Abstract: | The hypothesis that the intracellularNa+ concentration([Na+]i)is a regulator of the epithelialNa+ channel (ENaC) was tested withthe Xenopus oocyte expression systemby utilizing a dual-electrode voltage clamp.[Na+]iaveraged 48.1 ± 2.2 meq (n = 27)and was estimated from the amiloride-sensitive reversal potential.[Na+]iwas increased by direct injection of 27.6 nl of 0.25 or 0.5 MNa2SO4.Within minutes of injection,[Na+]istabilized and remained elevated at 97.8 ± 6.5 meq(n = 9) and 64.9 ± 4.4 (n = 5) meq 30 min after theinitial injection of 0.5 and 0.25 MNa2SO4,respectively. This increase of[Na+]icaused a biphasic inhibition of ENaC currents. In oocytes injected with0.5 MNa2SO4(n = 9), a rapid decrease of inwardamiloride-sensitive slope conductance(gNa) to 0.681 ± 0.030 of control within the first 3 min and a secondary, slowerdecrease to 0.304 ± 0.043 of control at 30 min were observed.Similar but smaller inhibitions were also observed with the injectionof 0.25 MNa2SO4.Injection of isotonicK2SO4(70 mM) or isotonicK2SO4made hypertonic with sucrose (70 mMK2SO4-1.2M sucrose) was without effect. Injection of a 0.5 M concentration ofeitherK2SO4,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. Thusincreases of[Na+]ihave multiple specific inhibitory effects on ENaC that can betemporally 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. |