Regulation of the epithelial Na+ channel by extracellular acidification

Abstract

The effect of extracellular acidification was tested on the native epithelial Na+ channel (ENaC) in A6 epithelia and on the cloned ENaC expressed in Xenopusoocytes. Channel activity was determined utilizing blocker-induced fluctuation analysis in A6 epithelia and dual electrode voltage clamp in oocytes. In A6 cells, a decrease of extracellular pH (pHo) from 7.4 to 6.4 caused a slow stimulation of the amiloride-sensitive short-circuit current ( I Na) by 68.4 ± 11% ( n = 9) at 60 min. This increase of I Na was attributed to an increase of open channel and total channel ( N T) densities. Similar changes were observed with pHo 5.4. The effects of pHo were blocked by buffering intracellular Ca2+ with 5 μM 1,2-bis(2-aminophenoxy)ethane- N,N,N′,N′-tetraacetic acid. In oocytes, pHo 6.4 elicited a small transient increase of the slope conductance of the cloned ENaC (11.4 ± 2.2% at 2 min) followed by a decrease to 83.7 ± 11.7% of control at 60 min ( n = 6). Thus small decreases of pHostimulate the native ENaC by increasing N T but do not appreciably affect ENaC expressed in Xenopus oocytes. These effects are distinct from those observed with decreasing intracellular pH with permeant buffers that are known to inhibit ENaC.