摘要:

The presence of cellular permeation pathways in human fetal membranes at term was evaluated. Electrical parameters (transepithelial potential [TEP], and conductance [Gt], and intracellular potentials [cell PD]), and water and urea diffusional coefficients (Pdw, Pdu), were determined in Ussing‐like chambers.In amniochorion, the TEP was practically 0 (0.1±0.03 mV), and the Gtvery high (144±14 mS/cm2). The Cell PD of amnion cells was −37±3 mV. Increasing the [K+] of the amniotic perfusate between 5.8 and 125.8 mM depolarized the cells with a slope of 23 mV. The deletion of Na+hyperpolarized the cells, whereas amiloride and ouabain depolarized them.The Pdw and Pdu were determined in intact amnion and chorion and in their epithelial cell layers. The Pdw/Pdu ratio in amnion was 4.0, and 7.0 in its cell layer; the ratio in chorion was 2.5, and 3.3 in its cell layer.The amniochorion is a leaky structure, but its cellular layers possess definite transcellular permeation pathways. The ionic conductances in amnion cells are complex, with the Cell PD being determined by at least K+and Na+conductances, and ouabain‐ and amiloride‐sensitive pathways. The amnion is a more effective diffusional barrier to water and urea than chorion is; its diffusional characteristics are comparable to those of nystatin‐treated lecithin: cholesterol bilayers and the membranes of human

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00828.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Many homopteran insects feed on plant sap which contains solutes in very low concentration. Their digestive tract presents a complex called the “filter chamber” where the excess dietary water is believed to flow directly from the initial part of the midgut to the terminal part of the midgut and the proximal reions of the Malpighian tubules.Freeze‐fracture experiments carried out on the filter chamber ofCicadella viridisrevealed the presence of intramembrane particles on the whole surface of the microvilli and of basal membrane infoldings of the cells.Examination of negatively stained isolated membranes and of freeze‐dried shadowed membranes revealed that the inner surface of the membrane is covered with particles protruding into the cytoplasm; they correspond to the numerous intramembrane particles observed on the P fracture face of the membrane. The outer surface of the membrane exhibits a regular network which corresponds to that observed on the E fracture face.SDS‐PAGE analyses were performed on purified membranes of the filter chambers ofC. viridisandPhilaenus spumarius. In both cases 2 major components, 25 kDa and 75 kDa, were detected. These 2 components appear to be specific for the filter chambers since they were not found in membranes isolated from the other parts of the midgut.Thus, the membranes of these filter chambers, though to be water‐shunting complexes, possess structural and biochemical peculiarities which are probably related to water p

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00829.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Freeze‐fracture studies of the lamprey gill epithelium reveal structural differences of the luminal and basolateral plasma membrane of the pavement cells. The luminal membrane is characterized by only a few intramembrane particles on the P face and numerous large (10–13 nm) particles on the E face, whereas the basolateral membrane shows the majority of intramembrane particles (6–8 nm) on the P face. The structural specialization of the luminal membrane and the differences between the luminal and basolateral membranes of the pavement cell are similar to those previously demonstrated for the unstimulated granular cell of the amphibian urinary bladder. Because of this similarity, it is suggested that the 2 cell types are analogous and that the luminal membrane of the pavement cell in the lamprey gill epithelium is functionally characterized by a low water permeability. A possible role of sodium uptake by the pavement cells from freshwater and putative differences of osmoregulatory mechanisms in the gills of lampreys and teleosts in freshwater environments are disc

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00830.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

As a rule, chloride movement (JCl−) across amphibian skin is considered to be passive; this is implied in fact for preparations incubated in Ringer's fluid, since short‐circuit current (Isc) is the quantitative expression of net, active sodium transport (JNa+). The nature of the Cl−pathway(s) was investigated by incubating amphibian skin (mostlyBufo marinus) with Cl−present on the epithelial side only, and after blocking JNa+by combined treatment with ouabain and amiloride. In such conditions, JCl−was found to be equal to (reversed) Isc; furthermore, when JCl−was “translated” in terms of conductance, gCl−, the latter accounted almost quantitatively for transepithelial conductance, gt. When residual intratissue (i.e.intracellular) electronegativity was eliminated by replacing Na+with K+, JCl−was larger but Iscand JCl−were still found to reflect each other, and gCl−again accounted for most, if not all, of gt. JCl−in the opposite direction, as a result of Cl−being present only on the dermal side, was negligible, and gtwas very low.Thus, in the absence of sodium transport, when experimental conditions are such that a net inward JCl−obtains, the anion apparently flows only through (a) conductive pathways(s). Aldosterone is probably involved in the regulation of this pathway, as JCl−was much lower when toads were maintained in dilute saline than in water or on moist peat; so was the fraction of the apical surface correspon

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00831.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Urea fluxes across the urinary bladder ofBufo viridiswere studiedin vitroafter modification of the mounting technique. The fluxes increased as a function of the bath urea concentration, saturating near 200 mmol/l. The apparent Kmwas 88 mmol/l in the bladders from tapwater‐acclimated toads, and 107 mmol/l in toads acclimated to 500 mOsm NaCl. TheVmaxwhich was 300 μmol·h−1·cm−2at room temperature in bladders from tapwater acclimation, changed to more than 1500 μmol·h−1·cm−2upon salt acclimation. It is suggested that urea movement across the urinary bladder of this species occurs by facilitated diffusion and that salt acclimation induces an increase in the density of this pathway, but not of its chara

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00832.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Whole skins and isolated epithelia were bathed with isotonic media (‐ 244 mOsm) containing sucrose or glucose. The serosal osmolality was intermittently reduced (to ‐ 137 mOsm) by removing the nonelectrolyte. Transepithelial and intracellular electrophysiological parameters were monitored while serosal osmolality was changed. Serosal hypotonicity increased the short‐circuit current (Isc) and the basolateral conductance, hyperpolarized the apical membrane (Φmc), and increased the intracellular Na+concentration. The increases in apical conductance and apical Na+permeability (measured from Goldman fits of the relationship between amiloride‐sensitive current and Φmc) were not statistically significant.To verify that the osmotically induced changes in Iscwere mediated primarily at the basolateral membrane, the basolateral membrane potential of the experimental area was clamped close to 0 mV by replacing the serosal Na+with K+in Cl−‐free media. The adjoining control area was exposed to serosal Na+. Serosal hypotonicity produced a sustained stimulation of Iscacross the control, but not across the adjoining depolarized tissue area.The current results support the concept that hypotonic cell swelling increases Na+transport across frog skin epithelium by increasing the basolateral K+permeability, hyperpolarizing the apical membrane, and increasing the electrical driving force for ap

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00833.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

The (Na,K)‐pump develops and maintains ionic gradients that are of fundamental importance for proper function of most animal cells. These gradients are utilized in the form of ionic leak pathways by a number of special and general cell processes (e.g., nerve conduction, nutrient transport, pH regulation). As the sodium gradient in particular energizes many vital cell processes, alterations in cell activity will often be manifest as changes in sodium entry. The (Na,K)‐pump rate varies accordingly, in order to maintain balance between Na entry and exit thereby maintaining the potential energy of the cell.Acute changes in sodium influx are balanced by increases in activity of existing pump units, with only a small change in intracellular sodium concentration. This is posible because intracellular is normally poised on the steep limb of the concentrationversusactivity curve for the (Na,K)‐pump, at a point well below maximal activity, allowing large increases in (Na,K)‐pump rate with only small changes in sodium concentration. If the increase in sodium influx is prolonged, it appears that the cell responds by synthesizing new pumps, allowing intracellular sodium concentration to return to its original values.Though increases in (Na,K)‐pump activity must be accompanied by increases in potassium leak rates, in the experiments we have presented, there does not appear to be direct functional coupling between the (Na,K)‐pump and the K leak pathways. In these situations the matching of active influx and passive efflux of K short‐term appears to occur by mechanisms not directly related to (Na,K)‐

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00834.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00835.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00836.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0248-4900    

DOI:10.1111/j.1768-322X.1989.tb00807.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY