Characterization of calcium signaling by purinergic receptor-channels expressed in excitable cells

ATP-gated purinergic receptors (P2XRs) are a family of cationpermeable channels that conduct Ca2+ and facilitate voltage-sensitive Ca2+ entry in excitable cells. To study Ca2+ signaling by P2XRs and its dependence on voltage-sensitive Ca2+ influx, we expressed eight cloned P2XR subtypes individually in gonadotropin-releasing hormone-secreting neurons. In all cases, ATP evoked an inward current and a rise in [Ca2+](i). P2XR subtypes differed in the peak amplitude of [Ca2+](i) response independently of the level of receptor expression, with the following order: P2X(1)R, P2X(3)R, P2X(4)R, P2X(2b)R < P2X(2a)R, P2X(7)R. During prolonged agonist stimulation, Ca2+ signals desensitized with different rates: P2X3R. P2X(1)R > P2X(2b)R > P2X(4)R >> P2X(2a)R >> P2X(7)R. The pattern of [Ca2+](i) response for each P2XR subtype was highly comparable with that of the depolarizing current, but the activation and desensitization rates were faster for the current than for [Ca2+](i). The P2X(1)R, P2X(3)R, and P2X(4)R-derived [Ca2+](i) signals were predominantly dependent on activation of voltage-sensitive Ca2+ influx, both voltage-sensitive and -insensitive Ca2+ entry pathways equally contributed to [Ca2+](i) responses in P2X(2a)R- and P2X(2b)R-expressing cells, and P2X(7)R operated as a nonselective pore capable of conducting larger amounts of Ca2+ independently on the status of voltage-gated Ca2+ channels. Thus, Ca2+ signaling by homomeric P2XRs expressed in an excitable cell is subtype-specific, which provides an effective mechanism for generating variable [Ca2+](i) patterns in response to a common agonist.