Ammonium and bicarbonate transport in isolated perfused rodent ascending limbs of the loop of Henle.

Ammonium accumulates in the renal medullas of antidiuretic animals, presumably due to countercurrent multiplication, driven by NH4+ absorption from ascending loops of Henle in the outer and inner medulla. Active absorption of NH4+ occurs in the thick ascending limb (TAL). But the passive transport properties of NH3 in the TAL and the transport of ammonium or HCO3- in the ascending thin limb (ATL) have not been studied in rodents. To investigate the potential role of the ascending limb segments in medullary accumulation of ammonium, we perfused isolated subsegments of the chinchilla ATL and of the rat ATL and TAL. After imposing concentration gradients of total ammonia or total CO2 across ATL subsegments, we found very high rates of transfer of both substances, implying that at physiological flow rates the tubule luminal fluid quickly equilibrates with the interstitium. In the medullary TAL, we found a passive NH4+ permeability of 17 x 10(-5) cm/s but a relatively low NH3 permeability of < 0.003 cm/s. The low NH3 permeability prevents backleak of NH3 when NH4+ is actively transported from the lumen. We conclude that the ATL acts as an equilibrating segment and the TAL has special permeability properties that enhance net ammonium absorption and therefore enhance medullary ammonium accumulation.