The inositol trisphosphate receptor of Xenopus oocytes

Xenopus laevis oocytes (stages V and VI) are a widely used model system for the study of Ca²⁺ signaling. The properties of the Xenopus oocyte InsP₃ receptor (InsP₃R) are of paramount importance for our thinking about this system and for our efforts to model Ca²⁺ dynamics in the oocyte cytosol. The recent data regarding the molecular structure, the regulation and the functional properties of the Xenopus oocyte InsP₃R are summarized in this review. The main properties of the Xenopus oocyte InsP₃R are compared with the properties of the cerebellar InsP₃R and are shown to be remarkably similar. The density of the InsP₃R in Xenopus oocyte cytoplasm is estimated to a value between 1.1-4.1 × 10¹⁴ tetrameric InsP₃Rs/I. The use of these numbers in a quantitative model of Ca²⁺ wave propagation leads to values of Ca²⁺ wave amplitude (0.8-1.5 μM Ca²⁺) and velocity of the wave propagation (12-24 μm/s) that are in excellent agreement with the values observed experimentally. The density of InsP₃Rs in Purkinje cells of the cerebellum is estimated to be about 20,000-fold higher, but in other types of neurons and in peripheral tissues the InsP₃R density is estimated to be of the same order of magnitude as, or up to 20-fold higher than, in Xenopus oocytes. The implications of differences in InsP₃R density for Ca²⁺ signaling are discussed.