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

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Abstract
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 butter concentrations. In this study microelectrodes were used to measure in situ and equilibrium pH (pHis and pHeq) and PCO2 in control and bicarbonate loaded rats before and after infusion of carbonic anhydrase. The disequilibrium pH method (pHdq = pHis - 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 duct and the adjacent vasa recta suggests trapping of CO2 in the medullary countercurrent system. The rise in PCO2 occurs both along the collecting duct and after exit from the papilla.