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.

A B S T R A C T Previous studies evaluating the mechanism of renal HCO-reabsorption have assumed equilibrium between systemic arterial blood and tubular fluid Pco2. We have recently reported that the Pco2 in proximal and distal tubular fluid as well as the stellate vessel significantly exceeded arterial Pco2 by 25.9 +0.92 mm Hg. The purpose of this study was to determine directly, for the first time, pH, Pco2, and total CO2 concentration in the accessible structures of the rat renal cortex with both microelectrodes and microcalorimetry. In addition, the concentrations of chloride and total CO2 were compared in the stellate vessel. The data demonstrate that: (a) values for total [CO2]

A B S T R A C T The mechanism by which the kidney reabsorbs sodium bicarbonate could be a result of: (a) H+ secretion, (b) direct HCO-

Previous micropuncture studies utilizing indirect methods to estimate bicarbonate transport in the rat superficial distal tubule have indicated that the distal bicarbonate reabsorptive process normally operates well below the saturation level. Recent studies from our laboratory failed to demonstrate a spontaneous acid disequilibrium pH in this segment, implying that the bicarbonate reabsorptive rate was less than previously estimated. The purpose of the present experiments were 1) to measure the rate of absolute bicarbonate reabsorption by the rat superficial distal tubule while controlling bicarbonate delivery, and 2) to examine the effects of alterations in acid-base status on the rate of bicarbonate reabsorption. Five groups of rats in different states of acid-base balance were studied. No significant bicarbonate reabsorption was detected in the control hydropenic, combined respiratory acidosis-metabolic alkalosis, acute respiratory acidosis, or acute metabolic acidosis groups. In contrast, metabolic acidosis of 3 days duration resulted in a significant bicarbonate reabsorptive rate of 52.6 +/- 13.9 pmol . mm-1 . min-1. The observation of significant bicarbonate reabsorption in the distal tubule only during chronic metabolic acidosis of 3 days duration is compatible with adaptation of this normally low-capacity segment to chronic changes in systemic acid-base states.

The purpose of this study was to determine and compare pH, PCO2, and fractional bicarbonate delivery in both superficial and juxtamedullary nephrons by microelectrode techniques and microcalorimetry in the rat in vivo in order to define more clearly the role of deeper nephron segments in urinary acidification. Values for pH and total CO2 concentration ([tCO2]) at the bend of Henle's loop (LOH) (7.39 +/- 0.04 units and 20.5 +/- 1.5 mM) were significantly greater and the PCO2 was significantly less (36.6 +/- 1.5 mmHg) than values for these same parameters in the superficial late proximal tubule (LPT) (6.78 +/- 0.03 units, 8.1 +/- 1.2 mM, and 63.2 +/- 1.0 mmHg, P less than 0.001). The fraction of filtered bicarbonate delivered to the LPT and LOH did not differ, however (12.2 +/- 2.5 vs. 9.0 +/- 0.8%). The pH and PCO2 values in the late distal tubule (6.59 +/- 0.04 units and 64.0 +/- 1.3 mmHg) were significantly greater than at the base (6.24 +/- 0.07 units and 34.5 +/- 1.5 mmHg) and tip (6.12 +/- 0.03 units and 35.2 +/- 1.2 mmHg) of the papillary collecting duct. The [tCO2] in the LOH and an adjacent vasa recta was compared and did not differ significantly (20.5 +/- 1.5 vs. 21.2 +/- 1.3 mM, P greater than 0.05). In summary, we have demonstrated significant alkalinization of tubule fluid in the deep LOH as a result of water abstraction and CO2 diffusion from the nephron. Our results suggest that a spontaneous disequilibrium pH may not exist in the LOH. Furthermore, similar values for [tCO2] in vasa recta and the LOH suggest that passive HCO-3 reabsorption in the thin ascending limb of Henle would be unlikely and does not contribute to the "loop" component of bicarbonate reabsorption.