DOWN-REGULATION AND DIFFERENTIAL RESTORATION OF CAMP RESPONSES UPON TRANSIENT PHORBOL ESTER TREATMENT OF PRIMARY OSTEOBLASTIC CELLS

We studied cAMP responses induced by parathyroid hormone (PTH), prosta glandin E(2) (PGE(2)) and forskolin in foetal rat calvariae-derived os teoblastic cells after 24 h treatment with a protein kinase C (PKC) ac tivating phorbol ester. After this treatment, meant to down-regulate P KC activity, all tested cAMP responses were attenuated and were indeed accompanied by a decline in PKC activity. PTH receptor affinity was n ot altered and PTH receptor number was only slightly lowered after 24 h phorbol ester treatment. These results indicate that modulation of t he cAMP responses by 24 h PMA treatment was mainly caused by a general impairment of adenylyl cyclase activity. Removal of the phorbol ester and subsequent culture for 2 days rendered the cells hyper-responsive to PTH: the PTH-induced cAMP response was 2 to 3 times higher than in control cells. Again no change in binding affinity of the PTH recepto r was observed and receptor number was just 10% lower than in control cells. The PGE(2)- and forskolin-induced cAMP responses were not highe r than normal. So, transient phorbol ester treatment leads to a differ ential, agonist-dependent restoration of the cAMP signalling system.