Cellular response to acute respiratory acidosis in rat medullary collecting duct

The collecting duct of the mammalian kidney is involved in urine acidification. Recent studies in the turtle bladder suggest that hydrogen ion secretion in response to elevated CO2 is regulated by insertion of hydrogen pumps into the luminal membrane of the mitochondria-rich cells. Because intercalated cells of the collecting duct are structurally similar to mitochondria-rich cells of the amphibian bladder, we studied the rat outer medullary collecting duct (OMCD) during respiratory acidosis to determine whether changes compatible with hydrogen ion secretion occur in the intercalated cells. Rats were studied during normal acid-base conditions and after 4-5 h of respiratory acidosis. After collection of physiologic data, the kidneys were fixed by in vivo perfusion and processed for electron microscopy. No changes were observed in the principal cells of the OMCD. Morphometric analysis revealed a significant increase in the surface density of the apical plasma membrane and a decrease in the number of tubulovesicular profiles in the apical region of the intercalated cells throughout the OMCD with respiratory acidosis. There were no changes in surface density of the basolateral membrane. These findings suggest that in response to respiratory acidosis there is transport of membrane from the tubulovesicular membrane compartment to the apical plasma membrane of the intercalated cells.