STREAMING POTENTIAL MEASUREMENTS IN ALPHA-BETA-GAMMA-RAT EPITHELIAL NA+ CHANNEL IN PLANAR LIPID BILAYERS

Streaming potentials across cloned epithelial Na+ channels (ENaC) inco rporated into planar lipid bilayers were measured. We found that the e stablishment of an osmotic pressure gradient (Delta pi) across a chann el-containing membrane mimicked the activation effects of a hydrostati c pressure differential (Delta P) on alpha beta gamma-rENaC, although with a quantitative difference in the magnitude of the driving forces. Moreover, the imposition of a Delta pi negates channel activation by Delta P when the Delta pi was directed against Delta P. A streaming po tential of 2.0 +/- 0.7 mV was measured across alpha beta gamma-rat ENa C (rENaC)-containing bilayers at 100 mM symmetrical [Na+] in the prese nce of a 2 Osmol/kg sucrose gradient. Assuming single file movement of ions and water within the conduction pathway, we conclude that betwee n two and three water molecules are translocated together with a singl e Na+ ion. A minimal effective pore diameter of 3 Angstrom that could accommodate two water molecules even in single file is in contrast wit h the 2-Angstrom diameter predicted from the selectivity properties of alpha beta gamma-rENaC. The fact that activation of alpha beta gamma- rENaC by Delta P can be reproduced by the imposition of Delta pi sugge sts that water movement through the channel is also an important deter minant of channel activity.