NH4 + conductance in Xenopus laevis oocytes

 Current-clamp and voltage-clamp techniques were used to study the effects of NH₄ ⁺ on the cell membrane conductance in Xenopus laevis oocytes. Superfusing the oocytes with NH₄Cl resulted in a depolarization of the oocyte’s cell membrane potential and, at a clamp potential of –70 mV, in an inward current. The magnitude of the inward current was proportional to the NH₄Cl concentration in the extracellular solution and on membrane potential. The reversal potential, E _rev , was –35.5 ± 11.6 mV under control conditions and –3.1 ± 11.0 mV (n = 19) in the presence of NH₄Cl (10 mmol/l). Superfusion of the oocytes with nominally Ca²⁺-free solution affected the NH₄Cl-evoked response only marginally. Replacement of extracellular Na⁺ by N-methyl-D-glucamine⁺ markedly reduced, but did not eliminate, the NH₄Cl-sensitive current and shifted the reversal potential to more negative potentials. The NH₄Cl-induced current was substantially inhibited by 0.1 mmol/l flufenamate, and was less affected by blockers of the endogenous K⁺ conductance, Ba²⁺ and isosorbiddinitrate (ISDN). The results are compatible with the activation of a conductance by NH₄Cl for Na⁺ and NH₄ ⁺. The mechanism by which NH₄Cl activates the conductance remains unknown. Received: 9 December 1996 / Received after revision: 10 March 1997 / Accepted: 13 March 1997