Calcium sequestering activities of reticulum vesicles from Xenopus laevis oocytes

Abstract Membrane vesicles isolated from Xenopus laevis full-grown stage VI and mature oocytes accumulate 45 Ca in the presence of ATP and oxalate. The Ca 2+ -pumping activity measured in vitro does not appear to be modified during meiotic maturation; it is not affected by the complex Ca 2+ -calmodulin. Preliminary experiments have shown that the addition of Na + (30 mM) rapidly discharges accumulated 45 Ca into oocyte vesicles indicating that a Na + Ca 2+ exchange system occurs in this membrane fraction. During progesterone-induced maturation, the different intracellular membranes undergo morphological changes. We suggest that intracellular movement of membrane vesicles could be involved in the local regulation of Ca 2+ levels.