DIFFERENTIAL REGULATION OF RECEPTOR-STIMULATED CYCLIC ADENOSINE-MONOPHOSPHATE PRODUCTION BY POLYVALENT CATIONS IN MC3T3-E1 OSTEOBLASTS

Extracellular cations have paradoxical trophic and toxic effects on os teoblast function. In an effort to explain these divergent actions, we investigated in MC3T3-E1 osteoblasts if polyvalent cations differenti ally modulate the agonist-stimulated cyclic adenosine monophosphate (c AMP) pathway, an important regulator of osteoblastic function. We foun d that a panel of cations, including gadolinium, aluminum, calcium, an d neomycin, inhibited prostaglandin E(1) (PGE)-stimulated cAMP accumul ation but paradoxically potentiated parathyroid hormone (PTH)-stimulat ed cAMP production. In contrast, these cations had no effect on forsko lin- or cholera toxin-induced increases in cAMP, suggesting actions pr oximal to adenylate cyclase and possible modulation of receptor intera ctions with G proteins. Phorbol 12-myristate 13-acetated (PMA) mimicke d the effects of cations on PGE(1)- and PTH-stimulated cAMP accumulati on in MC3T3-E1 cells, respectively, diminishing and augmenting the res ponses. Moreover, down-regulation of protein kinase C (PKC) by overnig ht treatment with PIMA prevented gadolinium (Gd3+) from attenuating PG E(1)- and enhancing PTH-stimulated cAMP production, indicating involve ment of PKC-dependent pathways. Cations, however, activated signal tra nsduction pathways not coupled to phosphatidylinositol-specific phosph olipase C (PI-PLC), since there was no corresponding increase in inosi tol phosphate formation or intracellular calcium concentrations. In ad dition, pertussis toxin treatment failed to prevent Gd3+-mediated supp ression of PGE(1)-stimulated cAMP, suggesting actions independent of G (alpha i). Thus, polyvalent cations may either stimulate or inhibit ho rmone-mediated cAMP accumulation in osteoblasts. These differential ac tions provide a potential explanation for the paradoxical trophic and toxic effects of cations on osteoblast function that occur in vivo und er different hormonal conditions.