Ammonium transport in medullary thick ascending limb of rabbit kidney: Involvement of the Na+, K+, Cl−-cotransporter

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Abstract
Summary 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 apparentK i 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.