Selenium (selenate) transport by human placental brush border membrane vesicles

1. Selenate uptake by human placental brush-border-membrane vesicles was studied in order to establish whether this anion shares a pathway with sulphate.2. Selenate uptake was found to be saturable with respect to medium selenate and was inhibited by the anion exchange inhibitor 4,4′-diisothiocyano-stilbene-2,2′-disulphonate (DIDS).3. Anions which have a similar tetrahedral shape to selenate, e.g. chromate, molybdate, tungstate and sulphate, were effective inhibitors of selenate uptake when added to the incubation medium.4. Sulphate inhibited selenate influx in a dose-dependent fashion; moreover sulphate was found to be a competitive inhibitor of selenate uptake.5. It is concluded that selenate and sulphate share a pathway for transport in the human placental microvillus membrane.

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Ca2+ and Zn2+ are transported by the electrogenic 2Na+/1H+ antiporter in echinoderm gastrointestinal epithelium

Journal of Experimental Biology ◽

10.1242/jeb.198.5.1207 ◽

1995 ◽

Vol 198 (5) ◽

pp. 1207-1217 ◽

Cited By ~ 1

Author(s):

Z Zhuang ◽

J Duerr ◽

G Ahearn

Keyword(s):

Brush Border ◽

Brush Border Membrane ◽

Divalent Cations ◽

Membrane Vesicles ◽

Homarus Americanus ◽

Competitive Inhibitor ◽

Brush Border Membrane Vesicles ◽

Carrier Mediated Transport ◽

Shift Experiment ◽

Exchange Function

45Ca2+ uptake by purified brush-border membrane vesicles of starfish (Pycnopodia helianthoides) pyloric ceca was stimulated by an outwardly directed H+ gradient and this stimulation was enhanced by the simultaneous presence of an induced membrane potential (inside negative; K+/valinomycin). External amiloride (competitive inhibitor; Ki=660 µmol l-1) and a monoclonal antibody raised against proteins associated with the lobster (Homarus americanus) electrogenic 2Na+/1H+ antiporter both inhibited approximately half of the proton-gradient-stimulated 45Ca2+ uptake. These results suggested that Ca2+ might be transported by the electrogenic antiporter and that the crustacean antibody was inhibitory to the exchange function in echinoderms, as was recently shown in crustacean epithelial brush-border membrane vesicles. Carrier-mediated 45Ca2+ influx by amiloride-sensitive and amiloride-insensitive systems displayed the following kinetic constants: (amiloride-sensitive) Kt=66±2 µmol l-1; Jmax=0.173±0.002 pmol µg-1 protein 8 s-1; (amiloride-insensitive) Kt=18±0.3 µmol l-1; Jmax=0.100±0.001 pmol µg-1 protein 8 s-1. Zn2+ was a mixed inhibitor of 45Ca2+ influx by carrier-mediated transport, displaying a Ki of 920 µmol l-1. Mn2+, Cu2+, Fe2+ and Mg2+ also inhibited 45Ca2+ uptake, but the mechanism(s) of inhibition by these other cations was not disclosed. An equilibrium shift experiment showed that both Na+ and Zn2+ were able to exchange with equilibrated 45Ca2+ in these vesicles, suggesting that both monovalent and divalent cations were able to enter pyloric cecal cells through a common carrier-mediated transport system. In addition, the echinoderm electrogenic system appeared to exhibit a molecular component recognized by the crustacean antibody that may imply a similar epitope in the two animals.

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Amiloride transport in rabbit renal brush-border membrane vesicles

AJP Renal Physiology ◽

10.1152/ajprenal.1989.256.3.f462 ◽

1989 ◽

Vol 256 (3) ◽

pp. F462-F468

Author(s):

S. H. Wright ◽

T. M. Wunz

Keyword(s):

Brush Border ◽

Brush Border Membrane ◽

Exchange Process ◽

Membrane Vesicles ◽

Competitive Inhibitor ◽

Brush Border Membrane Vesicles ◽

Ph Gradient ◽

Renal Brush Border Membrane ◽

Renal Brush Border ◽

Active Accumulation

Rabbit renal brush-border membrane vesicles (BBMV) were used to study amiloride transport across the luminal membrane of proximal tubular cells. An outwardly directed H+ gradient (pHi 6.0; pHo 7.5) stimulated 8 microM [14C]-amiloride uptake into BBMV and supported a transient "active" accumulation of substrate consistent with the presence of an amiloride-H+ exchange process. Uptake was inhibited, in the presence or absence of a pH gradient, by 1 mM unlabeled amiloride or 20 mM tetraethylammonium (TEA). Amiloride transport was not directly affected by the presence of 100 mM Na+ in the extravesicular medium, suggesting that Na-H exchange did not mediate amiloride flux. Amiloride transport was a saturable process with a maximal flux (under pH gradient conditions) of 3 nmol.mg-1.min-1 and an apparent Kt of 8 microM. TEA acted as a competitive inhibitor of this process with an apparent Ki of approximately 80 microM, similar to the Kt of TEA transport via the TEA-H+ exchanger. Likewise, amiloride acted as a competitive inhibitor of TEA uptake with an apparent Ki of approximately 11 microM. Preloading BBMV with 1-2 mM TEA stimulated the rate of amiloride uptake and supported a transient active accumulation of amiloride. We conclude that amiloride and TEA are transported by a common pathway in BBMV, which involves a carrier-mediated exchange with H+ and which may play a role in the tubular secretion of these compounds.

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Low-affinity transport of pyroglutamyl-histidine in renal brush-border membrane vesicles

AJP Cell Physiology ◽

10.1152/ajpcell.1989.257.5.c971 ◽

1989 ◽

Vol 257 (5) ◽

pp. C971-C975 ◽

Cited By ~ 3

Author(s):

H. A. Skopicki ◽

K. Fisher ◽

D. Zikos ◽

G. Flouret ◽

D. R. Peterson

Keyword(s):

Brush Border ◽

Brush Border Membrane ◽

Membrane Vesicles ◽

Brush Border Membrane Vesicles ◽

Proximal Tubular Cells ◽

Tubular Cells ◽

Luminal Membrane ◽

Renal Brush Border Membrane ◽

Proximal Tubular ◽

Renal Brush Border

These studies were performed to determine if a low-affinity carrier is present in the luminal membrane of proximal tubular cells for the transport of the dipeptide, pyroglutamyl-histidine (pGlu-His). We have previously described the existence of a specific, high-affinity, low-capacity [transport constant (Kt) = 9.3 X 10(-8) M, Vmax = 6.1 X 10(-12) mol.mg-1.min-1] carrier for pGlu-His in renal brush-border membrane vesicles. In the present study, we sought to demonstrate that multiple carriers exist for the transport of a single dipeptide by determining whether a low-affinity carrier also exists for the uptake of pGlu-His. Transport of pGlu-His into brush-border membrane vesicles was saturable over the concentration range of 10(-5)-10(-3) M, yielding a Kt of 6.3 X 10(-5) M and a Vmax of 2.2 X 10(-10) mol.mg-1.min-1. Uptake was inhibited by the dipeptides glycyl-proline, glycyl-sarcosine, and carnosine but not by the tripeptide pyroglutamyl-histidyl-prolinamide. We conclude that 1) pGlu-His is transported across the luminal membrane of the proximal tubule by multiple carriers and 2) the lower affinity carrier, unlike the higher affinity carrier, is nonspecific with respect to other dipeptides.

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