Low Passive Permeability Research Articles

Sodium exclusion mechanisms at the root surface of two maize cultivars

Sodium exclusion at the root surface of intact plants was investigated in two maize cultivars (Zea mays L. cv. Pioneer 3906 and cv. DeKalb XL75). The strong sodium exclusion by Pioneer 3906 relative to DeKalb XL75 was not caused by larger efflux rates. In contrast, this cultivar showed reduced influx rates resulting in about half the net uptake in comparison with DeKalb XL75. It is concluded that the cultivar difference in sodium exclusion at the root surface is based on a relatively low passive sodium permeability of the epidermal and cortex plasmalemma of Pioneer 3906. For both cultivars, the flux ratio equation indicated an active component of sodium efflux.

Erythrocyte Sodium and Sodium Flux in Relation to Hypertension in Chronic Renal Failure

Erythrocyte sodium and sodium fluxes in plasma and in physiological buffer solution were studied in normotensive and hypertensive uraemic patients and normal subjects. Erythrocyte sodium was reduced in the uraemic patients due primarily to low sodium influx, further supported by low passive membrane permeability. These differences were much more marked in normotensive patients. There was no evidence for sodium pump inhibition in the erythrocytes. The low erythrocyte sodium influx in the uraemic patients appeared to be due to a plasma factor which could be reversed in young cells but not in old. However, erythrocyte sodium flux in both plasma and physiological buffer was lower in normotensive than in hypertensive uraemic patients. Therefore, a membrane change to compensate for the effects of the plasma factor on sodium influx may be related to the development of hypertension in the uraemic patients.

Sodium-Independent Proline Transport in the Locust Rectum

ABSTRACT Recta of Schistocerca gregaria possess a high-affinity (Kt = 10 mmol l−1) and high-capacity (Vmax = 4·2 μequiv cm−2h−1) active absorptive mechanism for proline second in magnitude only to stimulated Cl− transport. Transcellular and paracellular pathways have extremely low passive permeability to proline, resulting in very high flux ratios (40:1) compared with those for other.solutes (less than 6:1). Net epithelial transport of proline is largely independent of luminal Na+, K+ and Cl−. Sodium influx does not change when proline net fluxes are varied 14-fold. Therefore Na+ cotransport is not a principal mechanism of proline uptake in this tissue. Prolonged absence of Na+ and K+ from the haemocoel side partially inhibits proline transport, probably indirectly, by affecting the general transport capacity of the rectum. Inhibition is irreversible. A component of the net proline flux (Jpronet) is electrogenic, located in the apical membrane, and may be due to proline/proton cotransport.

Apical Membrane Permeability of Mytilus Gill: Influence of Ultrastructure, Salinity and Competitive Inhibitors on Amino Acid Fluxes

ABSTRACT The apical membrane of gill integumental cells from the mussels Mytilus edulis and M. californianus serves as a permeability barrier separating sea water from a cytoplasm rich in amino acids and other small organic molecules. Morphometric analysis of transmission electronmicrographs indicates that the membrane area of these cells is increased between 10-and 18-fold by the presence of a microvillous brush border. The microvilli do not appear to influence the kinetics of solute transport across the cell apex, as determined using a mathematical model of the relationship between membrane structure and the kinetics of transport. Rates of amino acid loss from the integument were low, and estimates of the upper limit of the passive permeability of the apical membrane to amino acids ranged from 0·5 to 10 × 10−10 cm s−1. Abrupt exposure of intact mussels or isolated gill tissue to 60 % sea water (19%o salinity) resulted in a transient, 40-to 80-fold increase in the rate of loss of all amino acids from integumental tissues. Upon exposure to full-strength sea water, efflux rates returned to near control values. Exposure to 60 % sea water also inhibited the carrier-mediated accumulation of amino acid: uptake of 0·5 μmol l−1 [l4C]alanine and [14C]taurine was reduced by 80% compared to control uptake in 100% sea water. This inhibition was not adequate to account for the increase in net efflux of taurine from gill tissue into 60% artificial sea water (ASW), though the inhibition of alanine uptake may have contributed significantly to the increased loss of this amino acid. Efflux of discrete structural classes of amino acid occurred when integumental tissues were exposed to 50 μmol l−1 concentrations of structurally related analogues. It is concluded that the apical membrane of gill cells has a very low passive permeability to amino acids, and that the overall permeability of the gill can be increased in a reversible fashion by exposure to reduced salinity or to high external concentrations of amino acid.

Alanine and taurine transport by the gill epithelium of a marine bivalve: Effect of sodium on influx

Marine mussels can accumulate amino acids from seawater into the epithelial cells of the gill against chemical gradients in excess of 5×106 to 1. Uptake of both alanine and taurine into gill tissue isolated fromMytilus californianus was found to be dependent upon Na+ in the external solution. Uptake of these amino acids was described by Michaelis-Menten kinetics, and a reduction in external [Na+] (from 425 to 213mm) increased the apparent Michaelis constants (alanine, from 8 to 17 μm; taurine, from 4 to 39 μm) without a significant influence on theJmax's of these processes. Fivemm harmaline, an inhibitor of Na-cotransport processes in many systems, reduced both alanine and taurine uptake by more than 95%; this inhibition appeared to be competitive in nature, with an apparentK i of 43 μm for the interaction with alanine uptake. Increasing the external [Na+] from 0 to 510mm produced a sigmoid activation of alanine and taurine uptake withKNa's of approximately 325mm. The apparent Hill coefficients for this activation were 7.3 and 7.4 for alanine and taurine, respectively. These data are consistent with uptake mechanisms which require comparatively high concentrations of Na+ to activate transport, and which couple several Na+ ions to the transport of each amino acid. These characteristics, in conjunction with the previously demonstrated low passive permeability of the apical membrane to amino acids, result in systems capable of i) accumulating amino acids from seawater to help meet the nutritional needs of this animal, and ii) maintaining the high intracellular amino-acid concentrations associated with volume regulation in the gill.

Lack of selectivity to small ions in paracellular pathways in cerebral and muscle capillaries of the frog.

Selectivity to passive permeation of small ions through capillary walls was studied by measurements of diffusion potentials in response to ionic gradients established across capillary walls in frog brain and muscle in superfusion and perfusion experiments. Average dilution potentials in response to 2:1 or 10:1 gradients of NaCl across brain capillaries were 4.2 and 10.2 mV, respectively. The 'diluted' side was negative with respect to the 'undiluted' side, reflecting higher mobility of Cl- than of Na+ ions. Bi-ionic potentials in response to isosmotic KCl:NaCl gradients averaged 5.0 mV in brain capillaries, negative on the KCl side, reflecting higher mobility of K+ than of Na+ ions. The potential variations were symmetrical across the capillary wall. From Planck-Henderson formalism, the relative permeabilities in brain capillaries of Na+, K+ and Cl- were PCl/PNa:1.54 and PK/PNa:1.56, rather close to mobility ratios in free solution. Experiments on muscle capillaries showed similar results to those in brain. Streaming potentials created with excess (200 mosmol) mannitol or sucrose were congruent to 1 mV in brain capillaries and zero in muscle capillaries. It is concluded that the transcapillary permeation pathway in muscle and brain is neutral or weakly charged. The dominant ion permeation is paracellular in all 'continuous' capillaries. The large range of ion permeability of 'continuous' capillaries may be explained by variation in the length of the effective open portion of interendothelial junctions. The very low passive permeability of the blood-brain barrier may be due to an almost closed endothelial junction, leaving only about 0.1% of the length open. The open fraction is functionally similar to that in muscle and mesentery. This interpretation is in accordance with a finite, but very low, permeability to hydrophilic non-electrolytes. Such a brain capillary would still display strong preference for lipid-soluble solutes, but its behaviour cannot be satisfactorily understood from a simple analogy to a cellular plasma membrane.

TRANSPORT MECHANISMS IN THE PLACENTA

The mammalian conceptus is surrounded by, and entirely supported by, maternal extracellular fluid, an environment whose stability is well regulated by the mother's own homeostatic mechanisms. However, it appears to be necessary for a fetus to maintain some independence from its mother. The placenta provides such a barrier; the fetus can enjoy a separate 'milieu interieur', which may be an advantage from immunological, endocrinological or nutritional aspects.Animal experiments suggest that the placental barrier is epithelial in nature. In common with other epithelia, it is relatively permeable to substances of high lipid solubility, such as gases. There is good evidence for the presence of special transfer mechanisms for those lipid-insoluble substances of biological importance such as sugars, aminoacids and at least some electrolytes. The development of such transfermechanisms would have been an evolutionary necessity in the presence of an otherwise relatively impermeable barrier, as transfer of adequate quantities of anabolic substrates needs to be guaranteed. Little is yet known of the selectivity of these mechanisms or of what controls them - or if they are controlled at all; and there is no evidence concerning whether their failure contributes to materno-fetal disease. At present, the placental epithelium may be considered to have a transport function in search of a pathology.Most experimental data have been obtained in animals, but observations in man suggest that, while there are differences in detail, there is considerable similarity in the overall pattern of placental epithelial function. Experiments made by man occur whenever medication is prescribed. The different patterns of permeability to drugs of differing properties provide support for the relative impermeability to lipid insoluble molecules, while the neonatal hyponatremia following intravenous glucose infusions to women in labour reflects the high placental permeability to water.Experiments of nature often depend on the constitutional differences between mother and the genetically different fetus. The markedly different protein phenotypes in mother and fetus implies a very low passive permeability to protein. Similarly the relative impermeability to thyroxine is evident from the delayed osseous maturation of the hypothyroid fetus in spite of normal maternal thyroxine levels. In some cases placental transfer functions contribute to fetal disease; for example, the placenta fails to protect the fetus from the hostile environment of maternal diabetes, phenylketonuria or rhesus isoimmunisation.Most such observations pose more questions than they answer. Yet these answers must be sought if we are fully to understand, and perhaps modify, the contribution of placental transfer mechanisms to fetal health and disease.

Intact vesicles of canine cardiac sarcolemma: evidence from vectorial properties of Na+, K+-ATPase.

Most biological membranes are functionally asymmetric. To study biochemical control of cardiac transsarcolemmalion fluxes, it would be of obvious advantage to use isolated vesicles of sarcolemma which retains the low passive permeability characteristics of intact sarcolemma because in such vesicles the membrane should exhibit its normal asymmetric character with respect to enzymic activities. The purpose of this investigation was to attempt identify such vesicles in a cardiac microsomal (membrane vesicular) preparation. We studied activation by Na+ and K+ of Na+, K+-ATPase and its associated K+-phosphatase activities, using as substrates ATP or p-nitrophenylphosphate (pNPP) in the presence of Mg2+. Optimal concentrations of K+ alone (10 mM) stimulated p-nitrophenylphosphatase (pNPPase) activity 1.8-fold, and over 80% of the increase could be inhibited by ouabain. Optimal Na+ plus K+ concentrations (100 mM and 10 mM, respectively) stimulated the rate of ATP hydrolysis 2-fold, but only 11 +/- 1.1% of the increased activity was ouabain-sensitive. Optimal pretreatment with sodium dodecyl sulfate (SDS) (0.3 mg/ml) rendered both activities completely sensitive to inhibition by ouabain and reduced the basal Mg2+-ATPase activity by 70-90%. The K+-stimulated pNPPase activity doubled after preincubation in SDS, but the ATPase activity stimulated by Na+ plus K+ fell by 50% under these conditions. A similar pattern of apparent activation was produced by preincubation with deoxycholate (DOC), except that basal Mg2+-dependent activities were resistant to destruction by this detergent. The incremental responses to activation by ions and substrates, and inhibition by oubain, are consistent with the hypothesis that permeability-intact vesicles of sarcolemma are present in the isolated preparation, and that detergent activation renders the vesicles highly permeable to the ions, substrates, and ouabain.

Calcium movements across the membrane of human red cells.

1. A study has been made of the cellular content and movement of Ca across the membrane of human red blood cells.2. The [Ca] in the cellular contents of fresh red cells is 4.09 x 10(-2) mM. The intracellular concentration of free ionic Ca ([Ca(2+)]) is considered to be less than this value and therefore less than extracellular [Ca(2+)] under normal conditions.3. Observation of unidirectional Ca fluxes with (45)Ca confirms previous reports of low permeability of the red cell membrane for Ca. After nearly 1 week of loading in the cold, intracellular (45)Ca content is 1.8% of extracellular (45)Ca content. Appearance in extracellular fluid of (45)Ca from coldloaded cells can be considered to arise from two compartments. Efflux of (45)Ca from the ;slower compartment' is accelerated by the addition of glucose.4. Starved red cells, incubated at 37 degrees C, after reversible haemolysis for loading with Ca and Mg-ATP, exhibit an outward net transport of Ca against an electrochemical gradient. The transport is associated with the appearance of inorganic phosphate (P(i)). Cells treated similarly, but without ATP show no transport and no appearance of P(i).5. During the initial phase of transport, 1.3 mole P(i) appear per mole Ca transported.6. The transport of Ca from ATP-loaded cells is highly temperature-dependent, with a Q(10) of 3.5.7. Cell membrane adenosine triphosphatase (ATPase) activity of reversibly haemolysed cells is stimulated only by intracellular, and not by extracellular Ca.8. Neither Ca transport in reversibly haemolysed cells, nor the Ca-Mg activated ATPase of isolated cell membranes is sensitive to Na, K, ouabain or oligomycin.9. Mg is not transported under the conditions which reveal Ca transport, but Mg appears to be necessary for Ca transport.10. Sr is transported from reversibly haemolysed Mg-ATP-loaded cells. Sr also can substitute for Ca, but not for Mg, in the activation of membrane ATPase.11. It is concluded that, in addition to a low passive permeability, an active extrusion mechanism for Ca exists in the human red cell membrane. This extrusion mechanism, in addition to a low passive membrane permeability for Ca, may represent the means by which intracellular Ca content is maintained at a low level. It is suggested that the Ca-Mg activated membrane ATPase and the active transport of Ca are two manifestations of the same process.

Transport of sodium and secretion of potassium and bicarbonate by the colon of normal and sodium‐depleted rats

1. Ascending and descending colonic segments of normal and Na-depleted rats were perfused in vivo with isotonic solutions of varying Na concentration and the unidirectional Na fluxes and secretion rate of K and bicarbonate and the transmucosal electrical p.d. were measured.2. Potential difference was greater in Na-depleted rats, especially towards the distal end of the descending colon. With reduction of luminal Na concentration, p.d. was reduced.3. The ascending and descending segments were similar in regard to Na transport except that the latter had lower passive permeability. Na depletion caused an increase of Na influx rate, Na net flux rate and Na exchange diffusion whilst the mucosal passive Na permeability decreased. These changes resulted in a reduction in the critical luminal Na concentration, i.e. the concentration at which the unidirectional fluxes were equal.4. K secretion rate was similar in the ascending and descending colon and was increased by Na depletion. In all rats, it was reduced when the luminal Na concentration was low.5. Bicarbonate secretion rate was unaffected by the Na depletion and all solutions remained isotonic during perfusion.6. The results confirmed that active Na transport was stimulated by Na depletion but indicated that this was probably not the only factor in the elevation of transmucosal p.d.