Activation of the Ca²⁺ message system by parathyroid hormone is dependent on the cell cycle

Putative osteoblastic UMR 106-01 cells exhibit heterogeneous intracellular calcium [Ca²⁺](i) responses to parathyroid hormone (PTH). To determine whether this phenomenon is secondary to cell cycle asynchrony, UMR 106-01 cultures were synchronized at the G1/S boundary using a sequential thymidine- aphidicolin treatment. Five hours after release from the block >80% of the cells are in S phase, while nontreated confluent cultures are in G1. Using video image analysis of single cells loaded with fura-2, PTH (10^-7 M) induced transient increases in [Ca²⁺](i) preferentially in cells in S phase, with 82% response frequency whereas cells in G1 phase responded poorly to PTH with only 10% response frequency. In contrast, cell response to fetal calf serum was more frequent in G1 than in S phase. Pretreatment with La³⁺, nifedipine or pertussis toxin reduced both the frequency and amplitude of the PTH response in S phase to values comparable to those observed in cells in G1 phase. There was no significant difference in inositol trisphosphate generated by PTH stimulation in either phase. Thus, the heterogeneous osteoblastic [Ca²⁺](i) responsive to PTH is cell cycle-dependent with the change in the G1 to the S phase mode of response dependent on active coupling between the PTH receptor and a Ca²⁺ channel via a pertussis toxin-sensitive G protein.