Elevated urinary PCO2 in the rat: an intrarenal event

During a bicarbonate diuresis, final urine Pco2 considerably exceeds systemic Pco2, an effect thought to reflect the postpapillary delayed dehydration of carbonic acid. To test this explanation, Pco2 tensions along the inner medullary collecting duct (IMCD) of bicarbonate-loaded rats were measured directly using Pco2 microelectrodes. With systemic Pco2 held at 40 mm Hg, IMCD Pco2 exceeded systemic Pco2 in every measurement by an average of 20 to 30 mm Hg. A significant increment in Pco2 was seen between 50% IMCD length and the papilla tip. During the infusion of carbonic anhydrase, IMCD Pco2 was reduced but not to systemic levels. Finding elevated Pco2 along the terminal IMCD deemphasizes the importance of postpapillary delayed dehydration and suggests the possibility that bicarbonaturia is associated with papillary accumulation of carbon dioxide.