We coexpressed Kaposi's sarcoma-associated herpes-virus G protein-coupled receptors (KSHV-GPCRs) with thyrotropin-releasing hormone (TRH) receptors or m1-muscarinic-cholinergic receptors in Xenopus oocytes and in mammalian cells. In oocytes, KSHV-GPCR expression resulted in pronounced (81%) inhibition (heterologous desensitization) of Ca2+ -activated chloride current responses to TRH and acetylcholine. Similar inhibitions of cytoplasmic free Ca2+ responses to TRH were observed in human embryonic kidney HEK 293 EM cells and in mouse pituitary AtT20 cells. Further study of oocytes showed that this inhibition was partially reversed by interferon-γ -inducible protein 10 (IP-10), an inverse agonist of KSHV-GPCR. The basal rate of 45Ca2+ efflux in oocytes expressing KSHV-GPCRs was 4.4 times greater than in control oocytes, and IP-10 rapidly inhibited increased 45Ca2+ efflux. In the absence of IP-10, growth-related oncogene α caused a further 2-fold increase in 45Ca 2+ efflux. In KSHV-GPCR-expressing oocytes, responses to microinjected inositol 1,4,5-trisphosphate were inhibited by 74%, and this effect was partially reversed by interferon-γ-inducible protein 10. Treatment with thapsigargin suggested that the pool of calcium available for mobilization by TRH was decreased in oocytes coexpressing KSHV-GPCRs. These results suggest that constitutive signaling by KSHV-GPCR causes heterologous desensitization of responses mediated by other receptors, which signal via the phosphoinositide/calcium pathway, which is caused by depletion of intracellular calcium pools.