title: Intracellular calcium measurements as a method in studies on activity of purinergic P2X receptor channels.
abstract: Extracellular nucleotide-activated purinergic receptors (P2XRs) are a family of cation-permeable channels that conduct small cations, including Ca2+, leading to the depolarization of cells and subsequent stimulation of voltage-gated Ca2+ influx in excitable cells. Here, we studied the spatiotemporal characteristics of intracellular Ca2+ signaling and its dependence on current signaling in excitable mouse immortalized gonadotropin-releasing hormone-secreting cells (GT1) and nonexcitable human embryonic kidney cells (HEK-293) cells expressing wild-type and chimeric P2XRs. In both cell types, P2XR generated depolarizing currents during the sustained ATP stimulation, which desensitized in order (from rapidly desensitizing to nondesensitizing): P2X3R > P2X2b + X4R > P2X2bR > P2X2a + X4R > P2X4R > P2X2aR > P2X7R. HEK-293 cells were not suitable for studies on P2XR-mediated Ca2+ influx because of the coactivation of endogenously expressed Ca2+-mobilizing purinergic P2Y receptors. However, when expressed in GT1 cells, all wild-type and chimeric P2XRs responded to agonist binding with global Ca2+ signals, which desensitized in the same order as current signals but in a significantly slower manner. The global distribution of Ca2+ signals was present independently of the rate of current desensitization. The temporal characteristics of Ca2+ signals were not affected by voltage-gated Ca2+ influx and removal of extracellular sodium. Ca2+ signals reflected well the receptor-specific EC50 values for ATP and the extracellular Zn2+ and pH sensitivities of P2XRs. These results indicate that intracellular Ca2+ measurements are useful for characterizing the pharmacological properties and messenger functions of P2XRs, as well as the kinetics of channel activity, when the host cells do not express other members of purinergic receptors.
authors.affiliation: Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4510, USA.
authors.forename: Mu-Lan
authors.initials: ML
authors.lastname: He
authors.forename: Hana
authors.initials: H
authors.lastname: Zemkova
authors.forename: Taka-aki
authors.initials: TA
authors.lastname: Koshimizu
authors.forename: Melanija
authors.initials: M
authors.lastname: Tomić
authors.forename: Stanko S
authors.initials: SS
authors.lastname: Stojilkovic
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D005057:Eukaryotic Cells / Q000187:drug effects / Q000378:metabolism*; D005110:Extracellular Space / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects / Q000235:genetics; D006801:Humans; D007424:Intracellular Fluid / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D018048:Receptors, Purinergic P2 / Q000187:drug effects / Q000378:metabolism*; D058469:Receptors, Purinergic P2X; D011993:Recombinant Fusion Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D012964:Sodium / Q000172:deficiency; D015032:Zinc / Q000378:metabolism
chemical_list: D018048:Receptors, Purinergic P2; D058469:Receptors, Purinergic P2X; D011993:Recombinant Fusion Proteins; D000255:Adenosine Triphosphate; D012964:Sodium; D015032:Zinc; D002118:Calcium