STUDIES ON THE MECHANISM OF DESENSITIZATION OF THE PARATHYROID HORMONE-STIMULATED CALCIUM SIGNAL IN UMR-106 CELLS - REVERSAL OF DESENSITIZATION BY ALKALINE-PHOSPHATASE BUT NOT BY PROTEIN-KINASE-C DOWN-REGULATION

The involvement of protein kinase C (PKC), cAMP-dependent protein kina se (PKA), and other phosphorylation mechanisms in the rapid desensitiz ation of the [Ca2+](i) response to parathyroid hormone (PTH) stimulati on was investigated in osteoblast-like UMR-106 cells. A 5 minute prein cubation of the cell suspension with phorbol 12,13-dibutyrate (PDB) de creased the response to PTH in a concentration-dependent manner. 1-Ole oyl-2-acetyl-r-glycerol (OAG) pretreatment likewise decreased the PTH response. Staurosporine, a potent protein kinase inhibitor, completely prevented the desensitization caused by PDB. These PDB and staurospor ine effects were also observed in 3 mM EGTA-containing medium ([Ca2+]( free) < 10(-8) M). A 5 minute pretreatment of cells with 1 mu M forsko lin had no effect on the calcium response to PTH. Homologous and PDB-i nduced desensitizations differed in several respects. Staurosporine pr etreatment resulted in only a slight restoration of the PTH response u nder conditions of homologous desensitization. Chronic treatment with phorbol ester prevented the desensitization of the PTH response by acu te phorbol treatment but not the homologous desensitization. Both homo logous and PDB-induced desensitization were relieved by alkaline phosp hatase treatment, consistent with the involvement of phosphorylation i n the desensitization. This alkaline phosphatase effect on desensitiza tion was inhibited by L-phenylalanine. These results suggest that PTH receptor homologous desensitization involves phosphorylation process(e s) other than or in addition to those of PKC.