Expression of human parathyroid hormone receptor (hPTHR) was obtained in Xenopus oocytes. Receptor function was detected by hormone stimulation of endogenous Ca-activated Cl current. This current was blocked by injected, but not by extracellular, EGTA, confirming that the hPTHR activates cytosolic Ca signaling pathways. PTH responses were acutely desensitized but were regained in 6-12 h. Injection of cAMP or analogues had no effect on either responsiveness or desensitization to hPTH. The hPTH response was more sluggish than seen with serotonin 5-hydroxytryptamine (5-HT) receptor. In oocytes co-expressing both hPTHR and 5-HT receptors, homologous desensitization was seen, but cross-desensitization was not observed. Injection of inositol 1,4,5-trisphosphate (InsP) elicited a fast inward current similar to that induced by serotonin, and complete cross-desensitization occurred between the InsP and 5-HT responses. Desensitization by hPTH did not affect responses to either InsP or serotonin, but cells desensitized to injected InsP still responded strongly to PTH. Oocytes did not respond to either cADPR or NAADP, but NADP and analogues were found to be potent inhibitors of PTH signaling. We suggest that PTH cytosolic Ca signaling in oocytes either involves a novel signaling system or proceeds through a Ca compartment whose responsiveness is regulated in a novel way.

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