Developmental changes in the intracellular Ca2+ release mechanisms in porcine oocytes

The presence of different intracellular Ca2+ release mechanisms in porcine oocytes and their involvement in mediating Ca2+ transients in different developmental stages were investigated. Metaphase II arrested oocytes showed an increase in intracellular Ca2+ concentration after injection of inositol 1,4,5-trisphosphate (InsP3), the InsP3 receptor agonist. Similar Ca2+ spikes could be detected after injection of ryanodine and cyclic ADP ribose, the ryanodine receptor agonists. The InsP3-induced Ca2+ release was inhibited by heparin, the InsP3 receptor antagonist, whereas procaine, the ryanodine receptor antagonist, blocked the Ca2+ transients generated by ryanodine and cyclic ADP ribose. In germinal vesicle-stage oocytes, intracellularly stored Ca2+ could also be mobilized by agonist treatment, though the effective concentration to generate the Ca2+ spikes was higher. After in vitro fertilization, repetitive Ca2+ transients were generated in oocytes starting 2.5-3 h after insemination. They ceased around the time of pronuclear formation when the oocytes entered first interphase. At this stage, the receptors were still capable of mediating Ca2+ release upon agonist treatment; in many cases these spikes were of longer duration, suggesting that in interphase it takes a longer time for the Ca2+ stores to resequester the mobilized Ca2+ from the cytosol. These results suggest that porcine oocytes possess both InsP3 and ryanodine Ca2+ channel receptors and that the properties of the Ca2+ release mechanisms change during oocyte development.