CELLULAR HETEROGENEITY OF AMMONIUM ION-TRANSPORT ACROSS THE BASOLATERAL MEMBRANE OF THE HAMSTER MEDULLARY THICK ASCENDING LIMB OF HENLES LOOP

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 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. rhe basolateral membrane voltage (V(B)) Was measured by impaling cells with conventional microelectrodes. Addition of NH4+ to the bath depolarized V(B) 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 (DELTAVB(K+) or DELTAVB(NH4+)) Were compared, DELTAVB(NH4+) was almost the same as DELTAVB(K+) 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 DELTAVB(K+) and DELTAVB(NH4), whereas in the LBC cell it suppressed DELTAVB(K+) with a small effect on DELTAVB(NH4+), indicating that NH4+ is transported via a pathway distinct from Ba2+-sensitive K+ conductance. The VB deflection by NH4+ was unaffected by addition of 0.1 mM ouabain or 10 muM 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 muM inhibited DELTAVB(NH4+) 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.