Characterization of a 1,25(OH)2-vitamin D3-responsive capacitative Ca2+ entry pathway in rat osteoblast-like cells

We investigated the existence of a capacitative Ca2+ 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 Ca2+ stores with thapsigargin or 1,25(OH)(2)D-3 in the absence of extracellular Ca2+ transiently elevated cytosolic Ca2+ ([Ca2+](i)); after recovery of basal values, Ca2+ re-addition to the medium markedly increased Ca2+ entry, reflecting pre-activation of a CCE pathway. Recovery of the Ca2+ overshoot that followed the induced CCE was mainly mediated by the plasma membrane Ca2+-ATPase. Addition of 1,25(OH)(2)D-3 to the declining phase of the thapsigargin-induced CCE did not modify further [Ca2+](i), indicating that steroid activation of CCE was dependent on store depletion. Pre-treatment with I muM Gd3+ inhibited 30% both thapsigargin- and 1,25(OH)(2)D-3-Stimulated CCE, whereas 2.5 muM Gd3+ was required for maximal inhibition (similar to 85%). The activated CCE was permeable to both Mn2+ and Sr2+. Mn2+ entry sensitivity to Gd3+ was the same as that of the CCE. However, 1-muM Gd3+ completely prevented capacitative Sr2+ influx, whereas subsequent Ca2+ 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 Ca2+/Mn2+ permeable route insensitive to very low micromolar (1 muM) Gd3+ accounting for most of the CCE and a minor Ca2+/Sr2+/Mn2+ permeable route highly sensitive to 1 muM Gd3+. The Ca2+-mobilizing agonist ATP also stimulated CCE resembling the Ca2+/Sr2+/Mn2+ 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 Ca2+ influx route through which osteotropic agents influence osteoblast Ca2+ homeostasis. (C) 2002 Wiley-Liss, Inc.