Transport of NH3/NH+4 in oocytes expressing aquaporin-1

The aim of this study was to determine whether expressing aquaporin (AQP)-1 could affect transport of NH(3). Using ion-selective microelectrodes, the experiments were conducted on frog oocytes (cells characterized by low NH(3) permeability) expressing AQP1. In H(2)O-injected oocytes, exposure to NH(3)/ NH(4)(+) (20 mM, pH 7.5) caused a sustained cell acidification and no initial increase in pH(i) (as expected from NH(3) influx), and the cell depolarized to near 0 mV. The absence of Na(+), the presence of Ba(2+), or raising bath pH (pH(B)) did not inhibit the magnitude of the pHi decrease or result in an initial increase in pH(i) when NH(3)/NH(4)(+) was added. However, after the cell was acidified (because of NH(3)/NH(4)(+)), raising pH(B) to 8.0 caused a slow increase in pHi but had no effect on membrane potential. The changes in pHi with raising pHB did not occur in the absence of NH(3)/NH(4)(+). In AQP1 oocytes, exposure to NH(3)/NH(4)(+) usually resulted in little or no change in pHi, and in the absence of Na(+) there was a small increase in pHi (the cell still depolarized to near 0 mV). However, after exposure to NH(3)/NH(4)(+), raising pH(B) to 8.0 caused pH(i) to increase more than two times faster than in control oocytes. This increase in pHi is likely the result of increased NH(3) entry and not the result of NH(4)(+) transport. These results indicate that 1) the oocyte membrane, although highly permeable to NH(4)(+), has a significant NH(3) permeability and 2) NH(3) permeability is enhanced by AQP1.