title: Cellular heterogeneity of ammonium ion transport across the basolateral membrane of the hamster medullary thick ascending limb of Henle's loop.
abstract: The epithelia of the medullary thick ascending limb (MAL) consists of two cell types, high (HBC) and low basolateral conductance (LBC) cell, depending on the K+ conductance of the basolateral membrane. The NH4+ conductance distinct from the K+ conductance has been suggested to exist in the proximal tubule, MAL cell, and Xenopus oocyte. The present study was designed to examine whether there is a conductive NH4+ transport system distinct from K+ conductance in two different cell types of the hamster MAL perfused in vitro. The basolateral membrane voltage (VB) was measured by impaling cells with conventional microelectrodes. Addition of NH4+ to the bath depolarized VB in a dose-dependent manner in both cell types. The response was maintained in the absence of HCO3-. When the VB deflection elicited upon 50 mM KCl or NH4Cl in the bath (delta VBK+ or delta VBNH4+) were compared, delta VBNH4+ was almost the same as delta VBK+ in the HBC cell, whereas the former was greater than the latter in the LBC. In the HBC cell, 10 mM Ba2+ in the bath equally suppressed both delta VBK+ and delta VBNH4+, whereas in the LBC cell it suppressed delta VBK+ with a small effect on delta VBNH4+, indicating that NH4+ is transported via a pathway distinct from Ba(2+)-sensitive K+ conductance. The VB deflection by NH4+ was unaffected by addition of 0.1 mM ouabain or 10 microM 5-nitro-2-(3-phenylpropylamino)-benzoate (a Cl- channel blocker) to the bath, excluding the contribution of the Na+, K+ pump or Cl- channel. An abrupt reduction of Na+ in the bath from 200 to 20 mM did not cause any changes in VB, suggesting that a nonselective cation channel may not account for the NH4+ transport. Amiloride at 10 microM inhibited delta VBNH4+ with a higher efficacy in the LBC cell. We conclude that a rheogenic NH4+ transport system independent from the K+ conductance exists in the basolateral membrane of the LBC cell of the hamster MAL, and may play some roles in the regulation of NH4+ transport.
authors.affiliation: Department of Pharmacology, Jichi Medical School, Tochigi, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Tsuruoka
authors.forename: M
authors.initials: M
authors.lastname: Takeda
authors.forename: K
authors.initials: K
authors.lastname: Yoshitomi
authors.forename: M
authors.initials: M
authors.lastname: Imai
mesh_terms: D000641:Ammonia / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000494:pharmacology; D001692:Biological Transport; D002462:Cell Membrane / Q000502:physiology; D006224:Cricetinae; D005260:Female; D066298:In Vitro Techniques; D007679:Kidney Medulla / Q000502:physiology*; D007700:Kinetics; D008138:Loop of Henle / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D008647:Mesocricetus; D011188:Potassium / Q000494:pharmacology
chemical_list: D001639:Bicarbonates; D000641:Ammonia; D011188:Potassium