Adaptation of Cortical Collecting Duct (CCD) Beta Intercalated Cells(β-ICCs) to Metabolic Acidosis • 1824

Alpha and beta ICCs of the renal collecting duct play a significant role in the regulation of acid-base transport and both have previously been shown through functional studies to adapt under conditions of metabolic acidosis.β-ICCs secrete bicarbonate via an apical Cl-HCO₃ exchanger and basolateral H⁺ pump. They are further characterized by two unique surface antigen markers -- an apical glycoprotein which binds peanut agglutinin (PNA) and one which binds monoclonal antibody (Mab) B63, a probe that we have previously shown inhibits Cl-HCO₃ exchange (AJP 266: F901, 1994). β-ICCs adapt to metabolic acidosis by decreasing bicarbonate secretion. We hypothesize that the adaptation may involve cell surface remodeling of β-ICCs. Isolated CCDs from control and one day acid-fed rabbits were perfused with Mab B63 and two fluorescent probes (FITC-PNA and Texas Red coupled anti-mouse secondary antibody vs. Mab B63). After microperfusion of the CCDs, labeling of β-ICCs, and fixation in a 3D state, specimens were examined using fluorescence microscopy. PNA(+) and B63(+) cells exhibited coincident staining under control and acid-fed conditions. The pattern of staining resembled a cap when the cell's luminal surface was orthogonal to the plane of the camera. Cell surface area was assessed by measuring cap lengths. With acid treatment, the cap length decreased in both the PNA(+) cells (7.2 ± 0.3 μm, n=27 cells vs. 5.6± 0.3, n=14; p<0.01) and B63(+) cells (7.8 ± 0.3, n=33 vs. 5.6 ± 0.3, n=19; p<0.01). The number of PNA(+) cells per high power field identified in one focal plane of the tubule also decreased with acid treatment (5.6 ± 0.3 cells per field, n=17 fields vs. 3.7 ± 0.3, n=7; p<0.01) as did the number of B63(+) cells (6.1 ± 0.3, n=15 vs. 3.5 ± 0.7, n=8; p<0.01). Findings of decreased B63 staining with acid treatment were confirmed with immunocytochemistry on PLP fixed tissue sections. These studies indicate that the β-ICC adaptation to metabolic acidosis involves apical cell surface remodeling probably by endocytosis. The observed decrease in bicarbonate secretion with acidosis may be mediated by internalization of apical Cl-HCO₃ exchangers or associated regulatory proteins. (Data reported as mean ± SEM.)