Bone
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Elevated extracellular calcium (Ca(e)) stimulates both chemotaxis and mitogenesis of MC3T3-E1 osteoblasts via a calcium-sensing receptor (CasR). Ca(e)-mediated chemotaxis of these bone-forming cells is dependent on phospholipase C (PLC) and blocked by the Gi-protein inhibitor pertussis toxin. In this study, we examine the signaling mechanisms by which the CasR stimulates PLC activity in MC3T3-E1 osteoblasts. ⋯ Inhibition of protein kinase C (PKC) disrupted Ca(e)-stimulated tyrosine phosphorylation of PLC-gamma1. In addition, exposure to pertussis toxin or exogenous activation of protein kinase A (PKA) inhibited PLC-gamma1 tyrosine phosphorylation in response to Ca(e). The results indicate that: (a) the osteoblast CasR activates PLC-gamma1 downstream of PLC-beta in a PKC-dependent manner; (b) PKA is a negative regulator of Ca(e)-promoted PLC-gamma1 phosphorylation; and (c) Gq and Gi are both involved in the CasR-mediated phosphorylation of PLC-gamma1.