A long isoform of the epithelial sodium channel alpha subunit forms a highly active channel

A long isoform of the human Epithelial Sodium Channel (ENaC) alpha subunit has been identified, but little data exist regarding the properties or regulation of channels formed by alpha(728). The baseline whole cell conductance of oocytes expressing trimeric alpha(728)beta gamma channels was 898.1 +/- 277.2 and 49.59 +/- 13.2 mu S in low and high sodium solutions, respectively, and was 11 and 2 fold higher than the conductances of alpha(669)beta gamma in same solutions. alpha(728)beta gamma channels were also 2 to 5 fold less sensitive to activation by the serine proteases subtilisin and trypsin than alpha(669)beta gamma in low and high Na+ conditions. The long isoform exhibited lower levels of full length and cleaved protein at the plasma membrane and a rightward shifted sensitivity to inhibition by increases of [Na+](i). Both channels displayed similar single channel conductances of 4 pS, and both were activated to a similar extent by reducing temperature, altogether indicating that activation of baseline conductance of alpha(728)beta gamma was likely mediated by enhanced channel activity or open probability. Expression of alpha(728) in native kidneys was validated in human urinary exosomes. These data demonstrate that the long isoform of alpha ENaC forms the structural basis of a channel with different activity and regulation, which may not be easily distinguishable in native tissue, but may underlie sodium hyperabsorption and salt sensitive differences in humans.