CALCIUM DYNAMICS DURING STARFISH OOCYTE MATURATION AND FERTILIZATION

Intracellular free calcium levels in starfish oocytes have been monito red during meiotic maturation and fertilization using calcium-sensitiv e fluorescent dyes combined with confocal laser scanning microscopy or fura ratioing techniques. In time-lapse analyses of prophase-arrested and maturing oocytes, calcium transients were elicited by inositol 1, 4,5-trisphosphate (IP3), ryanodine, or caffeine, indicating that both the IP3-sensitive and IP3-insensitive receptors of the oocyte's calciu m release channels could be stimulated to mobilize calcium ions. Ferti lization also triggered a global calcium wave that appeared to travel faster around the cortex than through the center of the oocyte, and ma turing oocytes developed normally after their fertilization-induced ca lcium waves had been imaged. Prophase arrested specimens, on the other hand, did not undergo germinal vesicle breakdown or cleavage after di splaying a fertilization-induced calcium transient throughout their cy toplasm and nucleus, confirming previous observations that calcium spi kes are not sufficient to induce development in immature oocytes. In a ddition, although the calcium spikes triggered by sperm or caffeine re ached similar normalized peak heights, fertilization-induced calcium w aves in maturing oocytes tended to be more prolonged than the fertiliz ation waves observed in prophase-arrested oocytes or the caffeine-trig gered spikes elicited at any stage of maturation. Collectively, such f indings suggest that the total amount of releasable calcium does not v ary appreciably during maturation, but the patterns of the calcium tra nsients can differ depending on the stage of maturation and/or the typ e of calcium-releasing agent. Possible artifacts affecting these findi ngs are assessed, and the results are discussed relative to the functi oning of calcium release pathways during starfish oocyte maturation an d fertilization. (C) 1994 Academic Press, Inc.