Regulation of the epithelial Na1 channel by intracellular Na1

Awayda, Mouhamed S. Regulation of the epithelial Na1 channel by intracellular Na1. Am. J. Physiol. 277 (Cell Physiol. 46): C216–C224, 1999.—The hypothesis that the intracellular Na1 concentration ([Na]i) is a regulator of the epithelial Na1 channel (ENaC) was tested with the Xenopus oocyte expression system by utilizing a dual-electrode voltage clamp. [Na]i averaged 48.1 6 2.2 meq (n 5 27) and was estimated from the amiloride-sensitive reversal potential. [Na]i was increased by direct injection of 27.6 nl of 0.25 or 0.5 M Na2SO4. Within minutes of injection, [Na]i stabilized and remained elevated at 97.8 6 6.5 meq (n 5 9) and 64.9 6 4.4 (n 5 5) meq 30 min after the initial injection of 0.5 and 0.25 M Na2SO4, respectively. This increase of [Na]i caused a biphasic inhibition of ENaC currents. In oocytes injected with 0.5 M Na2SO4 (n 5 9), a rapid decrease of inward amiloridesensitive slope conductance (gNa) to 0.681 6 0.030 of control within the first 3 min and a secondary, slower decrease to 0.304 6 0.043 of control at 30 min were observed. Similar but smaller inhibitions were also observed with the injection of 0.25 M Na2SO4. Injection of isotonic K2SO4 (70 mM) or isotonic K2SO4 made hypertonic with sucrose (70 mM K2SO41.2 M sucrose) was without effect. Injection of a 0.5 M concentration of either K2SO4, N-methyl-D-glucamine (NMDG) sulfate, or 0.75 M NMDG gluconate resulted in a much smaller initial inhibition (,14%) and little or no secondary decrease. Thus increases of [Na]i have multiple specific inhibitory effects on ENaC that can be temporally separated into a rapid phase that was complete within 2–3 min and a delayed slow phase that was observed between 5 and 30 min.