Desensitization of parathyroid hormone receptors on cultured bone cells

Administration of excessive amounts of parathyroid hormone (PTH) in the treatment of osteoporosis can reverse the beneficial effects of a low-dose, intermittent regime. To investigate the direct actions and the possible cellular mechanisms of PTH in inducing desensitization of PTH receptors, we studied the effects of desensitization on rat osteoblastic UMR-106 cells. When the osteoblasts were preincubated with bPTH-(1–34), complete refractoriness to a subsequent challenge with the hormone developed within 1 h and at hormone concentrations as low as 5 nM. When osteoblasts thus desensitized were incubated in hormone-free medium, recovery of the cAMP responses began within 2 h and reached maximum after 16 h. Cycloheximide did not affect the process of desensitization. [NIe8, NIe18, Tyr34]bPTH-(3–34)amide significantly impaired the desensitization process by PTH-(1–34) but did not have stimulatory effect on cAMP responses. No significant heterologous desensitization was obvious after preincubation with isoprenaline (50 μM), prostaglandin E, (50 μM), or prostaglandin E2 (50 μM) for 2 h. Binding experiments with [125I]PLP-(1–36)amide after desensitization revealed that there was an approximate twofold decrease in receptor affinities as analyzed by Scatchard analysis, showing that the decrease in affinity was prominent in the process of desensitization. When the cells were treated with monensin during desensitization, PTH challenge after desensitization produced significantly lower cyclic AMP responses. Recovery after desensitization occurred over a period of 16 h. Inclusion of monensin, but not cycloheximide, impaired the recovery. The results show that homologous desensitization of rat osteoblasts to PTH is brought about by the occupancy of receptors by PTH-(1–34) but not by cAMP generation itself. It is not primarily dependent on protein synthesis or on activation of adenylate cyclase. A decrease in receptor affinity is important in the process of desensitization. Desensitization of parathyroid hormone receptors is potentiated and recovery impaired by monensin, an inhibitor of internalization and recycling, showing that both processes are important in the regulation of PTH receptors.