Cellular remodeling of HCO₃^--secreting cells in rabbit renal collecting duct in response to an acidic environment

The renal cortical collecting duct (CCD) consists of principal and intercalated cells. Two forms of intercalated cells, those cells involved in H⁺/HCO₃^- transport, have recently been described. H⁺-secreting cells are capable of apical endocytosis and have H⁺ ATPase on the apical membrane and a basolateral Cl^-/HCO₃^- exchanger. HCO₃^--secreting cells bind peanut agglutinin (PNA) to apical membrane receptors and have diffuse or basolateral distribution of H⁺ ATPase; their Cl^-/HCO₃^- exchanger is on the apical membrane. We found that 20 h after acid feeding of rabbits, there was a fourfold increase in number of cells showing apical endocytosis and a numerically similar reduction of cells binding PNA. Incubation of CCDs at pH 7.1 for 3-5 h in vitro led to similar, albeit less pronounced, changes. Evidence to suggest internalization and degradation of the PNA binding sites included a reduction in apical binding of PNA, decrease in pH in the environment of PNA binding, and incorporation of electron-dense PNA into cytoplasmic vesicles. Such remodeling was dependent on protein synthesis. There was also functional evidence for loss of apical Cl^-/HCO₃^- exchange on PNA-labeled cells. Finally, net HCO₃^- flux converted from secretion to absorption after incubation at low pH. Thus, exposure of CCDs to low pH stimulates the removal/inactivation of apical Cl^-/HCO₃^- exchangers and the internalization of other apical membranes components. Remodeling of PNA-labeled cells may mediate the change in polarity of HCO₃^- flux observed in response to acid treatment.