Proton compartmentation in rat renal cortical tubules

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Abstract
To study the control of renal ammoniagenesis, a technique was developed to estimate simultaneously intracellular (pHi) and intramitochondrial (pHm) pH in suspensions of rat renal cortical tubules. pHi was estimated with the fluorescent probe 2',7'biscarboxyethyl-5(6)-carboxy-fluorescein (BCECF). The intracellular distribution of the weak acid 5,5-dimethyloxazolidine-2,4-dione (DMO) allowed calculation of pHm with the use of values of pHi obtained with BCECF and tubule mitochondrial content. At medium pH (pHe) 7.4, pHi was 7.08 +/- 0.02. Over the pHe range 7.0-7.7, pHi was linearly related to pHe, but the pH gradient across the cell membrane decreased as pHe was lowered. No difference in the relationship between pHe and pHi was obtained when tubules were incubated in the presence of a nonbicarbonate or bicarbonate-buffered medium. Changes in pHe with bicarbonate-buffered media resulted in identical pHi values, whether the changes were induced by altered bicarbonate or CO2 content. At pHe 7.4, pHm was 7.78 +/- 0.6 in bicarbonate-buffered medium but was higher (0.2-0.3 pH units) when tubules were bathed in nonbicarbonate-buffered medium. pHm was linearly related to pHi in either buffer. The pH gradient across the inner mitochondrial membrane was also positively correlated with pHe. The present studies indicate the suitability of the techniques for estimating pHi and pHm simultaneously in suspensions of rat renal cortical tubules. Parallel changes occur in both intracellular compartments when pHe is altered. pHm, which is approximately 0.7 pH units greater than pHi, decreases in acute acidosis. This decrease may be important in stimulating renal ammoniagenesis, possibly by activation of alpha-ketoglutarate dehydrogenase.