Free-flow micropuncture experiments were performed to examine ammonia transport separately in early and late proximal convoluted tubule (PCT) of the rat. In control rats, ammonia was secreted along the early PCT but was reabsorbed along the late PCT. In rats with chronic metabolic acidosis, ammonia secretion along the early PCT was increased with controls, and ammonia absorption by the late PCT was converted to small net ammonia secretion. In the acidotic rats, ammonia secretion rate in the early PCT was six times higher than that in the late PCT. Thus, most or all of ammonia secretion by the PCT occurred along its early portion. In control and acidotic animals, luminal NH3 concentration in the early PCT was significantly higher than than in the late PCT, indicating that ammonia is not in diffusion equilibrium throughout the renal cortex. It is proposed that differences in ammonia transport rate in early vs. late PCT may be due to differences in ammonia production and/or to differences in the rate of an ammonia backflux that detracts from net ammonia secretion.
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