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[Formula: see text] (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 pH i decrease or result in an initial increase in pH i when NH 3 /NH[Formula: see text] was added. However, after the cell was acidified (because of NH 3 /NH[Formula: see text]), raising pH B to 8.0 caused a slow increase in pH i but had no effect on membrane potential. The changes in pH i with raising pH B did not occur in the absence of NH 3 /NH[Formula: see text]. In AQP1 oocytes, exposure to NH 3 /NH[Formula: see text]usually resulted in little or no change in pH i , and in the absence of Na + there was a small increase in pH i (the cell still depolarized to near 0 mV). However, after exposure to NH 3 /NH[Formula: see text], raising pH B to 8.0 caused pH i to increase more than two times faster than in control oocytes. This increase in pH i is likely the result of increased NH 3 entry and not the result of NH[Formula: see text] transport. These results indicate that 1) the oocyte membrane, although highly permeable to NH[Formula: see text], has a significant NH 3 permeability and 2) NH 3 permeability is enhanced by AQP1.