus of amphibian oocytes.

Potassium isotope exchange was studied in whole oocytes, with and without ovarian follicles, and in oocyte cytoplasm and nucleus. Cryomicrodissection was used to prevent solute redistribution during nuclear and cytoplasmic separation. Manual follicle removal causes a small decrease in the K+ of the preparation. No effect of follicle removal is seen on 42K+ exchange. Whole oocyte exchange is multiphasic and reflects the presence of two intracellular K+ fractions. One of these fractions is present in both nucleus and cytoplasm. It exhibits first-order exponential kinetics, apparently established at the cell membrane. The second fraction is restricted to cytoplasm and exchanges at an imperceptible rate. The fractions differ in pre- and posthibernation oocytes. These observations clarify the mechanism whereby nuclear/cytoplasmic K+ concentration asymmetries are maintained and the mechanisms responsible for the high K+-activity coefficient previously reported in these cells.

Animals↗
PubMed1979

Reference phase analysis of free and bound intracellular solutes. I. Sodium and potassium in amphibian oocytes.

A method is described for the quantitative determination of free and bound solute concentrations in the cytoplasm of intact cells. The method includes (a) introduction of a gelatin gel reference phase (RP) into the cytoplasm; (b) diffusion of dissolved substances between cytoplasm and RP, (c) cell quenching to - 196 degrees C to prevent subsequent solute redistributions, (d) ultra-low temperature microdissection to isolate RP and cytoplasm samples, and (e) analysis of isolates for solute and water content. In normal oocytes of the salamander, Desmognathus ochrophaeus, free or RP Na+ and K+ are 21.0 +/- 1.1 and 128.8 +/- 2.4 mu eq/ml, respectively, and vary stoichiometrically in altered oocytes. Overall cytoplasmic concentrations are 75.2 +/- 2.7 mu eq Na+/ml and 88.6 +/- 1.5 mu eq K+/ml. Cytoplasmic chemical activities are 16.2 mu eq Na+/ml and 99.2 mu eq K+/ml, corresponding to activity coefficients of 0.22 and 1.12, respectively. The results demonstrate unambiguously that (a) oocytes actively transport Na+ and K+, and (b) cytoplasm has important binding properties which differentiate it from an ordinary aqueous solution. These cytoplasmic properties are investigated in the following paper.

Animals↗
PubMed1979

Reference phase analysis of free and bound intracellular solutes. II. Isothermal and isotopic studies of cytoplasmic sodium, potassium, and water.

The intracellular reference phase (RP) method and ultra-low temperature micro-dissection were used for isothermal and isotopic phase distribution studies of Na(+), K(+), and water in amphibian oocyte cytoplasm. One-third of the cytoplasmic water is available as solvent for [(3)H]sucrose. This fraction, designated c1, quantitatively coincides with the water volume in which Na(+) and K(+) are freely diffusible. Two-thirds of the cytoplasmic water is inaccessible to sucrose and is designated c2. The Na(+) and K(+) associated with c2 are extremely slowly exchanging (bound) and at different concentrations than in c1. The cations in c1 are in mass-action equilibria with those in c2, each described by an equation of the formC(c) (i) = C(c) (1) (i) + C(c) (2) (i) = q(i).C(RP) (i) + (max)C(c) (2) (i).f(C(RP) (i)in which C(c) (i) is the cytoplasmic Na(+) or K(+) concentration, C(c) (1) (i) is the free, and C(c) (2) (i) the bound cation concentration averaged over the cytoplasmic water. q(i) is the fractional free solute space, C(RP) (i) the RP concentration, (max)C(c) (2) (i) the concentration of binding sites, and the function f is satisfied by the Langmuir isotherm. Numerical values for the variables of the isotherm are determined. Activity coefficients are calculated from RP data and provide a basis for generalizing the oocyte results to other cells. The conclusion is drawn that both c1 and c2 are widely distributed in cells, and that cellular ionic activities involve two distinct systems: the cell-membrane system and an adsorbed water ion-exchange-like buffering system. Alternative explanations for the two-component cytoplasm are considered. A model is proposed in which c1 is a normal intracellular aqueous phase controlled by the plasma membrane, whereas c2 consists of water and ions adsorbed in hydrate crystalline structures. In oocytes these structures are identified with yolk platelets.

Animals↗
PubMed1975

Nuclear envelope permeability.

The permeability of the amphibian oocyte nuclear envelope in situ has been determined for three tritiated dextrans. The envelope is a sieve, restricting molecular movement between the cytoplasm and nucleus. The patent radius of its pores is about 45å.

Amphibians↗
PubMed1975

Nucleocytoplasmic transport and distribution of an amino acid, in situ.

Ultra-low temperature techniques (microdissection and autoradiography) were used to study the nucleocytoplasmic distribution and transport of alpha-aminoisobutyric acid (AIB) in an amino acid-accumulating cell. In amphibiam oocytes incubated in AIB, the nuclear concentration of this non-metabolizable amino acid exceeds the cytoplasmic concentration by 45%, remaining constant both over time and variation in substrate concentration. The kinetics of uptake suggest that this nucleo-cytoplasmic asymmetry arises from solubility differences between the 2 compartments, and that the nuclear envelope plays a negligible role in amino acid transport. A solute exclusion model is offered to explain the nucleocytoplasmic asymmetry.

Aminoisobutyric Acids↗
PubMed1974

The nuclear permeability, intracellular distribution, and diffusion of inulin in the amphibian oocyte.

[(3)H]Inulin (mol wt approximately 5,500) solutions are microinjected into the cytoplasm of mature oocytes of Rana pipiens and the subsequent movement of the solute recorded by quantitative ultralow temperature autoradiography. The autoradiographs show transient cellular diffusion gradients, the influence of the nucleus on these gradients, and the nuclear:cytoplasmic distribution of inulin. Analysis leads to the following conclusions: (a) Inulin diffuses in cytoplasm at about 3 x 10(-6) cm(2)/s, or one-fifth as rapidly as in water. Most of this decrease is attributable to the increased tortuosity of the diffusional path due to the presence of inclusions and macromolecules. (b) The nuclear envelope is very permeable to inulin; its resistance to inulin's passage is similar to that of cytoplasm. The envelope appears to play a negligible role in regulating the nucleocytoplasmic movement of solutes smaller than macromolecules, (c) Inulin concentrates in the nucleus to four times its cytoplasmic level; this is attributed to solute exclusion from cytoplasmic water. Evidence is presented that among hydrophilic solutes the degree of exclusion increases with molecular size. The potential significance of cytoplasmic exclusion processes to understanding secretion and the intracellular movement of macromolecules is briefly discussed.

Animals↗
PubMed1972

The permeability of the amphibian oocyte nucleus, in situ.

Ultralow temperature radioautography, suitable for the quantitative localization of diffusible solutes, was used to study the permeability of the nuclear envelope in the intact amphibian oocyte Sucrose-(3)H solutions were injected into mature oocytes, in volumes of 0 016-0 14% of that of the cell, and the subsequent movement of the solute was recorded. The resultant radioautographs show diffusion gradients in the cytoplasm and nucleus, and concentration gradients across the nuclear envelope Analysis of these gradients discloses that the nuclear envelope is as permeable as a comparable structure composed of cytoplasm, and is about 10(8) times more permeable than the oocyte plasma membrane The diffusion coefficient of sucrose in cytoplasm is 2 x 10(-6) cm(2)/sec, or about one-third its diffusivity in pure water. This reduction can probably be accounted for by an effective lengthening of the diffusional path because of obstruction by cytoplasmic inclusions. The nuclear: cytoplasmic sucrose concentration ratio at diffusional equilibrium is about 3 05, or 1.6 times as great as expected from the water content of the two compartments This asymmetry is attributed to an unavailability of 36% of the cytoplasmic water as solvent Finally, sucrose entry into oocytes from a bathing solution was monitored by whole cell analysis and radioautography. These and the microinjection results are consistent with a model in which sucrose entry into the cell is entirely limited by the permeability of the plasma membrane. The results are inconsistent with cell models that hypothesize a short-circuit transport route from the extracellular compartment to the nucleus, and with models in which cytoplasmic diffusion is viewed as limiting the rate of solute permeation.

Animals↗
PubMed1970

The intracellular transport and distribution of cysteamine phosphate derivatives.

Radioautography and extractive techniques were used to analyze the transport of cysteamine phosphate and its derivatives in salamander oocytes. The quantitative relations among the processes involved - membrane permeation, enzymatic dephosphorylation, binding through mixed disulfide formation, and cytoplasmic diffusion - were elucidated. Within the detection limits, all of the intracellular material is present as dephosphorylated derivatives. Cytoplasmic diffusion is effectively slowed by binding (the "chromatographic" effect) and makes an appreciable contribution to cellular flux rates. As a consequence, one can observe by radioautography a cortical diffusion ring which spreads inward as a function of influx time, while also increasing in peak density because of the finite membrane permeability. Good agreement was found between the transport parameters determined by radioautography and those from influx data for the whole oocyte. The ratio of nuclear to cytoplasmic concentrations of the cysteamine phosphate derivatives at equilibrium is about 0.4. The nuclear membrane is, however, a negligible barrier to transport, and the asymmetry appears to arise primarily from the quantity and sulfhydryl content of the binding proteins in the two compartments.

Animals↗
PubMed1970

Analysis of sodium transport in the amphibian oocyte by extractive and radioautographic techniques.

The transport of Na(+) in mature Eurycea oocytes was studied by quantitative radioautography of (22)Na(+) using techniques suitable for localization of diffusible solutes, together with conventional extractive techniques. Intracellular Na(+) consisted of three kinetic fractions: a cytoplasmic fast fraction of about 8.5 microeq/ml H(2)O; a cytoplasmic slow fraction of about 58.7 microeq/ml H(2)O; and a nuclear fast fraction of about 11.1 microeq/ml H(2)O. A nuclear slow fraction, if it exists, does not exceed 5% of the cytoplasmic. The fast fractions represent freely diffusible Na(+) in the two compartments; the nuclear solvent space is 1.3 times the cytoplasmic. The flux of both fast fractions is determined by the permeability of the cortical membrane, with neither the nuclear membrane nor diffusion in the cytoplasm detectably slowing the flux. The cytoplasmic slow fraction is interpreted to represent Na(+) bound to nondiffusible constituents which are excluded from the nucleus; these may be yolk platelets, although the widespread observation of Na(+) binding in other cells, and the high Na(+)/K(+) selectivity, argues against simple ion-binding to the yolk phosphoprotein.

Animals↗
PubMed1968

Analysis of glycerol-3H transport in the frog oocyte by extractive and radioautographic techniques.

The efflux of glycerol-(3)H from mature R. pipiens oocytes was studied by extractive analysis and by quantitative radioautography using techniques suitable for diffusible solutes. Extractive analysis was used to determine the total cellular concentration of tracer, and radioautography, regional intracellular concentrations, at equilibrium and as a function of efflux time, t(E). The efflux was resolvable into four kinetic fractions: cytoplasmic fast and slow fractions, and nuclear fast and slow fractions. The fast fractions represent freely diffusible glycerol in the two compartments; the solvent space accessible to glycerol is unity in the nucleus (germinal vesicle), but only 0.73 in the cytoplasm. The efflux of both fast fractions from the cell is determined by the permeability of the cortical membrane, with neither the nuclear membrane nor diffusion in the cytoplasm detectably slowing the flux. The permeability at 13.6 degrees C is 2.2 x 10(-5) cm/sec. The slow fractions leave the cell at about one-tenth the rate of the fast; the interpretation is that these fractions represent glycerol bound to impermeant cellular constituents. The size of these constituents is below the radioautographic resolution; in the cytoplasm, they appear not to be the yolk platelets. The extent of binding is about fourfold greater, per milliliter of compartment water, in the cytoplasm than in the germinal vesicle.

Animals↗