Ammonium transport in medullary thick ascending limb of rabbit kidney: involvement of the Na+,K+,Cl(-)-cotransporter.

In order to investigate the question whether ammonium reabsorption in the thick ascending limb of Henle's loop (TALH) proceeds via the Na+,K+,Cl(-)-cotransporter, plasma membrane vesicles were prepared from TALH cells isolated from rabbit kidney outer medulla and the effect of NH+4 on their transport properties was investigated. It was found that, in the presence of a 78-mmol/liter NaCl gradient, 5 mmol/liter NH+4 inhibited bumetanide-sensitive rubidium flux by 86%; a similar decrease was observed for 5 mmol/liter, K+. Inhibition of bumetanide-sensitive rubidium uptake by NH+4 was competitive and an apparent Ki of 1.9 mmol/liter was found. Bumetanide-sensitive sodium uptake measured in the presence of a 83 mmol/liter KCl gradient was not inhibited by 5 mmol/liter NH+4. A 100-mmol/liter NH4Cl gradient was, however, capable of stimulating bumetanide-sensitive sodium uptake to the same extent as a KCl gradient. These data suggest that NH+4 is accepted by the K+ site of the Na+,K+,Cl-cotransport system and that the transporter can function in a Na+,NH+4,2Cl mode. Since the affinity of the transporter for NH+4 lies in the concentration range found in the TALH lumen in vivo, it is concluded that Na+,NH+4,2Cl-cotransport can contribute to the NH+4 reabsorption in this tubular segment.