Sodium dependence of the epithelial sodium conductance expressed in Xenopus laevis oocytes.
The epithelial Na+ conductance was expressed in Xenopus laevis oocytes by injection of size-fractionated mRNA of bovine tracheal epithelium. Fractionation was achieved by sucrose density gradient centrifugation. Successful expression was analysed by recording current/voltage (I/V) curves in the presence and absence of amiloride (10 mumol/l). The newly expressed conductance was half-maximally inhibited by 44 nmol/l amiloride and exhibited a selectivity for Na+ over K+ of 140:1. I/V curves obtained at different extracellular Na+ concentrations ([Na+]o) were subjected to a Goldman-fit analysis to obtain the relation between Na+ permeability (PNa) and [Na+]o. The data show that decreasing [Na+]o from 85 mmol/l to 0.85 mmol/l increased PNa by more than threefold, which is thought to reflect Na+ channel inhibition by increasing [Na+]o. This effect clearly exceeded what can be attributed to concentration saturation of single Na+ channel conductance (Palmer and Frindt (1986) Proc Natl Acad Sci USA 83:2767). No correlation of inhibition with intracellular Na+ concentration was observed. Preservation of the [Na+]o-dependent self-inhibition by the newly expressed Na+ conductance suggests that it is an intrinsic property of the Na+ channel protein, probably mediated by an extracellular Na+ binding site.
Kroll
B
B
Zentrum der Physiologie, Klinikum der J.W. Goethe-Universität, Frankfurt/Main, Federal Republic of Germany.
Bremer
S
S
Tümmler
B
B
Kottra
G
G
Frömter
E
E
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002499:Centrifugation, Density Gradient; D004553:Electric Conductivity; D004848:Epithelium / Q000502:physiology; D007267:Injections; D009865:Oocytes / Q000502:physiology*; D012333:RNA, Messenger; D012964:Sodium / Q000494:pharmacology* / Q000502:physiology; D015222:Sodium Channels / Q000235:genetics; D014982:Xenopus laevis
D012333:RNA, Messenger; D015222:Sodium Channels; D000584:Amiloride; D012964:Sodium