K+/NH4+ antiporter: a unique ammonium carrying transporter in the kidney inner medulla

AbstractThe mechanism of NH4+ transport in inner medulla is not known. The purpose of these experiments was to study the process that is involved in ammonium (NH4+) transport in cultured inner medullary collecting duct (mIMCD-3) cells. Cells grown on coverslips were exposed to NH4+ and monitored for pHi changes by the use of the pH-sensitive dye BCECF. The rate of cell acidification following the initial cell alkalinization was measured as an index of NH4+ transport. The rate of NH4+ transport was the same in the presence or absence of sodium in the media (0.052±0.003 vs 0.048±0.004 pH/min, P>0.05), indicating that NH4+ entry into the cells was independent of sodium. The presence of ouabain, bumetanide, amiloride, barium, or 4,4′-di-isothiocyanostilbene-2-2′-disulfonic acid (DIDS) did not block the NH4+-induced cell acidification, indicating lack of involvement of Na+:K+-ATPase, Na+:K+:2 Cl− transport, Na+:H+ exchange, K+ channel, or Cl−/base exchange, respectively, in NH4+ transport. The NH4+-induced cell acidification was significantly inhibited in the presence of high external [K+] as compared to low external [K+] (0.018±0.001 vs. 0.049±0.003 pH/min for 140 mM K+ vs. 1.8 mM K+ in the media, respectively, P