Calcium sequestering activities of reticulum vesicles from Xenopus laevis oocytes

Membrane vesicles isolated from Xenopus laevis full-grown stage VI and mature oocytes accumulate 45Ca in the presence of ATP and oxalate. The Ca2+-pumping activity measured in vitro does not appear to be modified during meiotic maturation; it is not affected by the complex Ca2+-calmodulin. Preliminary experiments have shown that the addition of Na+ (30 mM) rapidly discharges accumulated 45Ca into oocyte vesicles indicating that a Na+/Ca2+ 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 Ca2+ levels.