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 downregulation

The involvement of protein kinase C (PKC), cAMP‐dependent protein kinase (PKA), and other phosphorylation mechanisms in the rapid desensitization of the [Ca²⁺]_i response to parathyroid hormone (PTH) stimulation was investigated in osteoblast‐like UMR‐106 cells. A 5 minute preincubation of the cell suspension with phorbol 12,13‐dibutyrate (PDB) decreased the response to PTH in a concentration‐dependent manner. 1‐Oleoyl‐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 staurosporine effects were also observed in 3 mM EGTA‐containing medium ([Ca²⁺]_free < 10⁻⁸ M). A 5 minute pretreatment of cells with 1 μM forskolin had no effect on the calcium response to PTH. Homologous and PDB‐induced desensitizations differed in several respects. Staurosporine pretreatment resulted in only a slight restoration of the PTH response under conditions of homologous desensitization. Chronic treatment with phorbol ester prevented the desensitization of the PTH response by acute phorbol treatment but not the homologous desensitization. Both homologous and PDB‐induced desensitization were relieved by alkaline phosphatase treatment, consistent with the involvement of phosphorylation in the desensitization. This alkaline phosphatase effect on desensitization was inhibited by L‐phenylalanine. These results suggest that PTH receptor homologous desensitization involves phosphorylation process(es) other than or in addition to those of PKC.