Inhibitors of Protein Kinase-C Modulate Desensitization of the Parathyroid Hormone Receptor-Adenylate Cyclase System in Opossum Kidney Cells*

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Abstract
The mechanisms involved in mediating desensitization and down-regulation of renal PTH receptors have not been defined. Recent studies indicate that PTH binding promotes not only stimulation of adenylate cyclase and activation of protein kinase-A (PK-A), but also, stimulation of phospholipase-C, leading to activation of PK-C. PK-C has been shown to alter both receptor and adenylate cyclase function in other systems. Therefore, the present studies were conducted to test whether PK-C might play a role in the regulation of the PTH receptor-cyclase system after exposure to PTH. Exposure of confluent cultures of opossum kidney (OK) cells to rat PTH-(1-34) (100 nm) for 6 h resulted in a 48 .+-. 8% (n = 5) decrease in stimulation of cAMP accumulation in response to further exposure to PTH. PTH receptor binding, assessed with 125I[Nle8,Nle21,Tyr34]rat PTH-(1-34)NH2 as radioligand, as decreased to a similar extent. Phorbol ester (4.beta.-12,13-didecanoate; 1 .mu.m) treatment of the cells in the absence of PTH caused a 58 .+-. 3% decrease in PTH-stimulated cAMP production, but equilibrium PTH receptor binding was not different from the control value. Both 50 .mu.m H-7 and 0.5 .mu.m Staurosporine (inhibitors of PK-C) completely blocked the effects of phorbol ester. Pretreatment with PTH, however, in the presence of H-7 or Staurosporine resulted in a completely normal cAMP response to restimulation with PTH. Thus, two inhibitors of PK-C completely prevented desensitization to PTH. The decrease in equilibrium PTH binding, seen after incubation with PTH alone, was also blunted by the inhibitors of PK-C. These data indicate that activation of PK-C by stimulation with PTH may play a role of the regulation of the PTH receptor-cyclase system in OK cells.

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