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Biomedical subjects

J Fischbarg

Publications and source records attributed to J Fischbarg.

At least 91 records · Page 5Linked to original sources
PubMed1985

A central role for cell osmolarity in isotonic fluid transport across epithelia.

Previous theoretical models for solute-solvent coupling in epithelia that dealt only with the intercellular channel did not predict isotonic transport except when very high cell membrane permeabilities were assumed. To study this issue, we have developed the formalisms for osmotic equilibration at an alternative location, the apical cell membrane (including its adjacent unstirred layer), which are somewhat simpler than those for the channel. Much as in other models, we confirm that only rather unrealistically high values of the cell membrane permeability lead to isotonic transport. We have also found, however, that isotonic transport can occur at much lower values of the cell membrane permeability if the concentration within the cell differs slightly from that in the ambient medium. This emphasizes the importance of incorporating the intracellular concentration as an integral part to any transport model, such as in the present apical membrane version of local osmosis.

Animals↗
PubMed1984

Electrical potential, resistance, and fluid secretion across isolated ciliary body.

Rabbit ciliary epithelium was mounted with the sclera and lens still attached. The potential measured in physiological bathing medium was 1.29 +/- 0.18 mV initially, but dropped to 0.66 +/- 0.34 mV, then became stable for 2 hr or more. The resistance of the epithelium was measured by comparing total trans-tissue resistance before and after treatment with Triton X-100, ethanol, or distilled water. The calculated resistance with correction for epithelial surface area was found to be 1504 +/- 452 omega, in six experiments. Fluid secretion was measureable against a hydrostatic gradient when identical solutions bathed both sides of the preparation. Ouabain caused opposite responses when applied to either side of the preparation, indicating that it does not readily cross the epithelial barrier. The evidence presented suggests that the ciliary epithelium is a 'tight' rather than a 'leaky' epithelium, and that fluid secretion in the normal ciliary epithelium occurs by active transport rather than by ultrafiltration.

Animals↗
PubMed1983

Ionic selectivity of the paracellular shunt path across rabbit corneal endothelium.

We have measured the dilution and biionic potentials across the isolated rabbit corneal endothelium in order to learn about the ionic selectivity of its intercellular junctions. Single-salt dilution potentials have been measured as a function of [NaCl] or [NaHCO3] gradients across the tissue. Biionic potentials were similarly measured by replacing Na+ with K+ on either side of the tissue. The potentials thus measured were fit to the constant field equation and to an approximation of it to obtain the ionic permeabilities for K+, HCO-3 and Cl- relative to Na+. The permeability sequence obtained was PK greater than PNa greater than PHCO3 approximately equal to PCl. Potentials were also measured after imposing an osmotic gradient across the preparation using sucrose. The results obtained with all these methods are consistent and suggest that this tissue is slightly more permeant to cations than anions, but that the selectivity of the intercellular junction is relatively low. From these experiments, a 30 mM gradient of salt across the endothelial layer would be needed in order to explain the observed spontaneous potential difference (about 1 mV, aqueous negative) across that layer if the potential was due to the selectivity of the intercellular junctions. Such a value for the gradient is much larger than theoretical estimates of it; therefore, we favor electrogenic transport of HCO-3 as a better explanation for the origin of the spontaneous potential difference.

Animals↗
PubMed1983

Pyridine nucleotides of rabbit cornea with histotoxic anoxia: chemical analysis, non-invasive fluorometry and physiological correlates.

The pyridine nucleotides from both the epithelium and the endothelium of rabbit cornea were measured by the cycling assay. Sodium azide (10 mM) applied for 1 hr to induce histotoxic anoxia decreased the endothelial NAD+/NADH ratio from 4.62 to 1.49 and decreased the epithelial NAD+/NADH ratio from 2.56 to 1.08. The larger NAD+/NADH ratio for the endothelium as compared to the epithelium corresponds to a more oxidized state. The corresponding ratios for NADP+/NADPH were 1.2 for the endothelium and 0.70 for the epithelium. Sodium azide had no effect on the NADP+/NADPH ratio for the endothelium, but decreased the epithelial ratio to 0.62. Pyridine nucleotide fluorescence was measured with a difference corneal fluorometer on the perfused whole cornea preparation and the perfused everted corneal preparation. Sodium azide (10 mM) for 30 min resulted in a 19.4 +/- 0.7% increase in the pyridine nucleotide fluorescence from the whole corneal preparation and a 4.5 +/- 0.6% increase from the everted endothelial preparation. Corneal anoxia induced by stopping the perfusion on the endothelial side resulted in a 18.7 +/- 0.6% increase in pyridine nucleotide fluorescence for the whole corneal preparation. Sodium azide (10 mM) resulted in a 35% decrease in the transendothelial potential difference and a 76% decrease in the rate of transendothelial fluid transport. A comparison is made between invasive chemical analysis and real time, non-invasive fluorometry to measure histotoxic corneal anoxia.

Animals↗
PubMed1982

Osmotic permeabilities across corneal endothelium and antidiuretic hormone-stimulated toad urinary bladder structures.

Osmotic permeabilities of several epithelial structures have been determined with novel optical procedures based on specular microscopy. The osmotic permeabilities of several tissue layers were determined by continuously monitoring the position of the apical tissue borders while an osmotic flow was imposed across those layers. The values found were (in micrometer/s; mean +/- SE): corneal epithelium, 137 +/- 30 (n = 5): antidiuretic hormone stimulated toad bladder, 429 +/- 64 (n = 6); and corneal endothelium, 711 +/- 34 (n = 7). In addition, the osmotically-induced transient change in thickness of the corneal endothelial cells was determined with the help of a computer, and the apparent osmotic permeability measured for the apical membrane was 1420 +/- 160 micrometer/s (n = 5). It is concluded that the osmotic permeability across the endothelial layer is sizably larger than had been previously detected and that osmotic flows across such layer largely traverse the cellular membranes. With osmotic permeability values (per unit of cell membrane area) as large as presently reported, isotonic fluid transport by epithelia can be explained simply on the basis of local osmotic gradients.

Animals↗
PubMed1981

Osmotic water permeability of rabbit corneal endothelium and its dependence on ambient concentration.

We measured the fluid flow osmotically induced by sucrose concentration differences across the isolated rabbit corneal endothelium to determine if its osmotic water permeability, Pos, depends on the concentrations on both sides as well as on the concentration difference across that tissue. We found that when the osmotically induced fluid flow went from stroma to aqueous, Pos decreased from 35 +/- 4 mum/s to 20 +/- 3 mum/s when an additional 20 mosM sucrose was added to the solutions on both sides of the endothelium. However, when the osmotically induced fluid flow was towards the stroma, Pos remained practically unchanged. (28 +/- 4 mum/s vs. 31 +/- 5 mum/s), when additional 20 mosM sucrose was present on both sides. These changes in the measured permeability are consistent with the possibility that sucrose would be swept into the intercellular channels by the osmotically induced fluid flow. We also confirmed that an osmotic gradient can 'prime' the fluid pump. After a gradient was removed, gradients which had previously induced flow from stroma to aqueous caused an increase in the basal fluid pump rate, while gradients in the opposite direction decreased that fluid pump rate.

Animals↗
PubMed1981

Electrical properties of rabbit corneal endothelium as determined from impedance measurements.

Alternating- and direct-current electrical characteristics of rabbit corneal endothelium were studied under varying experimental conditions. The measurements were performed by sending a 10-microA current (AC or DC) across the tissue layer. Maximal values of transendothelial potential difference and resistance were 1.3 +/- 0.1 mV and 73 +/- 6 omega . cm2, respectively. The short-circuit current was estimated from the potential and resistance values. Impedance loci were obtained for the frequency range 0.5-100 kHz. A capacitive reactance (C = 0.63 +/- 0.02 microF/cm2) was observed in the 100 Hz-100 kHz range. To relate the impedance data to the electrical parameters of the cell membranes, the voltage-divider ratio was determined by sending square pulse across the tissue and measuring voltage responses across the apical and basal membranes with an intracellular microelectrode. The intracellular potential difference was on the average -61 +/- 1 mV, and the voltage-divider ratio was found to be between 0.33 and 4. Impedance data were fit by a computer to an equivalent circuit representing a "lumped" model, and the agreement between the model and the data was satisfactory. The results are discussed in terms of both the morphological characteristics and properties of the fluid transport mechanism across the preparation.

Animals↗
PubMed1980

Priming of the fluid pump by osmotic gradients across rabbit corneal endothelium.

The present study shows that the inclusion of 5% Dextran (average mol. wt. 40 000) in solutions to preserve in vitro rabbit corneal endothelium induces a sizable osmotic flow across the preparation which is superimposed on the existing fluid transport. Furthermore, even after fluid transport ceases due to in vitro deterioration, the Dextran-induced flow remains for some addition time. The osmotic permeability was 162 +/- 17 micrometer/s in the presence of glucose and 451 +/- 84 micrometer/s in its absence. The latter, comparatively high value suggests that such osmotic flow traverses the intracellular junctions. In addition, temporary (10--15 min) imposition of an osmotic gradient has a separate stimulatory 'priming' effect on the rate of fluid transport. Thus, the rate of fluid pumping increased by about 40% after challenge with Dextran. It was further noted that, after addition of Dextran, preparations in the absence of glucose escape gross deterioration for a time longer than those in the presence of glucose. On the other hand, mere addition of Dextran to a glucose-containing solution does not appear to prolong the estimated 'survival time' of the pumping mechanism. The sizable osmotic flows and the priming effect described here may provide a physiological context with which previously described Dextran effects on cornea preservation can now be compared.

Animals↗
PubMed1980

Unstirred layer effects in osmotic water flow across gallbladder epithelium.

The standard one-dimensional model of the unstirred layer is applied in a re-examination of the experimental results of Wright, Smulders and Tormey (Wright, E.M., Smulders, A.P., Tormey, J. McD., 1972, J. Membrane Biol. 7:198) who reported large transients in the osmotic flux of water from the serosal to the mucosal side of rabbit gallbladder epithelium. They initiated osmosis by the addition of sucrose to the mucosal bathing solution (initially, approximately 300 nOsm NaCl) and observed that the initial flux was more than ten times its eventual steady-state value; they interpreted this as a consequence of the piling-up of NaCl in the unstirred tissue layer on the serosal side of the epithelium. The present analysis (both steady-state and unsteady) shows that if measured values of layer thickness delta are used, together with reasonable values of the reduced diffusivity of NaCl in the tissue and of the fraction of tissue available for water flow, then one would predict a discrepancy of only about 10%, not tenfold, between the initial and final values of the flux. Thus the standard model is inconsistent with the observations. Furthermore, Wright et al's results cannot be used to infer that the osmotic permeability of epithelial cell membranes is much larger than steady-state measurements on whole epithelia would indicate. Mucosal-to-serosal flow is also analyzed, and in this case a considerably greater osmotic permeability is predicted; this result is consistent with the observed changes in structure of the lateral intercellular spaces when the direction of flow is reversed.

Animals↗
PubMed1980

Noninvasive measurements of pyridine nucleotide fluorescence from the cornea.

The autofluorescence of reduced pyridine nucleotides (NADH and NADPH) and oxidized flavoproteins within the rabbit cornea were noninvasively measured as a function of depth. This was accomplished by combining a corneal specular microscope with a time-shared spectrofluorometer. When either 8 mM sodium pentobarbital or sodium sulfide, known inhibitors of mitochondrial respiration were applied to cornea, the autofluorescence at 440 nm (excited at 366 nm) increased and that at 540 nm (excited at 460 nm) decreased. No autofluorescence was measurable following destruction of the cellular membranes by freezing and leaching of the cellular constituents. The 440 nm autofluorescence is from reduced pyridine nucleotides, whereas the 540 nm autofluorescence is from the oxidized flavoproteins. The time course of the pyridine nucleotide autofluorescence after the application of the pentobarbital to either the endothelial or epithelial bathing solutions made it possible to measure the diffusion properties of this drug through the cornea. The method used is useful studying the diffusion and effects of metabolically active drugs upon the cornea.

Animals↗
PubMed1978

Biphasic effects of insulin and ouabain on fluid transport across rabbit corneal endothelium.

1. Low levels of insuling stimulate transendothelial fluid transport from preswollen stroma to aqueous in rabbit corneal preparations. The rate of stromal thinning at the end of the first hour averages 30% faster with insulin, 3.5 x 10(-22) M (4.8 micromicron/ml.), than that of the paired control. This concentration is about the physiological level in rabbit aqueous. 2. The stimulation with insulin is transient. Rates of thinning average higher but not significantly different from control rates by the second hour. 3. High levels of insulin between 3.5 x 10(-9) M (480 micromicron/ml.) and 2.0 x 10(-6) M (2.75 X 10(5) micromicron/ml.) inhibit fluid transport. The inhibition at the low end of this range of concentrations becomes more pronounced with longer perfusion times but appears not to exceed ca. 50% of the control rate. 4. Ouabain also induces a biphasic effect on fluid transport which is characteristically different from that with insulin. The maximal stimulation observed at all times occurred with a fixed concentration of 10(-10) M. The stimulation is not transient but increases throughout the duration of the perfusion; the average rate is elevated 50% above the control rate by the third hour. 5. The transition from a stimulatory to an inhibitory effect occurs consistently at ca. 10(-8) M with ouabain, while a similar transition with insulin occurs at ca. 10(-9) M and appears to shift towards slightly higher concentrations during a 3 hr perfusion period. 6. Inhibition of fluid transport with ouabain, 3 x 10(-7) M, is increased from ca. 50% after 1 hr to more than 70% at the end of the third hour of perfusion. 7. The combined presence of stimulatory concentrations of ouabain and insulin affects tromal thinning in a manner resembling the effect of ouabain alone more than that of insulin; additive effects could not be discriminated. Progressively raising the concentration of insulin to a level (10(-8) M) that alone inhibits stromal thinning, ultimately abolishes the stimulatory effect of ouabain. Based on other evidence and current models of drug/hormone-membrane interaction, these results can be interpreted to indicate a concentration-dependent interaction between receptor complexes of ouabain and insulin with (Na+ + K+)-ATPase.

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