Search PubMed⌕ Search

Biomedical subjects

J Fischbarg

Publications and source records attributed to J Fischbarg.

108 records · Page 6Linked to original sources
PubMed1978

The effect of external pH on osmotic permeability, ion and fluid transport across isolated frog skin.

1. The rate of volume flow across frog skin induced by an osmotic gradient was measured when normal (7.4) and low pH (2.28) solutions bathed the outside. The osmotic permeabilities (Pos) were 2.4 +/- 0.4 and 4.8 +/- 1.0 micrometer/sec, respectively. The change in Pos induced by low pH was reversible. 2. Volume flow in the absence of an osmotic gradient was measured at normal and low pH. Values were 0.69 +/- 0.13 and 1.1 +/- 0.2 microliter/hr. cm2, respectively; the paired differences were significant (P less than 0.0025). This change in rate was partially reversible upon return to normal pH. 3. The potential difference (V) and short-circuit current (Is) across skins were measured under several conditions and the following equivalent parameters in a simplified electrical model were computed: total resistance (Rt); shunt resistance (Rs); electromotive force of the pump (ENa); and salt transport at open circuit (JNaCl). Representative figures were (a), at pH 7.4: Is = 14 +/- 1.6 microampere/cm2; Rt = 3.3 +/- 0.4 komega.cm2; Rs = 7.2 +/- 1.0 komega.cm2; ENA = 103 +/- 38 mV; JNaCl = 7.2 +/- 1.2 microampere/cm2; (b) at pH 2.28: Is = 8.3 +/- 2.1 microampere/cm2; Rt = 0.46 +/- 0.12 komega. cm2; Rs = 0.65 +/- 0.06 komega.cm2; ENa = 59 +/- 25 mV; JNaCl = 9.4 +/- 3.3 microampere/cm2. 4. From the electrical parameters measured concomitantly with the rate of fluid transport in given experiments, the expected salt concentration of the transported fluid was 0.30 +/- 0.08 and 0.38 +/- 0.08 mole/l. at normal and low pH, respectively, or some 3-4 times hyperosmotic with respect to the medium. 5. Treatment with low pH on the outside has been found to open the intercellular junctions in previous studies. The present results suggest that, if such an effect occurs, it is localized only to a small fraction of the cell perimeter. Making certain assumptions that fraction could be as low as 0.003. 6. Low pH on the outside reversibly changes the electrical parameters of a 'tight' epithelium like the frog skin into values more typical of 'intermediate' epithelia; both the total and shunt resistances decrease to about 0.1 of their normal values. These changes do not apparently affect the osmolarity of the transported fluid.

Animals↗
PubMed1978

A method for preparing a viable corneal endothelial layer, completely denuded of overlying stroma.

A method for denuding Descemet's membrane by dissection of all overlying stroma, while preserving the corneal endothelial layer, was developed and evaluated. The technique consists in treating the stroma with trypsin and surgically removing the softened stromal layers. With an automated thickness-measuring technique, the endothelium-Descemet preparation was found to range from 23 to 42 micron in thickness. Endothelial cell morphology was normal under specular and light-transmission microscopic examinations. Under electron microscopy, the endothelial cells appeared intact, except for an increase in the number of intracellular vacuoles. Occasionally, small portions of an intercellular space were found to be mildly dilated, but the over-all integrity of the junctional complex was intact. In vitro, the viability of the preparation was comparable with that of a cornea with all layers intact. Endothelial resting membrane potentials, measured with intracellular microelectrodes, were found to be within the normal (33 +/- 2 mV) range. It is thus possible to obtain a viable endothelial layer, completely stripped of stroma.

Animals↗
PubMed1977

Adenosine stimulation of fluid transport across rabbit corneal endothelium.

The rate of fluid transport across rabbit corneal endothelium has been measured with an automatic volumetric method. The present resolution of the procedure is 1-3 nanoliters, and intervals of measurement can be made as small as seconds. In the presence of glucose, oxidized glutathione (GSSG), and adenosine, the maximal rates were 6.2+/-1.0 microliter/hr cm2, and 8.2+/-0.8 microliter/hr cm2 if a large portion of the stroma was dissected away. In the presence of glucose and GSSG only, the rates were lower, namely 3.7+/-0.5 microliter/hr cm2. The rates consistently increased or decreased when adenosine was added or deleted, respectively, during given experiments. The stimulation of fluid transport by adenosine was in the order of 40-50%. The results raise the possibility that this transport mechanism might be subject to metabolic control.

Adenosine↗
PubMed1976

Standing-gradient osmotic flow. Examination of its validity using an analytical method.

The solutions to the non-linear differential equations governing solute-solvent coupling in the intercellular spaces of epithelial layers have been obtained by using an analytical method, rather than the usual numerical ones. When the present series solution includes second-order correction terms, the concentration and velocity profiles obtained by the analytical method agree very well with those coming from numerical solutions. This method has further allowed us to examine the standing-gradient hypothesis when applied to the backwards fluid transport system of the corneal endothelium. With the information presently available for the relevant parameters (osmotic permeability, rate of transport, radius and length of the spaces, and location of the pumping sites), near-isotonicity of the transported fluid would not be explained by the standing-gradient model.

Biological Transport↗
PubMed1975

The effect of vitreous humor on fluid transport by rabbit corneal endothelium.

The effects of vitreous humor and some of its components on the function of rabbit corneal endothelium were investigated and compared to those of artificial and natural aqueous humor. The rate of fluid pumping across the endothelium was measured while that layer was in contact with: (1) complete medium, an artificial aqueous solution; (2) pooled rabbit aqueous humor; (3) filtered and (4) unfiltered pooled rabbit vitreous humor; and (5) solid residue from vitreous humor. The relative magnitudes of the rates were: complete medium greater than aqueous larger than or equal to filtered vitreous greater than vitreous larger than or equal to solid residue. A definite adverse effect of unfiltered vitreous and of its solid residue on endothelial pumping was observed. It is suggested that the solid (collagenous) elements in vitreous humor may be responsible for these effects and perhaps for those generically referred to as the "vitreous touch syndrome" in clinical studies.

Animals↗
PubMed1974

Role of cations, anions and carbonic anhydrase in fluid transport across rabbit corneal endothelium.

1. A small electrical potential difference (541 +/- 48 muV, aqueous side negative) across rabbit corneal endothelium has been recently found. Its dependence on ambient [Na(+)], [K(+)], [H(+)] and metabolic and specific inhibitors was examined.2. Changes in concentration of the ions above either were known or were presently shown to affect the rate of fluid transport across this preparation (normal value: 5.2 +/- 0.4 mul./hr.cm(2)). Ionic concentration changes were also found here to influence potential difference in the same way as fluid transport. In the cases tested, the effects on both fluid transport and potential difference were reversible.3. Fluid transport and potential difference were both decreased or abolished in absence of Na(+), K(+) and HCO(3) (-), and when [H(+)] was decreased. Fluid transport and potential difference were saturable functions of [HCO(3) (-)] and half-saturation occurred in both cases at about 13 mM-HCO(3) (-). The potential difference was also a saturable function of [Na(+)] (half-saturation around 15 mM). There was a pH optimum for potential difference in the range 7.4-7.6. Lower pH values decreases the potential difference and the fluid transport, and a small (-100 muV) reversed potential was observed in the range of 5.3-5.5.4. Total replacement of Cl(-) by HCO(3) (-) or SO(4) (2-) produced no impairment on either fluid transport or potential difference.5. Carbonic anhydrase inhibitors (ethoxyzolamide 10(-5) or 10(-4)M and benzolamide 10(-3)M) produced a 40-60% decrease in the rate of fluid pumping. In contrast, ethoxyzolamide 10(-4)M or acetazolamide 10(-3)M did not produce any change in the potential difference. NaCN and Na iodoacetate (both 2 mM) eliminated the potential difference in 1-1.5 hr while in controls it lasted for 5-6 hr.6. Ouabain (10(-5)M) abolished the potential difference in less than 10 sec when added to the aqueous side, which suggests the existence of an electrogenic pump. This extremely fast time transient can be accounted for by the accessibility and simple geometry of the present monocellular layer. Ouabain abolished also the reversed potential difference observed at low pH.7. The data are interpreted in terms of a scheme similar to that advanced for other epithelia and in which (a) H(+) would be pumped into the intercellular spaces, while Na(+) and CO(2) would enter into the cells, and (b) Na(+) would be subsequently pumped into the aqueous humour, producing as a result the fluid movement observed. The actual origin of the potential difference is further discussed in terms of two contrasting possibilities: (i) one or more electrogenic pumps, and (ii) a neutral pump which would create a diffusion potential across ;leaky' intercellular junctions.

Animals↗
PubMed1972

Ionic permeability changes as the basis of the thermal dependence of the resting potential in barnacle muscle fibres.

1. The thermal dependence of the resting potential of isolated barnacle muscle fibres was larger (1-2 mV/ degrees C) than predicted by Nernst's equation (about 0.2 mV/ degrees C). A comparative study was made of the influence on thermal dependence of parameters related to (a) passive permeability and to (b) Na extrusion.2. High [K](o) decreased the thermal dependence reversibly. [K(i)], [Na](i) and [Cl](i) were determined by chemical analysis, and Goldman's equation was fitted to data relating V to [K](o) at different temperatures, in the presence and absence of ouabain 5 x 10(-5)M. In both cases the behaviour of V when T was lowered from 20 to 4 degrees C was accounted for by increases in the calculated P(Na/PK) and P(Cl/PK) (from 0.006 to 0.043 and from 0.17 to 0.34 on the average, respectively.)3. Other parameters related to passive permeability (and which caused reversible depolarization): decreased [Cl](o) (methanesulphonate or gluconate substituted), and decreased pH(o) (below 5.0), also decreased the thermal dependence reversibly.4. Inhibitors (ouabain 5 x 10(-5)M, cyanide 2-10 x 10(-3)M, 2,4-dinitrophenol 2 x 10(-4)M) externally applied did not affect either resting potential or its thermal dependence for several hours.5. Increasing [Na](i) three- to fourfold by intracellular injection decreased both resting potential and its thermal dependence.6. Although a small effect by a Na electrogenic pump cannot be excluded, the largest part of the thermal effect on the resting potential is concluded to depend on temperature-induced variations in relative ionic permeabilities to cations and anions. A model is proposed which can account for the data assuming that (a) each permeant ion associates to a separate site in the membrane, and (b) the ion-site equilibrium is temperature-dependent.

Animals↗
PubMedDate not supplied

On the possible permeation of water across the glucose transporter.

The possibility that the glucose transporter may serve as water channel is explored with the help of theoretical and experimental arguments. A model for a pore is drawn based on a hypothetical water channel structure, subject to the constraints that: molecules will bind to the channel wall in successive rings, forming a hollow sleeve; an integer number of molecules will exist in each ring; the pore radius will not be large enough to allow water molecules along its center, but will be large enough to allow glucose molecules across. The only configurations that meet these conditions exhibit either 5 or 6 water molecules abreast in each ring, with pore radii of 4.1 and 4.5 A, respectively. The kinetic characteristics of such pores are estimated and found to conform to available evidence.

Animals↗