Hydrogen ion secretion by the collecting duct as a determinant of the urine to blood PCO2 gradient in alkaline urine.

Abstract

A B S T R A C T Several theories have been advanced to explain the elevation in urinary Pco2 during bicarbonate loading and include: (a) H+ secretion, (b) countercurrent system for CO2, (c) the "ampholyte" properties of bicarbonate, and (d) mixing of urine of disparate bicarbonate and buffer concentrations. In this study microelectrodes were used to measure in situ and equilibrium pH (pHi, and pHeq) and Pco2 in control and bicarbonate loaded rats before and after infusion of carbonic anhydrase. The disequilibrium pH method (pHdq = pHi, -pHeq) was used to demonstrate H+ secretion. Control rats excreting an acid urine (pH = 6.04±0.06) failed to display a significant disequilibrium pH at the base (BCD), or tip (TCD) of the papillary collecting duct. Urine pH (7.54±0.12), and urine to blood (U-B) Pco2 increased significantly during NaHCO3 loading while Pco2 at the BCD and TCD also increased (95±4 and 122±4). Furthermore, an acid disequilibrium pH was present at both the BCD and TCD (-0.42±0.04 and -0.36±0.03) and was obliterated by carbonic anhydrase. Comparison of the Pco2 in the BCD or TCD with the adjacent vasa recta revealed similar values (r = 0.97). It is concluded that H+ secretion by the collecting duct into bicarbonate containing fluid with delayed dehydration of H2CO3, is the most likely determinant of the U-B Pco2 in alkaline urine. Similar values for Pco2 in the collecting

Effect of selective aldosterone deficiency on acidification in nephron segments of the rat inner medulla.

“…During acute bicarbonate loading urine pH was equally alkaline in adrenal intact controls and in aldosteronedeficient rats (Table VI) were significantly above arterial blood levels, as has been observed previously in our laboratory (14). Moreover, Pco2 increased from base to tip collecting duct (82±5 to 118±3 mmHg, P < 0.01).…”

“…Control-bicarbonate loaded rats (CON-HCO3) (n = 25) were adrenal intact normal controls that received a hypertonic sodium bicarbonate solution (300 mM NaHCO3, 25 mM KCl) at a rate equal to 1.8% body wt/h for 2.0 h before micropuncture or until stability of urine pH and Pco2 was assured as described previously (14). Rats in this group were subjected to papillary micropuncture and were employed to determine urinary and papillary Pco2 as described previously ( 14).…”

“…Surgical exposure of the extrarenal papilla and papillary micropuncture was accomplished exactly as described previously (9,14). Arterial blood acid-base status was carefully monitored and maintained as described previously (9).…”

“…Arterial blood gases were monitored and maintained as described previously (9,14). Tubule fluid samples from the bend of Henle's loop (ascending and descending limbs) and from the base and tip ofthe papillary collecting duct were collected as described previously (15).…”

“…Pco2 was measured with Pco2 microelectrodes of 15-20 Am tip diameter as routinely manufactured in our laboratory and described in detail previously (14). Calibration in three analytically balanced gases was performed as described previously ( 14) before and after each set of data collections (n = 4-6) by direct puncture ofthe base papillary collecting duct or by insertion into the opening of the tip of the papillary collecting duct.…”