EFFECTS OF BARIUM AND 5-(N-ETHYL-N-ISOPROPYL)-AMILORIDE ON PROXIMAL TUBULE AMMONIA TRANSPORT

Ionic NH4+ transport is an important mode of ammonia transport in the proximal convoluted tubule (PCT). NH4+ transport via the Na-H exchanger has been previously demonstrated. Potassium channels are present in the proximal tubule, but their role in ammonia transport has not been evaluated. We studied rat PCTs perfused in situ at 30 nl/min with solutions containing 5 mM HCO3; ammonia was measured with a fluorometric method. When perfused with the potent amiloride analogue 5-(N-ethyl-N-isopropyl)-amiloride (EIPA, 500-mu-M) or with BaCl2 (10 mM), ammonia entry (14.3 +/- 1.7 and 11.8 +/- 1.5 pmol.min-1.mm-1, respectively) was unaffected compared with control (12.8 +/- 1.0 pmol.min-1.mm-1). However, the combination of EIPA and barium inhibited entry (7.4 +/- 1.0 pmol.min-1.mm-1, P < 0.02 vs. other groups). Also, when perfused with 10 mM ammonia, neither EIPA nor BaCl alone blocked ammonia loss (70.5 +/- 9.1 and 60.5 +/- 5.9 pmol.min-1.mm-1, respectively) compared with control (53.4 +/- 6.0 pmol.min-1.mm-1). However, the combination inhibited ammonia loss (29.2 +/- 6.3 pmol.min-1.mm-1, P < 0.025 vs. other groups). Thus blocking both the Na-H exchanger and potassium channels decreases PCT ammonia transport. As the combination was required, this implies that multiple pathways exist for NH4+ transport in the PCT. This is the first demonstration that a mode of NH4+ transport other than via the Na-H exchanger is important in this segment, and the data are most consistent with transport of ammonia via potassium channels.