SINGLE-CHANNEL BEHAVIOR OF A PURIFIED EPITHELIAL NA+ CHANNEL SUBUNIT THAT BINDS AMILORIDE

The apical membrane of high electrical resistance epithelia, which is selectively permeable to Na+, plays an essential role in the maintenance of salt balance. Na+ entry from the apical fluid into the cells is mediated by amiloride-blockable Na+-specific channels. The channel protein, purified from both amphibian and mammalian sources, is composed of several subunits, only one of which, the 150-kDa polypeptide, specifically binds the Na+ transport inhibitor amiloride. The goal of the present study was to investigate whether the isolated amiloride-binding subunit of the channel could conduct Na+. The patch-clamp technique was used to study the 150-kDa polypeptide incorporated into a lipid bilayer formed on the tip of a glass pipette. Unitary conductance jumps averaged 4.8 pS at 100 mM Na2HPO4. Open times ranged from 24 ms to several seconds. The channel spent most of the time in the closed state. Channel conductance and gating were independent of voltage between -60 and +100 mV. Amiloride (0.1 muM) decreased the mean open time of the channel by 98%. We conclude that the 150-kDa subunit of the amiloride-blockable Na+ channel conducts current and may be sufficient for the Na+ transport function of the whole channel.