The recent demonstration of elevated PCO2 in structures of the rat renal cortex indicated that previous determinations of disequilibrium pH (pHDq), and thus the differentiation of H+ secretion from
bicarbonate reabsorption per se, required further evaluation. A new aspiration pH
electrode was developed to allow tubule fluid to achieve chemical equilibrium at the PCO2 prevailing in vivo. In control and
bicarbonate-loaded rats a pHDq was not observed in either proximal or distal tubules. After intravenous
benzolamide a significant
acid pHDq was observed in the proximal (but not the distal) nephron, and increased further during metabolic
alkalosis. During combined metabolic
alkalosis and
respiratory acidosis a significant pHDq was present in the distal but not in the proximal tubule.
Aldosterone administration to
bicarbonate-loaded, hypercapnic rats did not alter the distal pHDq further. When present, the pHDq in the distal tubule was obliterated by
carbonic anhydrase infusion. We conclude that proximal but not distal tubule fluid is in functional contact with
carbonic anhydrase; the
enzyme is in excess in the proximal lumen and H2CO3 did not accumulate even during conditions associated with increased H+ secretion; the basal rate of H+ secretion in the distal nephron accessible to cortical
micropuncture is less than previously assumed. The data support the view that H+ secretion is the major mechanism of renal
bicarbonate reabsorption.