Fertilization of mature mouse oocytes triggered highly repetitive Ca²⁺ oscillations lasting 2-3 h. However, immature oocytes generated only two or three oscillations, which ceased within 1 h. Development of repetitive Ca²⁺ transients to sperm occurred late in oocyte maturation and was dependent on cytoplasmic modifications that were independent of cell cycle progression from metaphase I to metaphase II. Immature oocytes released significantly less Ca²⁺ from stores than mature oocytes in response to ionomycin and thapsigargin. Ryanodine had no effect on intracellular Ca²⁺ in maturing oocytes but stimulated an increase in Ca²⁺ in mature oocytes. The ability of ryanodine to increase Ca²⁺ levels was, however, strain-dependent. Preincubation of oocytes with thapsigargin or ryanodine significantly attenuated the normal fertilization Ca²⁺ response, causing a decrease in the number and the rate of rise of the transients. The inhibition of sperm-induced Ca²⁺ transients by ryanodine was independent of its ability to cause an immediate Ca²⁺ increase. Low concentrations of ryanodine had no effect on resting Ca²⁺ levels but inhibited Ca²⁺ oscillations at fertilization. Similarly Ca²⁺ oscillations were blocked in oocytes from a strain of mouse that showed no immediate Ca²⁺ increase with ryanodine. These results suggest that modifications in Ca²⁺ stores and ryanodine-sensitive Ca²⁺ release mechanisms during oocyte maturation play an important role in Ca²⁺ oscillations at fertilization.
Journal of Biological Chemistry Vol. 270, Issue 12, p. 6671-6677