INTRACELLULAR MODULATION OF ACID-SECRETION IN RAT INNER MEDULLARY COLLECTING DUCT CELLS

In this study we defined some of the important elements in the acidifi cation process of rat inner medullary collecting duct (IMCD) cells in culture. After cell acidification, i.e., cell pH (pH(i)) = 6.51 +/- 0. 02, pH(i) increased 0.046 +/- 0.003 units/min. N-ethylmaleimide, N,N' -cyclohexylcarbodiimide, and bafilomycin reduced this rate by over 85% . In contrast, omeprazole and Sch-28080 had no effect. 1,2-Bis(2-amino phenoxy)ethaneN,N,N,N'-tetraacetic acid, which prevents a rise in cell Ca2+ concentration ([Ca2+](i)), reduced the rate of pH(i) recovery to 0.013 +/- 0.002 units/min. Calmodulin inhibitors or disruption of cyt oskeletal elements with cytochalasin B and colchicine also reduced pH( i) recovery significantly. In addition, these cells contain acidic ves icles and undergo pH(i)-regulated endocytosis and exocytosis, which ar e inhibited by disrupting the cytoskeleton. We conclude that, in our c ultured line of rat IMCD cells, proton secretion is mediated by an H+- adenosinetriphosphatase. Changes in pH(i) produce alterations in acid secretion through a signal cascade that requires changes in [Ca2+](i), activation of calmodulin, an intact cytoskeleton, and alteration in t he rate of exocytosis and endocytosis.