Ammonium and bicarbonate transport in isolated perfused rodent long-loop thin descending limbs

Ammonium accumulates in the renal medullas of antidiuretic mammals. The accumulation process is thought to involve countercurrent multiplication, energy-dependent recycling between the ascending and descending limbs of Henle's loop. To investigate the role of the long-loop thin descending limb (LDL) in countercurrent multiplication of ammonium, we have perfused outer medullary and inner medullary subsegments of the chinchilla LDL (and inner medullary subsegments of rat LDL) in vitro and measured the fluxes of total ammonia and total CO2. No spontaneous fluxes of total ammonia or total CO2 occurred in the absence of imposed concentration gradients. When transepithelial concentration gradients were imposed, passive total ammonia and total CO2 transport were observed in all subsegments, although the permeabilities varied with distance along the descending limb. Passive total ammonia transport occurred through a combination of NH3 and direct NH4+ permeation. The outer medullary segment was the most permeable to NH4+. The deep inner medullary segment was the most permeable to bicarbonate. Addition of carbonic anhydrase to the lumen accelerated passive NH3 entry in the outer medullary LDL, indicating that little or no luminal carbonic anhydrase is endogenously present. The passive secretion of NH4+ and NH3 into the LDL may contribute to the countercurrent multiplication of ammonium in the rodent renal medulla.