Effect of prostaglandin F_2α on Ca²⁺ influx in osteoblast‐like cells: Function of tyrosine kinase

We previously reported that pertussis toxin–sensitive GTP‐binding protein is involved in prostaglandin F_2α (PGF_2α)‐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 PGF_2α‐induced Ca²⁺ influx in MC3T3‐E1 cells. PGF_2α‐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 PGF_2α‐induced inositol 1,4,5‐trisphosphate formation. PGF_2α stimulated ⁴⁵Ca²⁺ influx dose dependently, attaining a maximum effect at 10 nM. Dose of PGF_2α 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 PGF_2α‐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 PGF_2α‐induced ⁴⁵Ca²⁺ influx. Genistein also suppressed the PGF_2α‐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 PGF_2α‐induced inositol 1,4,5‐trisphosphate formation. The pretreatment with pertussis toxin had little effect on the PGF_2α‐induced ⁴⁵Ca²⁺ influx. These results strongly suggest that PGF_2α stimulates Ca²⁺ mobilization from extracellular space and PI hydrolysis via independent pathways in osteoblast‐like cells, and the PGF_2α‐induced Ca²⁺ influx is regulated by protein tyrosine kinase, resulting in the promotion of PI hydrolysis.