A Cl- cotransporter selective for NH4+ over K+ in glial cells of bee retina

Marcaggi, P. and Coles, J. A. (2000) A Cl- cotransporter selective for NH4+ over K+ in glial cells of bee retina. Journal of General Physiology, 116(2), pp. 125-142. (doi: 10.1085/jgp.116.2.125)

Abstract

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 (pHi) in isolated bundles of glial cells using a fluorescent indicator. These changes in pHi 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 pHi 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 Km ≈ 7 mM. The inhibition of NH4+ flux by extracellular K+ appeared to be competitive, with an apparent Ki of ∼15 mM. A simple standard model of the transport process satisfactorily described the pHi 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+.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Coles, Dr Jonathan
• Authors: Marcaggi, P., and Coles, J. A.
• College/School: College of Medical Veterinary and Life Sciences > School of Infection & Immunity
• Journal Name: Journal of General Physiology
• Journal Abbr.: JGP
• Publisher: Rockefeller University Press
• ISSN: 0022-1295
• ISSN (Online): 1540-7748
• Copyright Holders: Copyright © 2000 The Authors
• First Published: First published in Journal of General Physiology 116(2):125-142
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher