Hydrostatic pressure inhibition of hormone-induced resumption of meiotic maturation in isolated oocytes.

Prior to ovulation, fully grown oocytes of both the amphibian (Rana pipiens) and the starfish (Piaster ochraceus), like those of many other organisms, are arrested in late prophase I of meiosis. Reinitiation of meiotic maturation in oocytes from either of these organisms is hormonally induced. Although the meiosis-inducing substance (MIS) for each organism is chemically dissimilar (a steroid in the frog and a purine in the starfish) induction of oocyte maturation in both biological systems appears to be initiated by the interaction of the MIS with the plasma membrane of the oocyte. The objective of this investigation was to determine if elevated hydrostatic pressure affected hormonal induction of oocyte maturation and to compare the effect of pressure on oocytes stimulated with different meiosis-inducing substances. In isolated oocytes from either the frog or the starfish, increasing ambient pressure reduced the percentage of oocytes which matured, and a stepwise increase in pressure resulted in a corresponding shift in the dose response curves of hormone-induced, oocyte maturation to the right. In experiments using only starfish oocytes, this inhibitory effect of pressure was found to be reversible and limited to the initial period of the maturation event. Taken together, these data suggest that elevated ambient pressure inhibits an early cellular event in the hormonal induction of meiotic maturation which is common to both amphibian and starfish oocytes.