Effect of respiratory acidosis on acidification by the medullary collecting duct

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Abstract
The effect of acute respiratory acidosis (ARA) on inner medullary collecting duct (IMCD) acidification was studied and the results were compared with previously obtained data from rats with acute metabolic acidosis (AMA). Microcatheterization was used to directly measure pH and PCO2 [partial pressure of CO2] with glass-membrane electrodes, and fluid samples were obtained for measurement of HCO3-, phosphate and ammonium. Arterial pH was 7.18 .+-. 0.01 and PCO2 was 88 .+-. 2 mmHg. The IMCD data were analyzed as a function of IMCD length (.apprx. 6 mm). pH decreased from 5.78 .+-. 0.07 to 5.27 .+-. 0.03 and PCO2 increased from 55 .+-. 4 to 75 .+-. 2 mmHg between origin and tip. HCO3- delivery decreased from 154 .+-. 34 to 25 .+-. 3 nmol/min but no change was noted in acid phosphate, NH4+ or net acid addition along the IMCD. Net acid excretion was not different from that found previously in AMA. Apparently, during ARA, acidification is augmented prior to, but not along, the IMCD. During AMA, IMCD plays a major regulatory role in urinary acidification, accounting for .apprx. 50% of the excreted H+.

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