Shunt Paths Research Articles

Sodium uptake across the apical border of the isolated turtle colon: confirmation of the two-barrier model.

The initial rate of Na uptake by the turtle colon from the mucosal bathing solution consists of two operationally distinct components. One component is a linear function of mucosal Na concentration, is unaffected by amiloride, and appears to represent Na uptake into the paracellular shunt path. The major component of Na uptake is abolished by amiloride and is virtually equal to the short-circuit current over a wide range of mucosal Na concentrations, suggesting that this portion of Na uptake represents Na movement into Na-transporting cells of the colon. The amiloride-sensitive component of Na uptake, at low mucosal Na concentrations, was unaffected if net Na transport was abolished by ouabain. Similarly, at low mucosal Na concentrations the amiloride-sensitive conductance of the colon was identical in the presence and in the absence of net Na transport. These results show that the isolated turtle colon behaves as two distinct barriers to transmural Na transport, an apical barrier blocked by amiloride and a more basal-lying barrier where active, transmural Na transport is blocked by ouabain. In addition, these experiments appear to provide the first unambiguous demonstration that the initial-rate isotope uptake technique can provide a direct measure of the properties of the amiloride-sensitive barrier to transmural Na movement, presumably the apical membranes of the Na-transporting cells. The results are consistent with the notion that the rate of transmural active Na transport and the conductance of the active Na-transport path are determined by the properties of the apical membrane.

The paracellular pathway in toad urinary bladder: permselectivity and kinetics of opening.

Determination of serosa-to-mucosa fluxes of Na, K, and Cl yields information about the properties of the shunt pathway in toad urinary bladder. We show that measurement of these fluxes at 30-sec intervals following an abrupt increase in mucosal osmolality yields evidence on the rate of opening of the path and of its permselectivity. The relationship between the fluxes of any pair of these ions indicates that the shunt is paracellular both before and after the increase in conductance effected by hyperosmolality and that the transepithelial PD affects the permselectivity properties (at 0 mV, PK/PNa/PCl=1:0.71:0.57; at + 25 mV, Pk/PNaPCl=1:0.71:0.99). The relationship between any of the fluxes and the total transepithelial conductance is linear and yields an estimate of cellular conductance (the intercept of this regression on the conductance axis) which is in accord with that measured electrically. These studies provide information on tight junction permeability to nonelectrolytes, as well. Finally, they provide new information about the role of the shunt path as a controlling influence on transepithelial sodium transport and raise the possibility that, in both leaky and tight epithelia, differences in transepithelial conductance from tissue to tissue, organ to organ, and species to species may be due, in the absence of edge damage, to changes in conductance of the paracellular pathway.

Luminescence decay kinetics in GaP:Bi and GaP:N

The lifetime of excitons bound to isoelectronic traps in GaP is studied as a function of temperature, excitation energy and intensity. The photoluminescence was excited both selectively and above the gap by using a pulsed, tunable dye laser with photon flux varying in the range of 1014–1019 photons/cm2 per pulse. GaP:Bi shows a strong intensity dependence of τ(T) in the temperature range of 40–60 K due to the thermal activation of the electron. Similarly, GaP:N shows this behavior in the range of 20–100 K. In this case, two activation processes can be identified: release of the bound exciton into the free exciton band and dissociation of the exciton. The observed dependence of τ(T) on both excitation energy and intensity indicate that saturable deep traps (shunt paths) deplete the excess free carriers. These traps can be completely saturated in GaP:N while only partial saturation is achieved in GaP:Bi. The kinetic equations are written for GaP:Bi and solved numerically assuming quasiequilibrium conditions. Fitting this model to the experimental results yields the capture cross sections for carriers by Bi and by the deep traps as well as the concentration of the latter.

Chloride dependence of active sodium transport in frog skin: the role of intercellular spaces.

1. In agreement with previous observations the replacement of Cl by a nonpenetrating anion in the solution bathing either the outside or both sides of the frog skin causes a fall in the short-circuit current. 2. When Cl is replaced by a non-penetrating anion in the solution bathing the outside of the frog skin the Isc is still a correct measure of the net Na transport. 3. Under the same conditions both active and shunt paths seem to be affected since there is a decrease in Isc, Na influx, amiloride-dependent conductance, and initial Na uptake across the external barrier, together with a decrease in Cl-backfluxes and amiloride-independent conductance. There is also a decrease in water permeability and a reduction in size of the intercellular spaces. 4. The removal of Cl does not appear to affect the entry step of Na but may have an effect on the shunt path. This in turn may change the active Na transport.

Analysis of Electrolyte Shunt Currents in Fuel Cell Power Plants

A general mathematical model is presented for calculating the shunt currents and effective operating potentials of the materials in multicell stacks which have electrolyte interconnections between the individual fuel cells. The analysis takes into account the electrochemical polarizations that occur at the positive and negative ends of each shunt path. Numerical results are presented for an acid electrolyte fuel cell stack design.

Thickness dependence of structural and electrical properties of Cds films for solar cells

Polycrystalline films of CdS were evaporated on zinc-plated foils of copper so that different thicknesses were obtained in the same evaporation. The different thicknesses were compared with respect to their surface morphologies, acid etching, grain sizes, grain structure, and orientation as well as crystallographic orientation. Concomitant measurements of the electrical properties as functions of thickness were taken. These included average resistance through the thickness, current–voltage characteristics of diodes prepared by evaporating gold dots on the surfaces, and measurements of capacitance as functions of reverse bias on the same diodes. It was established that grain size, the preferential grain orientation and conelike growth towards the source, the preferential c-axis orientation towards the source and the surface roughness, all increased as the film thickness increased. Furthermore, it was found that the concentration of free carriers increased with film thickness, the quality of the diode behavior improved considerably in the same manner, and the existence of voltage-dependent shunting paths diminished as the film thickness increased. An explanation is proposed which relates the observed electrical behavior as a function of thickness to the structural parameters.

Correlation between Dislocation Pits in GaP LPE Layers and LEC Substrates

In order to improve the efficiency of a GaP green-emitting diode, one essential point is to decrease the D-pit density in the epitaxial layer. Two types of D-pits on the LPE (111) phosphorus surfaces characterized by their respective sizes were studied to determine their correlation with LEC crystal characteristics. Both types of D-pits were found to have origins in the LEC substrates. Small-size D-pits in the LPE layer originate from the substrate dislocations which from a `star' pattern on the LEC wafer. On the other hand, large-size D-pits have an origin whose distribution is very different from that of small D-pits. The large-size D-pit density correlates with the roughness of the RC etched LEC surfaces, and its density levels differ on a large scale among the LEC crystals used. As both types of D-pits act as a shunt path for minority carriers, both origins should be eliminated in the LEC crystals.

Radiation Testing of PIN Photodiodes

PIN photodiodes representative of commercially available device types were radiation tested in total dose, neutron fluence, and ionization pulse environments to examine the vulnerability mechanisms that occur. Also included in the testing was a special epitaxial device that was designed to have a small ionization-sensitive volume. It was found that surface shunt paths were produced in all the devices, with the effects in the p-on-n devices with guard rings being particularly bad. These shunt paths produced a decreased optical response and an increased dark current. In devices biased to less than full depletion, lifetime degradation was observed. This caused significant optical response degradation, especially at longer wavelengths, and gave a significant contribution to the increased dark current. The epitaxial device had the predicted small ionizationsensitive volume, and no unusual vulnerability mechanism occurred in the device. In fact, it was generally at least as radiation-toleranct as any of the other devices, and thus is a good choice for use in applications requiring radiation hardness.

Response waveforms of vertebrate photoreceptors: what are the underlying mechanisms?

A class of models is investigated using computer simulation in which the inner and outer segments of the vertebrate photoreceptor are coupled through a pump. The outer segment membrane conductance is controlled by an internal transmitter, activated by photolysis of the photosensitive molecules in the cell. Several possibilities for the coupling dynamics are investigated. The analysis favors the conclusion that the hyperpolarizing transient at high intensity stimuli arises from the coupling dynamics, (unless there is an extracellular current shunt path). It predicts, moreover, that the transient should be observed intracellularly, but not extracellularly to the outer segment. This is, in fact, the case. It also predicts that the transient should become more marked, as the steady state ratio of inner to outer segment currents decreases. The computer simulations are concerned with the intracellularly recorded responses; the long term adaptation parallel to pigment bleaching and regeneration is not considered explicitly here. In conclusion, it is shown that the state conditions as well as the response waveforms can be related to physiologically significant variables.

Na and Cl transport across the isolated turtle colon: parallel pathways for transmural ion movement.

Transmural fluxes of3H-mannitol and22Na or36Cl were measured simultaneously in portions of isolated turtle colon stripped of serosal musculature. The relationships between mannitol flux and the flux of Na or Cl are characteristic of simple diffusion and suggest that transmural mannitol flow is largely confined to a paracellular pathway where Na, Cl and mannitol move much as in free solution. The contribution of “edge damage” to the transmural mannitol flow appears to be minimal. Mucosal hyperosmolarity causes “blisters” in epithelial tight junctions and increases the diffusional permeability to Na and mannitol, suggesting that the rate-limiting barrier in the shunt path is the tight junction. If the total mucosa to serosa flux of Na is corrected for the portion traversing the shunt pathway it is apparent that changes in the short-circuit current are completely accounted for by the mucosa to serosal movement of Na through a cellular path. In addition, the serosa to mucosa flux of Na appears to be restricted to the shunt. These observations suggest that there is no appreciable “backflux” of Na through the active, cellular path. In the presence of 10−4m amiloride the short-circuit current is markedly reduced and the mucosa to serosa Na flux is restricted to the shunt, so that the net Na flux is abolished. The small amiloride-insensitive short-circuit current is consistent with HCO3 secretion. Mucosa to serosa and serosa to mucosa fluxes of Cl appear to be largely restricted to the paracellular shunt path and there is no evidence for any net flow of Cl under short-circuit conditions. The total tissue conductance can be described as the sum of three components: a shunt conductance which is linearly related to the transmural mannitol flow, an “active” conductance which is linearly related to the short-circuit current and a small residual conductance. The shunt conductance is attributable to the diffusive movements of Na and Cl through the paracellular path. Variations in the active Na transport from tissue to tissue are largely attributable to variations in the apparent conductance of the active Na transport path. The driving force for active Na transport can be described as an apparent emf of approximately 130 mV. These results suggest that transmural mannitol flux provides a quantitative estimate of the ion permeability and electrical conductance of a paracellular shunt path across the isolated turtle colon and thereby facilitates the study of the transport characteristics and electrical properties of cellular paths for transepithelial solute movement.

Effect of plant cytokinins on microfilaments and tight junction permeability.

IN epithelia classified as “leaky”, net transport efficiency of NaCl and water may be determined not only by cellular metabolic activity, but also by tight junction permeability and the conjugate physical driving forces acting between lumen and intercellular space. The significance of paracellular ion movement across leaky epithelia would become more apparent if a cytoplasmic mechanism for regulation of tight junctional permeability could be defined. The structure–function observations reported here suggest that the microfilament system may have such a function. Rapid and reversible increases in shunt path electrical resistance were observed in Necturus gallbladder exposed to plant cytokinins. In electron micrographs zeiotic transformation of the luminal membrane was associated with disruption of microfilaments and in freeze fracture, with reorientation and disorder of tight junctional elements.

Electroluminescence in semiconductors

This brief review is restricted to a selection of very recent work on bulk properties of commercially important III–V compound semiconductors, mainly GaP and GaAs1-xPx. Some recent theoretical and experimental work on electronhole binding suggests the importance of correlation for isolated N in GaP. The band structure enhancement of the optical efficiency of N in GaAs1-xPx has received clarification. Recent advances in our understanding of the electronic binding energies of the dominant shunt path recombination centres and their relationship with dislocations are reviewed, mainly for LPE GaP.

Effects of dislocations on photoluminescent properties in liquid phase epitaxial GaP

Correlations between photoluminescent decay time and dislocation density were studied in undoped liquid phase epitaxial GaP. Dislocations strongly reduce the photoluminescent decay time, especially in samples in which nonradiative recombination times (τ0) due to shunt paths other than dislocations are long. Dislocations were found to correspond to photoluminescence dark spots whose diameter is correlated with τ0. A quantitative model is presented, which is in good qualitative agreement with the observed photoluminescent−decay−time−versus−dislocation−density relationships.

Solid composition and gallium and phosphorus vacancy concentration isobars for GaP

Thermodynamic calculations are presented which provide explicit expressions for the combined temperature and phosphorus partial pressure dependences of the Ga- and P-vacancy concentrations in GaP. This work extends the applicability of earlier results for the vacancy concentrations, obtained by an analysis of the solidus data, from solid-liquid (i.e., LPE and LEC growth) to solid-vapor equilibria (vapor growth, annealing, and diffusion). The calculations are represented in the form of Ga- and P-vacancy concentration and solid-composition isobars. In addition, the previously established correlation between the reciprocal of the nonradiative shunt-path lifetime (1/τn) and the relative Ga-vacancy concentration for GaP crystals prepared by a variety of techniques is transformed into a correlation with absolute vacancy concentrations. The electron-capture cross section of a Ga-vacancy behaving as a shunt path in p -type material is estimated to be ∼4.5×10−17 cm2. Finally, the Ga-vacancy concentration isobars and the 1/τn correlation are discussed in the context of LPE, vapor growth, and annealing processes for GaP.

Sodium conductance changes by aldosterone in the rat kidney.

To asses passive permeability properties of distal, and proximal tubules of the rat kidney the tubular lumen was perfused with solutions of 1.5 and 150 mM Na/l while transtubular potential differences were recorded. Sodium transport numbers (T Na) were calculated.T Na in the distal tubule of adrenalectomized rats was acutely increased from 0.21 to 0.27 by aldosterone (5 μg/100 g B.W.). This effect of aldosterone could not be reduced by concomitant injection of cycloheximide (100 μg/100 g B.W.). Aldosterone was also effective in control rats. In the proximal tubule similar data were obtained. However, the aldosterone-induced increase of conductance was slightly reduced with cycloheximide. These measurements of transepithelial sodium conductance indicate that aldosterone, in addition to the already known stimulation of active sodium transport, increases overall permeability of the tubular wall to sodium. In the distal tubule this effect indicates an increase of the luminal membrane permeability whereas in the proximal tubule aldosterone may facilitate the diffusion of sodium through the intercellular shunt path and/or the luminal membrane. The passive components of transepithelial electrolyte transfer seem to be less sensitive to inhibition of protein synthesis than the active transport components.

Current-induced voltage transients in Necturus proximal tubule.

Transients in the potential difference spontaneously developed by theNecturus proximal tubule were characterized during and after voltage or current clamp commands. These voltage transients were adequately fitted by an exponential function similar to that describing the ionic charging of a leaky fluid capacitance and were slower during clamp periods (t 1/2=0.98 min) than after release of the clamp (t 1/2-0.46 min). Changes in luminal ionic composition and cellular membrane potential were ruled out as sources of generation of the voltage transients. The volume of the fluid compartment in which concentration changes occurred was calculated from the electrical data and it was concluded that the extracellular shunt path was the principal site of the concentration changes which resulted in voltage transients. A fall in transepithelial resistance to nearly one-half its original value occurred during hyperpolarizing commands while depolarizing commands did not significantly alter resistance. The resistance changes were interpreted as indicative of the degree of widening of the lateral intercellular spaces caused by fluid accumulation or depletion. The important role of the lateral space dimensions in determining epithelial permeability, electrical resistance and voltage transients was pointed out, and a new electrical analogue model of the shunt path was proposed.

A parallel path model forNecturus proximal tubule

A parallel path model based on the principles of nonequilibrium thermodynamics was developed for theNecturus proximal tubule. The cellular path was represented as a luminal membrane followed by an irreversible active NaCl transport system in the peritubular barrier. The shunt pathway was described as three “coarse” barriers in series: tight junction, lateral intercellular spaces, and basement membrane with connective tissue. Volume and solute flows were predicted by the model equations as a function of applied electric current. Variations of the model parameters revealed the quantitative importance of the shunt path properties and the relative insensitivity of epithelial transport to changes in most cell parameters. Circulation of electric current and solute within the epithelium were shown to significantly influence the bahavior of the tubule in the presence of an electric field. Values for all transport parameters of the shunt path and epithelium were calculated and compared with available experimental evidence. Volume flow and electric currents predicted by the model compared favorably with experimental observations.

Simplified Analysis of Electron-Hole Recombination in Zn- and O-Doped GaP

An analysis of the recombination kinetics of three differently annealed GaP samples of a crystal doped with Zn and O is presented. The kinetics are described in terms of four parameters: a hole recombination lifetime ${T}_{1}$; an emission rate ${e}_{1}$ for electrons to be thermally excited from the Zn-O center back into the conduction band; a shunt-path lifetime ${T}_{\mathrm{SH}}$ and a rate ${c}_{1}$ for capture of electrons by the Zn-O centers. Rate ${e}_{1}$ had been previously determined by Jayson, Bachrach, Dapkus, and Schumaker. Two other parameters ${T}_{1}$ and (${T}_{\mathrm{SH}}^{\ensuremath{-}1}+{c}_{1}$) were directly measured. ${T}_{\mathrm{SH}}$ and ${c}_{1}$ were separately determined by evaluating the ratio $\frac{{T}_{\mathrm{SH}}^{\ensuremath{-}1}}{({T}_{\mathrm{SH}}^{\ensuremath{-}1}+{c}_{1})}$, which is the fraction of the carriers leaving the conduction band through the shunt path. This ratio was deduced in two independent ways from analysis of the time dependence of the red and green luminescence. In the most efficient sample, the parameters at 298 K were ${e}_{1}^{\ensuremath{-}1}=220\ifmmode\pm\else\textpm\fi{}10$ nsec, ${T}_{1}=630\ifmmode\pm\else\textpm\fi{}30$ nsec, ${T}_{\mathrm{SH}}=60\ifmmode\pm\else\textpm\fi{}10$ nsec, and ${c}_{1}^{\ensuremath{-}1}=10\ifmmode\pm\else\textpm\fi{}1$ nsec. About 0.60 \ifmmode\pm\else\textpm\fi{} 0.05 of the recombination in this sample was through the Zn-O centers. The measured radiative efficiency was 0.29 \ifmmode\pm\else\textpm\fi{} 0.03, indicating that about half the recombination was radiative. The radiative and nonradiative hole recombination times were ${({T}_{1})}_{\mathrm{rad}}=1310\ifmmode\pm\else\textpm\fi{}130$ nsec and ${({T}_{1})}_{\mathrm{nonrad}}=1210\ifmmode\pm\else\textpm\fi{}90$ nsec for this sample with $p\ensuremath{\approx}\frac{2\ifmmode\times\else\texttimes\fi{}{10}^{17}}{{\mathrm{cm}}^{3}}$ (${N}_{A}\ensuremath{-}{N}_{D}=\frac{2.5\ifmmode\times\else\texttimes\fi{}{10}^{17}}{{\mathrm{cm}}^{3}}$). These lifetimes are in good agreement with those deduced previously by Jayson, Bhargava, and Dixon. The effect of annealing is to change ${T}_{\mathrm{SH}}$ from about 5 to 60 nsec and to increase the Zn-O center concentration by 5 or 6 times. These changes increased the recombination through the Zn-O center by a factor of 25 \ifmmode\pm\else\textpm\fi{} 5. Annealing also appeared to decrease the majority carrier concentration by about 10%.

Influence of core gap in design of current-limiting transformers

Core gaps are used in the magnetic shunt paths of current-limiting transformers to limit secondary current under short-circuit conditions. It is shown here that for short-circuit current limited to twice normal a current limiting transformer has only 55% of the rating of a noncurrent-limiting transformer of the same physical size. Part of the reduction in rating stems from the space required for magnetic shunts, part from the greater space needed for winding insulation, and part from the increased loss due to the gaps. Reliability of a previously tested gap-loss equation is established, and design examples are given for both stamped laminations and strip-wound cores.

The route of passive ion movement through the epithelium of Necturus gallbladder.

Electrophysiological experiments were performed onNecturus gallbladder to determine whether the main route of passive ion flow was via the cells or via a paracellular shunt path. In the first approach the following values were determined: the transepithelial resistance, the ratio of the voltage deflections across the luminal and basal cell membrane during transepithelial current flow, and the voltage spread within the epithelial cell layer during intracellular application of current pulses. From these data the luminal and basal cell membrane resistances were calculated to be 4,500 and 2,900 Ωcm2, respectively, whereas the transepithelial resistance was only 310 Ωcm2, indicating that approximately 96% of the transepithelial current bypassed the cells. This result was confirmed in a second approach, in which the intracellular voltage deflections were found to remain approximately constant, when the current pulses were passed from a cell into the interstitial compartment with the luminal compartment being empty or when they were passed from the cell into both external compartments simultaneously. In the third approach current was passed through the epithelium and a voltage-scanning microelectrode was moved across the surface of the epithelium to explore the induced electrical field. Significant distortions of the field were observed in the immediate vicinity of the cell borders. This result indicated that the paracellular shunt, which carries the main part of the transepithelial current, leads through the terminal bars and that the terminal bars or “tight” junctions arenot tight for transepithelial movement of small ions in gallbladder epithelium.