Inositol 1,4,5-trisphosphate receptor function in human oocytes: Calcium responses and oocyte activation-related phenomena induced by photolytic release of InsP₃ are blocked by a specific antibody to the type I receptor

Type I inositol 1,4,5-trisphosphate-sensitive receptors (InsP₃R) are expressed in human oocytes and may be involved in operating the Ca²⁺ release triggered by the fertilizing sperm. This study examines the contribution of type I InSP₃R in operating Ca²⁺ release in human oocytes secondary to InsP₃ itself, using a specific function-blocking antibody in conjunction with photolytic release of microinjected InsP₃. Intracellular Ca²⁺ responses were assessed in oocytes microinjected with only caged InsP₃ in experiment set A, while in experiment sets B and C, sibling oocytes were injected with caged InsP₃ and the blocking antibody or a corresponding volume of medium, prior to flash photolysis. In experiment set C, certain fertilization-related phenomena (cortical granule exocytosis and chromatin configurations) were assessed using optical sections and three-dimensional image reconstructions obtained from a confocal laser scanning microscope. In experiment set A, photolytic release of InsP₃ triggered a Ca²⁺ response (increase from ∼100 to 220 nmol/l followed by an exponential recovery, n = 8) and a wave in the oocytes that spread from the stimulation point to the opposite pole. In set B, photolytic InsP₃ release generated Ca²⁺ responses in control oocytes (n = 9), but not in the antibody-injected oocytes (n = 7). In set C, cortical granule exocytosis and anaphase chromosome configurations were noted in the control oocytes; after flash photolysis (n = 6). These changes were completely absent in antibody injected oocytes as their cortical granules were intact and the chromosomes were in metaphase. These oocytes had also lacked Ca²⁺ responses as in set B (n = 5). This study demonstrates the functional presence of type I InsP₃R-operated Ca²⁺ channels in human oocytes and further suggests an active role of InsP₃ in triggering the Ca²⁺ rise and secondary activation phenomena at fertilization.