Feedback inhibition of epithelial Na+
 channels in Xenopus
 oocytes does not require G0
 or Gi2
 proteins

Degenerin/epithelial Na+ channels (DEG/ENaCs) are Na+ channels that are blocked by the diuretic amiloride. In general, they are impermeable for Ca2+ or have a very low permeability for Ca2+. We describe here, however, that a DEG/ENaC from the cnidarian Hydra magnipapillata, the Hydra Na+ channel (HyNaC), is highly permeable for Ca2+ (PCa/PNa = 3.8). HyNaC is directly gated by Hydra neuropeptides, and in Xenopus laevis oocytes expressing HyNaCs, RFamides elicit currents with biphasic kinetics, with a fast transient component and a slower sustained component. Although it was previously reported that the sustained component is unselective for monovalent cations, the selectivity of the transient component had remained unknown. Here, we show that the transient current component arises from secondary activation of the Ca2+-activated Cl− channel (CaCC) of Xenopus oocytes. Inhibiting the activation of the CaCC leads to a simple on–off response of peptide-activated currents with no apparent desensitization. In addition, we identify a conserved ring of negative charges at the outer entrance of the HyNaC pore that is crucial for the high Ca2+ permeability, presumably by attracting divalent cations to the pore. At more positive membrane potentials, the binding of Ca2+ to the ring of negative charges increasingly blocks HyNaC currents. Thus, HyNaC is the first member of the DEG/ENaC gene family with a high Ca2+ permeability.

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Blocker-induced noise of rat epithelial Na+ channels expressed in Xenopus oocytes

Comparative Biochemistry and Physiology Part A Molecular & Integrative Physiology ◽

10.1016/s1095-6433(00)80144-0 ◽

2000 ◽

Vol 126 ◽

pp. 73

Author(s):

A. Segal ◽

W. Van Driessche ◽

D. Jans ◽

J. Eggermont ◽

W-M. Weber

Keyword(s):

Xenopus Oocytes ◽

Na Channels ◽

Epithelial Na Channels

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The N terminus of α-ENaC mediates ENaC cleavage and activation by furin

The Journal of General Physiology ◽

10.1085/jgp.201711860 ◽

2018 ◽

Vol 150 (8) ◽

pp. 1179-1187 ◽

Cited By ~ 2

Author(s):

Pradeep Kota ◽

Martina Gentzsch ◽

Yan L. Dang ◽

Richard C. Boucher ◽

M. Jackson Stutts

Keyword(s):

Xenopus Oocytes ◽

Feedback Inhibition ◽

Brefeldin A ◽

Open Probability ◽

Α Subunit ◽

Naturally Occurring ◽

Epithelial Na Channels ◽

N Terminus ◽

Oocyte Membrane ◽

Pore Blocker

Epithelial Na+ channels comprise three homologous subunits (α, β, and γ) that are regulated by alternative splicing and proteolytic cleavage. Here, we determine the basis of the reduced Na+ current (INa) that results from expression of a previously identified, naturally occurring splice variant of the α subunit (α-ENaC), in which residues 34–82 are deleted (αΔ34–82). αΔ34–82-ENaC expression with WT β and γ subunits in Xenopus oocytes produces reduced basal INa, which can largely be recovered by exogenous trypsin. With this αΔ34–82-containing ENaC, both α and γ subunits display decreased cleavage fragments, consistent with reduced processing by furin or furin-like convertases. Data using MTSET modification of a cysteine, introduced into the degenerin locus in β-ENaC, suggest that the reduced INa of αΔ34–82-ENaC arises from an increased population of uncleaved, near-silent ENaC, rather than from a reduced open probability spread uniformly across all channels. After treatment with brefeldin A to disrupt anterograde trafficking of channel subunits, INa in oocytes expressing αΔ34–82-ENaC is reestablished more slowly than that in oocytes expressing WT ENaC. Overnight or acute incubation of oocytes expressing WT ENaC in the pore blocker amiloride increases basal ENaC proteolytic stimulation, consistent with relief of Na+ feedback inhibition. These responses are reduced in oocytes expressing αΔ34–82-ENaC. We conclude that the α-ENaC N terminus mediates interactions that govern the delivery of cleaved and uncleaved ENaC populations to the oocyte membrane.

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Epithelial Na Channels: Function and Diversity

Annual Review of Physiology ◽

10.1146/annurev.ph.54.030192.000411 ◽

1992 ◽

Vol 54 (1) ◽

pp. 51-66 ◽

Cited By ~ 135

Author(s):

L G Palmer

Keyword(s):

Na Channels ◽

Epithelial Na Channels

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Mechanisms and sequelae of increased alveolar fluid clearance in hyperoxic rats

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.1997.272.3.l407 ◽

1997 ◽

Vol 272 (3) ◽

pp. L407-L412 ◽

Cited By ~ 35

Author(s):

G. Yue ◽

S. Matalon

Keyword(s):

Alveolar Epithelial Cells ◽

Na Channels ◽

Alveolar Fluid Clearance ◽

Na Transport ◽

Alveolar Epithelial ◽

Lung Fluid ◽

Isotonic Fluid ◽

Epithelial Na Channels ◽

Alveolar Edema

We instilled 4 ml isotonic fluid containing trace amounts of fluorescently labeled dextran (molecular mass 150 kDa) in the lungs of rats exposed to either 85% O(2) for 7 days or to 85% O(2) for 7 days and 100% O(2) for 3 days. We withdrew the fluid every hour for a 3-h period and calculated alveolar fluid clearance (AFC) from changes in dextran concentration. Postinstillation (3 h), AFC values in the control and the two hyperoxic groups were 51 +/- 1, 63 +/- 2, and 62 +/- 3 (SE), respectively (%instilled volume; n > or = 5; P < 0.05). Addition of either 1 mM amiloride or N-ethyl-N-isopropyl amiloride (EIPA) in the instillate decreased the AFC values in all groups 3 h later to approximately 30% of instilled volume. Instillation of phenamil, an irreversible blocker of epithelial Na+ channels into the lungs of rats exposed to 85% O(2) for 7 days and 100% O(2) for 2 days, resulted in a significant increase of their extravascular lung fluid volumes 24 h later. These results demonstrate the existence of EIPA-inhibitable Na+ channels in alveolar epithelial cells in vivo and indicate that an increase in Na+ transport plays an important role in limiting the amount of alveolar edema in O(2)-damaged lungs.

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