ABSTRACT

Fertilization of mature mouse oocytes triggered highly repetitive Ca oscillations lasting 2-3 h. However, immature oocytes generated only two or three oscillations, which ceased within 1 h. Development of repetitive Ca 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 Ca from stores than mature oocytes in response to ionomycin and thapsigargin. Ryanodine had no effect on intracellular Ca in maturing oocytes but stimulated an increase in Ca in mature oocytes. The ability of ryanodine to increase Ca levels was, however, strain-dependent.

Preincubation of oocytes with thapsigargin or ryanodine significantly attenuated the normal fertilization Ca response, causing a decrease in the number and the rate of rise of the transients. The inhibition of sperm-induced Ca transients by ryanodine was independent of its ability to cause an immediate Ca increase. Low concentrations of ryanodine had no effect on resting Ca levels but inhibited Ca oscillations at fertilization. Similarly Ca oscillations were blocked in oocytes from a strain of mouse that showed no immediate Ca increase with ryanodine. These results suggest that modifications in Ca stores and ryanodine-sensitive Ca release mechanisms during oocyte maturation play an important role in Ca oscillations at fertilization.

FOOTNOTES

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()

The abbreviations used are: InsP, inositol trisphosphate; CICR, Ca-induced Ca release; GV, germinal vesicle; GVBD, GV breakdown; BSA, bovine serum albumin.

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