Abstract

ATP-gated purinergic receptors (P2XRs) are a family of cation-permeable 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: P2X₁R < P2X₃R < P2X₄R < P2X_2bR < P2X_2aR < P2X₇R. During prolonged agonist stimulation, Ca²⁺ signals desensitized with different rates: P2X₃R > P2X₁R > P2X_2bR > P2X₄R ≫ P2X_2aR ≫ P2X₇R. The pattern of [Ca²⁺]_iresponse 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 P2X₁R, P2X₃R, and P2X₄R-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_2aR- and P2X_2bR-expressing cells, and P2X₇R 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.