The role of ryanodine-sensitive Ca2+ stores in the Ca2+ oscillation machine of human oocytes.
This study was undertaken to localize ryanodine-sensitive Ca2+ stores in human oocytes and to evaluate their role in the Ca2+ oscillations responsible for oocyte activation at fertilization. The addition of ryanodine provoked a Ca2+ discharge from stores localized throughout the ooplasm with the exception of the cortical and subcortical peripheral regions. The ryanodine-induced discharge was typically followed by a short series of Ca2+ oscillations that only involved the cytoplasmic region populated by the ryanodine-sensitive stores. In contrast, the Ca2+ oscillations induced by the thiol reagent thimerosal or by spermatozoa at fertilization were of a much longer duration and also involved ryanodine-insensitive stores. Presumably, these ryanodine-insensitive stores are sensitive to inositol 1,4,5-trisphosphate (InsP3). The addition of ryanodine to oocytes during ongoing thimerosal- or sperm-induced Ca2+ oscillations inhibited the oscillations. These data suggest a co-operation between the ryanodine-sensitive and ryanodine-insensitive stores in maintaining the sperm-induced Ca2+ oscillations. In this two-store oscillation model, each periodic [Ca2+]i increase is triggered by a Ca2+ discharge from the peripheral, InsP3-sensitive stores inducing Ca(2+)-induced Ca2+ release from the ryanodine-sensitive stores. However, the pacemaker frequency of the Ca2+ discharges from the InsP3-sensitive stores is conditioned by the actual physiological state of the ryanodine-sensitive stores.
Sousa
M
M
Laboratory of Cell Biology, University of Oporto, Portugal.
Barros
A
A
Tesarik
J
J
D002118:Calcium / Q000502:physiology*; D002478:Cells, Cultured; D005260:Female; D006801:Humans; D017136:Ion Transport; D008297:Male; D018613:Microscopy, Confocal; D009865:Oocytes / Q000187:drug effects / Q000502:physiology*; D012433:Ryanodine / Q000494:pharmacology; D013084:Sperm-Ovum Interactions
D012433:Ryanodine; D002118:Calcium