Many G protein-coupled receptor agonists activate p42/p44 mitogen-activated protein kinase (MAPK), using signaling pathways that are a function of receptor, G protein-coupled, and effector complement. In opossum kidney (OK) cells, activation of endogenous PTH receptors caused a time- (peak within 15-30 min, sustained for approximately 2 h) and dose-dependent (EC50 approximately 3 x 10(-10) M) activation of MAPK. Immunoblot analysis with an activation- specific MAPK antibody indicated that PTH activated both p42 and p44 MAPK. Epidermal growth factor (EGF) also activated p42 and p44MAPK in a time- (peak at 5 min, return to basal within 2 h) and dose-dependent (EC50 approximately 3 ng/ml) fashion. PTH-dependent MAPK activation was mimicked by the protein kinase C activator (PKC) phorbol myristate acetate (PMA), and the protein kinase A activators 8 bromo-cAMP (8-Br-cAMP) and forskolin but was not affected by pertussis toxin pretreatment. PMA or 8-Br-cAMP pretreatment blocked MAPK activation by reexposure to each kinase activator but caused no significant reduction in MAPK activation by PTH. MAPK activation by PTH, EGF, and 8-Br-cAMP was inhibited by the MAPK kinase inhibitor PD98059 and an EGF receptor (EGFR)-selective inhibitor tyrphostin AG1478. AG1478 also blocked MAPK activation by insulin-like growth factor-1 and platelet-derived growth factor. EGF and PTH caused time- and AG1478-sensitive phosphorylation of the EGFR, but EGFR desensitization did not affect MAPK activation by PTH. EGF, PMA, and low doses of PTH (10(12) to 10(-9) M) stimulated while 8-Br-cAMP and high doses of PTH (10(-8) to 10(-6) M) inhibited [3H]thymidine uptake. These data demonstrate that PTH activates MAPK and suggest that PKC, protein kinase A, and the EGFR play roles in PTH signaling. The biphasic effect of PTH on DNA synthesis suggests that MAPK activation by the hormone leads to distinct cellular responses.