Human osteoblastic cells propagate intercellular calcium signals by two different mechanisms

Effective bone remodeling requires the coordination of bone matrix depositi on by osteoblastic cells, which may occur via soluble mediators or via dire ct intercellular communication. We have previously identified two mechanism s by which rat osteoblastic cell lines coordinate calcium signaling among c ells: autocrine activation of P2 (purinergic) receptors leading to release of intracellular calcium stores, and gap junction-mediated communication re sulting in influx of extracellular calcium. In the current work we asked wh ether human osteoblastic cells (HOB) were capable of mechanically induced i ntercellular calcium signaling, and if so, by which mechanisms. Upon mechan ical stimulation, human osteoblasts propagated fast intercellular calcium w aves, which required activation of P2 receptors and release of intracellula r calcium stores but did not require calcium influx or gap junctional commu nication. After the fast intercellular calcium waves were blocked, we obser ved slower calcium waves that were dependent on gap junctional communicatio n and influx of extracellular calcium. These results show that human osteob lastic cells can propagate calcium signals from cell to cell by two markedl y different mechanisms and suggest that these two pathways may serve differ ent purposes in coordinating osteoblast functions.