Streaming potential measurements in alphabetagamma-rat epithelial Na+ channel in planar lipid bilayers

Streaming potentials across cloned epithelial Na+ channels (ENaC) incorporated into planar lipid bilayers were measured. We found that the establishment of an osmotic pressure gradient (Deltapi) across a channel-containing membrane mimicked the activation effects of a hydrostatic pressure differential (DeltaP) on alphabetagamma-rENaC, although with a quantitative difference in the magnitude of the driving forces. Moreover, the imposition of a Deltapi negates channel activation by DeltaP when the Deltapi was directed against DeltaP. A streaming potential of 2.0 +/- 0.7 mV was measured across alphabetagamma-rat ENaC (rENaC)-containing bilayers at 100 mM symmetrical [Na+] in the presence of a 2 Osmol/kg sucrose gradient. Assuming single file movement of ions and water within the conduction pathway, we conclude that between two and three water molecules are translocated together with a single Na+ ion. A minimal effective pore diameter of 3 A that could accommodate two water molecules even in single file is in contrast with the 2-A diameter predicted from the selectivity properties of alphabetagamma-rENaC. The fact that activation of alphabetagamma-rENaC by DeltaP can be reproduced by the imposition of Deltapi suggests that water movement through the channel is also an important determinant of channel activity.