We previously reported that pertussis toxin–sensitive GTP‐binding protein is involved in prostaglandin F2α (PGF2α)‐induced phosphoinositide (PI) hydrolysis in osteoblast‐like MC3T3‐E1 cells [Miwa et al. (1990): Biochem Biophys Res Commun 171:1229–1235]. In the present study, we investigated the mechanism of PGF2α‐induced Ca2+ influx in MC3T3‐E1 cells. PGF2α‐induced formation of total inositol phosphates (IPs) was markedly reduced by the depletion of extracellular Ca2+ with EGTA. On the other hand, the depletion of extracellular Ca2+ had little effect on PGF2α‐induced inositol 1,4,5‐trisphosphate formation. PGF2α stimulated 45Ca2+ influx dose dependently, attaining a maximum effect at 10 nM. Dose of PGF2α above 10 nM caused less than maximal stimulation. Genistein, an inhibitor of protein tyrosine kinase, which by itself had little effect on 45Ca2+ influx, significantly suppressed the PGF2α‐induced 45Ca2+ influx in a dose‐dependent manner in the range between 1 μg/ml and 0.1 mg/ml. Sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, enhanced the PGF2α‐induced 45Ca2+ influx. Genistein also suppressed the PGF2α‐induced total IPs formation dose dependently in the range between 1 μg/ml and 0.1 mg/ml. However, it had little effect on the PGF2α‐induced inositol 1,4,5‐trisphosphate formation. The pretreatment with pertussis toxin had little effect on the PGF2α‐induced 45Ca2+ influx. These results strongly suggest that PGF2α stimulates Ca2+ mobilization from extracellular space and PI hydrolysis via independent pathways in osteoblast‐like cells, and the PGF2α‐induced Ca2+ influx is regulated by protein tyrosine kinase, resulting in the promotion of PI hydrolysis.
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