Characterization of acidification by the isolated perfused rat kidney: evidence for adaptation by the distal nephron to a high bicarbonate diet.

To characterize acidification by the distal nephron of the intact kidney independent of alterations in buffer availability, we subjected isolated rat kidneys perfused with glucose as the sole substrate to stepwise acidification of the perfusate. In response to progressive perfusate acidification with hydrochloric acid, a maximal pH gradient between urine and perfusate, which averaged 1.71 +/- 0.12, was achieved at a mean perfusate pH of 6.89 +/- 0.04. The maximum pH gradient was increased when sulfuric acid rather than hydrochloric acid was used to acidify the perfusate, and it was decreased by 0.67 when amiloride (10(-5) M) was added to the perfusate. Thus, hydrogen ion transport by the distal nephron of the intact rat kidney appears to be amenable to study, and it responds similarly to the hydrogen ion pump of anuran urinary epithelia. Kidneys from animals subjected to a variety of dietary regimens were studied in a response to stepwise perfusate acidification with hydrochloric acid. Ammonium excretion averaged 0.49 +/- .03 mumoles/min and did not differ significantly between any of the dietary groups. Chronic acidosis and the ingestion of either a low or high salt diet had no influence on the maximal pH gradient. Neither a low nor a high potassium diet affected the pH gradient, suggesting that the difference in urine pH between these two conditions in vivo is the result of differences in ammonia production. Ingestion of a high bicarbonate diet significantly decreased the pH gradient to 1.20 +/- 0.09. Thus, an adaptive change in distal nephron hydrogen ion transport occurs in the rat kidney in response to chronic ingestion of alkali.