Pathways of NH3/NH4+ permeation across Xenopus laevis oocyte cell membrane

While acid loading with extracellular NH4Cl solutions usually first alkalinizes the cells through NH3 influx, and acidifies only when NH4Cl is removed, Xenopus oocytes became immediately acidic upon NH4Cl addition and the cells did not acidify further upon its removal. Since NH4Cl solutions also collapsed the membrane potential (Vm) and resistance (Rm), we conclude that primarily NH4+ entered the cells where it liberated H+, with NH3 being trapped in intracellular lipid stores. To identify the NH4+ permeation pathway we have used K+ channel blockers (Ba2+, Cs+, tetraethylammonium, quinidine), various cation transport inhibitors (ouabain, bumetanide, amiloride) and other inhibitors, some of which block non-selective cation channels (La3+, diphenylamine-2-carboxylate, and p-chloromercuribenzoate). However, only the latter substances partially prevented the collapse of Vm and Rm. This suggests, that NH4+ passes through non-selective cation channels. In accordance with the voltage dependence and/or stretch activation of such channels NH4+ fluxes appeared to be asymmetric. NH4+ influx, which depolarized and swelled the cells, was large and acidified rapidly, while the efflux, which repolarized and shrank the cells, was slow and alkalinized only slowly.