A micropuncture evaluation of renal ammonia excretion in the rat

Abstract

Measurements of ammonia and inulin concentration were performed on samples of proximal or distal tubular fluid from 46 diuretic rats, some of which were pretreated with oral ammonium chloride. During mannitol diuresis alone, the ammonia concentration of midproximal tubular fluid averaged .77 ± .46 mm, while that of distal tubular fluid averaged 3.2 ± 1.1 mm. Midproximal ammonia was sufficient to account for as much as 62–70% of urine ammonia, while the distal tubular ammonia concentration theoretically provided 93–114%. After chronic ammonium chloride administration, the midproximal ammonia concentration rose to 1.5 ± .8 mm, while that of distal tubular fluid equaled 5.29 ± 1.3 mm. Net ammonia addition to collecting duct fluid was not obtained in either normal or acidotic rats. These observations demonstrate that the proximal tubule is an important potential source of final urine ammonia, and that increased proximal ammonia addition also occurs in response to chronic ammonium chloride acidosis. Although distal tubular ammonia addition probably occurs also, definite statements cannot be made regarding its magnitude in either normal or acidotic rats. However, it is apparent that the collecting duct is not an important source of final urine ammonia.