Effect of cortical-medullary gradient for ammonia on urinary excretion of ammonia

Previous studies suggested that a portion of ammonia secreted into the proximal tubule may diffuse directly from Henle's loop into the medullary collecting duct. Since water is absorbed along the course of the descending portion of the loop, it was proposed that the concentration of ammonia increased in loop fluid, and that rapid diffusibility of the free base would facilitate the delivery of ammonia into medullary interstitium where a high level could be maintained by the countercurrent exchange process. In this schema it was proposed that there was an ammonia concentration gradient between medullary structures and cortex, and recovery of ammonia by the medullary collecting duct due to the low pH in tubule fluid at that site. The present study was designed to evaluate this hypothesis by estimating ammonia concentrations in medullary and cortical tissue, and by correlating medullary levels with secretion rate into the inner medullary collecting duct. In control animals the concentration of total ammonia (NH4+ + NH3+) in inner medullary vasa recta was 9.2 +/- 1.5 mumoles/ml, a level 100-fold higher than the cortical level of 0.10 +/- 0.01. During acute acidosis the medullary level rose to 22.5 +/- 2.7 mumoles/ml, but in acute acidosis during mannitol infusion the level fell to 8.0 +/- 1.2. The rate of ammonia secretion into inner medullary collecting duct fluid correlated directly with medullary vasa recta ammonia concentration. These data provide evidence for a steep ammonia concentration gradient between the medulla and cortex, and suggest that the diffusion gradient across collecting duct epithelium governs the rate of the addition of ammonia to collecting duct fluid.