Functional expression of the parathyroid cell calcium receptor in Xenopus oocytes

Various studies suggest the existence of a plasma membrane receptor on parathyroid cells that senses changes in the concentration of extracellular Ca 2+. To test this hypothesis, Xenopus leavis oocytes were injected with poly(A) +-enriched mRNA from bovine parathyroid cells and examined for their ability to respond to increases in the concentration of extracellular Ca 2+ or other polycations. Cytosolic Ca 2+ concentrations were measured indirectly by recording Cl − currents through the endogenous, cytosolic Ca 2+-activated Cl − channel. Increasing the concentration of extracellular Ca 2+ (from 0.7 to 5 mM) or Mg 2+ (from 0.8 to 10 mM) elicited oscillatory increases in the CT current. Responses to either divalent cation were not observed in oocytes injected with water or with mRNA prepared from HL-60 cells or rat liver. Responses elicited by extracellular Mg 2+ persisted when extracellular Ca 2+ was reduced to low micromolar levels. La 3+, Gd 3+, or neomycin B also evoked oscillatory increases in the Cl − current in oocytes under conditions of low extracellular Ca 2+ levels. These extracellular polycations all cause the mobilization of intracellular Ca 2+ in oocytes injected with parathyroid cell mRNA like they do in intact parathyroid cells. The injection of parathyroid cell mRNA thus confers on oocytes the ability to detect and respond to changes in the concentration of extracellular polycations. The data provide compelling evidence for the existence of a cell surface Ca 2+ receptor protein(s) on parathyroid cells that regulates cellular function.