Ammonia addition into the medullary collecting duct of the rat.

The purpose of these studies was to determine if ammonia is added directly to the medullary collecting duct of the rat, and if so, to estimate its quantitative contribution to ammonium excretion. Samples of fluid were obtained along the length of the medullary collecting duct by retrograde microcatheterization. To document net addition of ammonium, we measured the ammonium concentration in tubular fluid by an enzymatic isotope technique, and we calculated the degree of fluid reabsorption from the ratio of the inulin concentration in tubular fluid and plasma. The ammonium concentration corrected for water reabsorption ([NH4+]/[TF/P]In) rose progressively from the beginning to the end of the medullary collecting duct, indicating net ammonia addition to this nephron segment. By calculation, it appears that about 40% of the excreted ammonium reached the urine by direct addition of ammonia to the duct fluid.