NH4 + conductance in Xenopus laevis oocytes. III. Effect of NH3

Abstract

Exposure of Xenopus laevis oocytes to NH(4)Cl caused intracellular acidification, cell membrane depolarization and the generation of an inward current. To determine the contribution of uncharged NH(3) and positively charged NH(4)(+), the NH(4)Cl-induced inward current was measured in the presence of increasing [NH(3)] at constant [NH(4)Cl] (10 mM) or increasing [NH(4)Cl] at constant [NH(3)] (0.045 mM) with pH varying in both cases. At -70 mV, the NH(4)Cl-induced current was barely detectable at pH 6.5, 0.01 mM NH(3), but increased successively at pH 7.5, 0.1 mM NH(3) and pH 8.5, 1 mM NH(3). In contrast, NH(4)Cl-associated currents were independent of changes of the [NH(4)Cl] at constant [NH(3)] and variable pH. Similar results with respect to acidification, depolarization and inward current in response to concentration and pH changes were obtained with trimethylamine HCl. Increasing concentrations of the weak acid propionate led to a reduction of the NH(4)Cl-induced current. These data suggest that NH(3) entry may induce local alkalinization that, in turn, may trigger the opening of a conductance for NH(4)(+) or trimethylamine-H(+) entry.

Amt/MEP/Rh proteins conduct ammonia

“…Electroneutral NH 4 + / H + exchange has been inferred mainly from the pH dependence of substrate uptake rates but, as stated by the authors, the data do not really distinguish between NH 3 influx and NH 4 + /H + exchange. The induction of current observed in some studies with frog oocytes most likely is due to the presence of endogenous NH 4 + permeable channels that are activated by local alkalinization upon NH 3 influx [5]. In the absence of heterologously expressed ammonia channels, this response to ammonia influx is seen only above about 1 mM external NH 4 + at pH 7.5 [5,23].…”

“…Monitoring this pH change via a pH sensitive fluorescent dye has been used to follow NH 3 equilibration processes in liposomes [22] or resealed red blood cell ghosts [34]. In oocytes, rapid alkalinization can apparently induce secondary responses like the activation of other transport systems, pumps, or channels [5].…”

“…The induction of current observed in some studies with frog oocytes most likely is due to the presence of endogenous NH 4 + permeable channels that are activated by local alkalinization upon NH 3 influx [5]. In the absence of heterologously expressed ammonia channels, this response to ammonia influx is seen only above about 1 mM external NH 4 + at pH 7.5 [5,23]. The significant currents seen in RhBG [32] and RhCG oocytes [2] at even lower concentrations may, therefore, be the consequence of RhBG/RhCG-enhanced NH 3 permeability rather than proof of NH 4 + transport.…”

Functional characterization of Rhesus glycoproteins from an ammoniotelic teleost, the rainbow trout, using oocyte expression and SIET analysis

“…These cells, when exposed to ammonia, display a classic biphasic rise and fall of intracellular pH (pH i ; basis of the pre-pulse method) caused by the influx of NH 3 followed by a lower influx of NH 4 + (Boron and DeWeer, 1976). Xenopus oocytes are exceptional in that exposure to high ammonia concentrations (≥1mmoll -1 ) leads to a paradoxical fall in pH i , depolarization, and an inward current (Bakouh et al, 2004;Boldt et al, 2003;Burckhardt and Burckhardt, 1997;Burckhardt and Fromter, 1992). These changes are thought to be caused by an initial rapid diffusion of NH 3 into the oocyte that causes an alkalinization close to the oocyte surface which subsequently activates nonselective cation channels through which NH 4 + could enter (Boldt et al, 2003;Cougnon et al, 1996).…”

“…Xenopus oocytes are exceptional in that exposure to high ammonia concentrations (≥1mmoll -1 ) leads to a paradoxical fall in pH i , depolarization, and an inward current (Bakouh et al, 2004;Boldt et al, 2003;Burckhardt and Burckhardt, 1997;Burckhardt and Fromter, 1992). These changes are thought to be caused by an initial rapid diffusion of NH 3 into the oocyte that causes an alkalinization close to the oocyte surface which subsequently activates nonselective cation channels through which NH 4 + could enter (Boldt et al, 2003;Cougnon et al, 1996). Low concentrations of ammonia (<1mmoll -1 ), however, cause little change in pH i and no inwardly induced current (Bakouh et al, 2004;Holm et al, 2005;Mayer et al, 2006).…”