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Original Article

^a Institut National de la Santé et de la Recherche Medicale U394 Neurobiologie intégrative, Institut François Magendie, 33077 Bordeaux cedex, France
^b Institut National de la Santé et de la Recherche Medicale U438 RMN Bioclinique, CHU Grenoble, 38043 Grenoble cedex 09, France
Department of Physiology, University College London, Gower Street, London WC1E 6BT, UK.0044 171 413 8395

p.marcaggi{at}ucl.ac.uk

There appears to be a flux of ammonium (NH₄⁺/NH₃) from neurons to glial cells in most nervous tissues. In bee retinal glial cells, NH₄⁺/NH₃ uptake is at least partly by chloride-dependant transport of the ionic form NH₄⁺. Transmembrane transport of NH₄⁺ has been described previously on transporters on which NH₄⁺ replaces K⁺, or, more rarely, Na⁺ or H⁺, but no transport system in animal cells has been shown to be selective for NH₄⁺ over these other ions. To see if the NH₄⁺-Cl^– cotransporter on bee retinal glial cells is selective for NH₄⁺ over K⁺ we measured ammonium-induced changes in intracellular pH (pH_i) in isolated bundles of glial cells using a fluorescent indicator. These changes in pH_i result from transmembrane fluxes not only of NH₄⁺, but also of NH₃. To estimate transmembrane fluxes of NH₄⁺, it was necessary to measure several parameters. Intracellular pH buffering power was found to be 12 mM. Regulatory mechanisms tended to restore intracellular [H⁺] after its displacement with a time constant of 3 min. Membrane permeability to NH₃ was 13 µm s^–1. A numerical model was used to deduce the NH₄⁺ flux through the transporter that would account for the pH_i changes induced by a 30-s application of ammonium. This flux saturated with increasing [NH₄⁺]_o; the relation was fitted with a Michaelis-Menten equation with K_m 7 mM. The inhibition of NH₄⁺ flux by extracellular K⁺ appeared to be competitive, with an apparent K_i of 15 mM. A simple standard model of the transport process satisfactorily described the pH_i changes caused by various experimental manipulations when the transporter bound NH₄⁺ with greater affinity than K⁺. We conclude that this transporter is functionally selective for NH₄⁺ over K⁺ and that the transporter molecule probably has a greater affinity for NH₄⁺ than for K⁺.

Key Words: ammonia • K-Cl cotransporter • neuroglia • pH • Apis

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