Effects of pH on calcium transport in turtle bladder

Abstract

This study was designed to examine the effect of apical and basolateral (ie, mucosal and serosal) pH on calcium (Ca) transport in turtle bladder, a nonmammalian analog of the distal nephron. Unidirectional Ca 45 fluxes were measured when serosal pH was 6.4, 7.4, or 8.4 (mucosal pH, 7.4) in the presence and absence of ouabain. When serosal pH was 8.4, M → S Ca 45 flux increased significantly, and when it was 6.4, M → S Ca 45 flux decreased markedly. Changes in serosal pH did not affect the S → M Ca 45 flux. When 5 × 10 −4 mol/L ouabain was added to inhibit sodium transport, M → S Ca 45 flux, at pH 7.4, was 221.6 ± 27.4 pmol/mg/h (n = 10), and low pH again inhibited this flux (∼50%). Lowering mucosal pH (with serosal pH 7.4) also decreased M → S Ca 45 flux. In stripped bladders, Ca 45 uptake increased linearly as medium pH was increased from 4.4 to 8.4. Total tissue Ca concentration did not change when serosal pH was varied, except at the extreme of pH 4.4, where tissue Ca decreased. By contrast, when apical pH was 6.4, tissue Ca rose substantially (∼1.5-fold). These results demonstrate that extracellular pH directly affects Ca homeostasis in the turtle bladder. Lowering the pH of either the serosal or mucosal medium directly inhibits apical Ca permeability. This change in Ca permeability is seen in the presence of ouabain. By contrast, alkalization of the serosal medium enhances apical permeability, but this effect is, in some manner, related to sodium transport. The hypercalciuria of metabolic acidosis universally seen in humans may be due to altered apical Ca permeability in the distal nephron.