Calcium-sensing receptor (CaSR) modulates vacuolar H + -ATPase activity in a cell model of proximal tubule.

BACKGROUND: The calcium-sensing receptor (CaSR) is localized in the apical membrane of proximal tubules in close proximity to the transporters responsible for proton secretion. Therefore, the aim of the present study was to analyze the effects of CaSR stimulation on the biochemical activity of the vacuolar H+ -ATPase in a cellular model of proximal tubule cells, OKP cells.

METHODS: Biochemical activity of H+ -ATPase was performed using cell homogenates, and the inorganic phosphate released was determined by a colorimetric method. Changes in cytosolic ionized calcium [Ca2+ ]i were also determined using Fluo-4.

RESULTS: A significant increase of vacuolar H+ -ATPase activity was observed when the CaSR was stimulated with agonists such as Gd3+ (300 µM) and neomycin (200 µM). This activity was also stimulated in a dose-dependent fashion by changes in extracellular Ca2+ (Ca2+ o ) between 10-4 and 2 mM. Gd3+ and neomycin produced a sustained rise of [Ca2+ ]i , an effect that disappears when extracellular calcium was removed in the presence of 0.1 µM thapsigargin. Inhibition of phospholipase C (PLC) activity with U73122 (5 × 10-8  M) reduced the increase in [Ca2+ ]i induced by neomycin.

CONCLUSION: CaSR stimulation induces an increase in the vacuolar H+ -ATPase activity of OKP cells, an effect that involves an increase in [Ca2+ ]i and require phospholipase C activity. The consequent decrease in intratubular pH could lead to increase ionization of luminal calcium, potentially enhancing its reabsorption in distal tubule segments and reducing the formation of calcium phosphate stones.