Permeabilities of rat collecting duct segments to NH3 and NH4+

We have measured NH3 and NH4+ permeability coefficients in collecting ducts from the cortex, outer medulla, and inner medulla of the rat kidney. Isolated collecting duct segments of the rat were perfused with bicarbonate-buffered solutions containing carbonic anhydrase to eliminate any pH disequilibrium in the tubule lumen. NH3 or NH4+ concentration gradients were set up between the bath and the lumen. By measuring the total CO2 and total ammonia concentrations in the bath, the perfusate, and collected fluid, the NH3 and NH4+ concentrations were determined. Then, using the flow rate in the tubule and the tubule dimensions, we calculated the apparent permeability in each collecting duct segment for NH3 and NH4+. The NH3 permeabilities were as follows: 0.002 cm/s in the inner medullary collecting duct, 0.012 cm/s in the outer medullary collecting duct, and 0.024 cm/s in the cortical collecting duct. The NH4+ permeabilities for all segments were on the order of 10(-5) cm/s or less. The relative permeability values for the two ammonium species are consistent with the view that the secretion observed in vivo in collecting ducts is due to passive diffusion of NH3 from the interstitium to the lumen of the duct, parallel with H+ secretion.