Characterization of a 1,25(OH) 2 ‐vitamin D 3 ‐responsive capacitative Ca 2+ entry pathway in rat osteoblast‐like cells

We investigated the existence of a capacitative Ca 2+ entry (CCE) pathway in ROS 17/2.8 osteoblast‐like cells and its responsiveness to 1,25‐dihydroxy‐vitamin D 3 [1,25(OH) 2 D 3 ]. Depletion of inner Ca 2+ stores with thapsigargin or 1,25(OH) 2 D 3 in the absence of extracellular Ca 2+ transiently elevated cytosolic Ca 2+ ([Ca 2+ ] i ); after recovery of basal values, Ca 2+ re‐addition to the medium markedly increased Ca 2+ entry, reflecting pre‐activation of a CCE pathway. Recovery of the Ca 2+ overshoot that followed the induced CCE was mainly mediated by the plasma membrane Ca 2+ ‐ATPase. Addition of 1,25(OH) 2 D 3 to the declining phase of the thapsigargin‐induced CCE did not modify further [Ca 2+ ] i , indicating that steroid activation of CCE was dependent on store depletion. Pre‐treatment with 1 μM Gd 3+ inhibited 30% both thapsigargin‐ and 1,25(OH) 2 D 3 ‐stimulated CCE, whereas 2.5 μM Gd 3+ was required for maximal inhibition (∼ 85%). The activated CCE was permeable to both Mn 2+ and Sr 2+ . Mn 2+ entry sensitivity to Gd 3+ was the same as that of the CCE. However, 1‐μM Gd 3+ completely prevented capacitative Sr 2+ influx, whereas subsequent Ca 2+ re‐addition was reduced only 30%. These results suggest that in ROS 17/2.8 cells CCE induced by thapsigargin or 1,25(OH) 2 D 3 is contributed by at least two cation entry pathways: a Ca 2+ /Mn 2+ permeable route insensitive to very low micromolar (1 μM) Gd 3+ accounting for most of the CCE and a minor Ca 2+ /Sr 2+ /Mn 2+ permeable route highly sensitive to 1 μM Gd 3+ . The Ca 2+ ‐mobilizing agonist ATP also stimulated CCE resembling the Ca 2+ /Sr 2+ /Mn 2+ permeable entry activated by 1,25(OH) 2 D 3 . The data demonstrates for the first time, the presence of a hormone‐responsive CCE pathway in an osteoblast cell model, raising the possibility that it could be an alternative Ca 2+ influx route through which osteotropic agents influence osteoblast Ca 2+ homeostasis. © Wiley‐Liss, Inc.