Transepithelial ammonia concentration gradients in inner medulla of the rat

Transport of NH₃ from loops of Henle to medullary collecting ducts has been proposed to play an important role in renal ammonia excretion. To determine whether transepithelial ammonia concentration gradients capable of driving this transport are present in the inner medulla, micropuncture experiments were performed in control rats and in rats with chronic metabolic acidosis. In situ pH and total ammonia concentrations were measured to calculate NH₃ concentrations ([NH₃]) for base and tip collecting duct, loop of Henle, and vasa recta. In control and acidotic rats, [NH₃] in the loop of Henle was significantly greater than [NH₃] in the collecting ducts. [NH₃] did not differ in loop of Henle and adjacent vasa recta in either group of rats, indicating that NH₃ concentration gradients between loop and collecting duct represent NH₃ gradients that are present between medullary interstitium and collecting duct. During acidosis, an increase in collecting duct ammonia secretion was associated with an increase in the NH₃ concentration difference between loop of Henle and collecting duct but occurred in the absence of a fall in collecting duct pH. The NH₃ concentration gradient favoring diffusion of NH₃ into the collecting ducts increased during acidosis because [NH₃] in the loop of Henle and medullary interstitium increased more than [NH₃] in the collecting duct. These findings indicate that transport processes involved in medullary ammonia accumulation play an important role in regulating ammonia secretion into the inner medullary collecting duct in vivo and that a fall in inner medullary collecting duct pH is not necessarily required for ammonia secretion by this segment to increase during chronic metabolic acidosis.