Effects of prostaglandin E 2 and F 2 α on cytoplasmic pH in a clonal osteoblast‐like cell line, MOB 3–4

Prostaglandin E 2 (PGE 2 , 5 ng/ml to 5 μg/ml) induced a dose‐dependent increase in cAMP accumulation, inositol phosphates (IPs) accumulation, and cytoplasmic free Ca 2+ ([Ca 2+ ]i) in a clonal osteoblast‐like cell line, MOB 3–4. In contrast, prostaglandin F 2 α (PGF 2 α, 5 ng/ml to 5 μg/ml) stimulated increases in IPs accumulation and [Ca 2+ ]i without stimulating an increase in cAMP accumulation. Both PGE 2 (>0.5 μg/ml) and PGF 2 α (≥0.5 μg/ml) increased cytoplasmic pH (pHi) from approximately 7.15 to 7.35 in BCECF‐loaded cells. A tumor promotor, phorbol 12‐myristate 13‐acetate (PMA, 0.1–100 nM) also increased pHi without effect on phosphoinositide hydrolysis. Both PGE 2 ‐(5 μg/ml) and PMA‐ (100 nM) induced cytoplasmic alkalinization was inhibited by removal of extracellular Na + , or by pretreatment of the cells with amiloride (0.5 mM), an inhibitor of Na + /H + exchange, or H‐7 (100 μM), a nonspecific inhibitor of protein kinase C. Thus, MOB 3–4 cells appeared to possess PGE 2 receptors and PGF 2 α receptors: the former are coupled to adenylate cyclase and phospholipase C, and the latter are predominantly coupled to phospholipase C. Also the cells appeared to possess an amiloride‐sensitive Na + /H + exchange activity, which increases pHi in response to PGE 2 and PGF 2 α, as well as to PMA. Long‐term (48 hr) exposure of the cells to PGE 2 at a high concentration (5 μg/ml), but not to PGF 2 α and PMA, decreased DNA synthesis in the serum‐deficient medium. Thus, cytoplasmic alkalinization appeared insufficient for cell replication. At least in MOB 3–4 cells, the inhibitory effect of PGE 2 on DNA synthesis may be due to the cAMP messenger system.