Pathways for HCO3– reabsorption in mouse medullary collecting duct segments

Abstract

To determine pathways of HCO3– reabsorption in the collecting duct of the mouse kidney, the outer medullary collecting duct (OMCD) and the terminal inner medullary collecting duct (IMCDt) were dissected and perfused at 1 to 2 nL/min, and total CO2 was measured by microfluorometry. In the OMCD, net HCO3– flux (JtCO2) was 12.2 ± 0.7 pmol/min/mm tubule length and decreased to 6.9 ± 0.6 pmol/min/mm tubule length (n = 5) with 10 μmol/L Schering 28080 (SCH) in perfusate (P <.001) and to 7.7 ± 0.6 pmol/min/mm tubule length (P <.004; n = 4) with 50 μmol/L diethylstilbestrol (DES), an inhibitor of H+-adenosine triphosphatase; together they reduced JtCO2 to 3.7 ± 0.2 pmol/min/mm tubule length (P =.0002; n = 4). In IMCDt, JtCO2 was 10.9 ± 1.1 pmol/min/mm tubule length, and it decreased to 4.3 ± 0.9 pmol/min/mm tubule length (n = 4) with 10 μmol/L SCH in perfusate (P <.05) and to 7.0 ± 1.1 pmol/min/mm tubule length (P <.05; n = 4) with 50 μmol/L DES; together they decreased JtCO2 to 2.3 ± 0.3 pmol/min/mm tubule length (P <.002; n = 4). Ouabain (1 mmol/L), an inhibitor of colonic H-K-adenosine triphosphatase (cHKA), in perfusate had no effect on JtCO2 in either segment. Northern hybridization studies showed a high level of expression of gastric HKA (gHKA) in outer medulla and a low level in inner medulla; cHKA expression was undetectable. Thus, in normal mouse OMCD and IMCDt, HCO3– reabsorption is predominantly mediated by gHKA and H+-adenosine triphosphatase and not cHKA. A third isoform of HKA could be present in mouse IMCDt. (J Lab Clin Med 2000;136:218-23)