Transport of NH3/NH4+ in oocytes expressing aquaporin-1

Abstract

The aim of this study was to determine whether expressing aquaporin (AQP)-1 could affect transport of NH3. Using ion-selective microelectrodes, the experiments were conducted on frog oocytes (cells characterized by low NH3 permeability) expressing AQP1. In H2O-injected oocytes, exposure to NH3/NH4+ (20 mM, pH 7.5) caused a sustained cell acidification and no initial increase in pHi (as expected from NH3 influx), and the cell depolarized to near 0 mV. The absence of Na+, the presence of Ba2+, or raising bath pH (pHB) did not inhibit the magnitude of the pHi decrease or result in an initial increase in pHi when NH3/NH4+ was added. However, after the cell was acidified (because of NH3/NH4+), raising pHB 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 NH3/NH4+. In AQP1 oocytes, exposure to NH3/NH4+ 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 NH3/NH4+, raising pHB to 8.0 caused pHi to increase more than two times faster than in control oocytes. This increase in pHi is likely the result of increased NH3 entry and not the result of NH4+ transport. These results indicate that 1) the oocyte membrane, although highly permeable to NH4+, has a significant NH3 permeability and 2) NH3 permeability is enhanced by AQP1.