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 InsP(3) receptor (InsP(3)R) 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 InsP(3)R are summarized in this review; The main properties of the Xenopus oocyte InsP(3)R are compared with the properties of the cerebellar InsP(3)R and are shown to be remarkably similar. The density of the InsP(3)R in Xenopus oocyte cytoplasm is estimated to a value between 1.1-4.1 x 10(14) tetrameric InsP(3)Rs/l, The use of these numbers in a quantitative model of Ca2+ wave propagation leads to values of Ca2+ wave amplitude (0.8-1.5 mu M Ca2+) and velocity of the wave propagation (12-24 mu m/s) that are in excellent agreement with the values observed experimentally. The density of InsP(3)Rs 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 InsP(3)R 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 InsP(3)R density for Ca2+ signaling are discussed.