276 THE EFFECT OF CUMULUS CELLS DURING MATURATION ON THE RISE IN THE CONCENTRATION OF INTRACELLULAR Ca²⁺ ([Ca²⁺]i) OF PORCINE OOCYTES INDUCED BY INOSITOL 1,4,5-TRISPHOSPHATE

Abstract

Increase of inositol 1,4,5-triphosphate (IP3) in the cytoplasm of mammalian oocytes is said to be responsible for [Ca²⁺]i oscillation observed in the oocytes immediately after sperm penetration, and the [Ca²⁺]i oscillation is known to be essential for the development of embryos. On the other hand, cumulus cells have been reported to play an important role in cytoplasmic maturation of oocytes and affecting the embryonic development. To obtain more information about the role of cumulus cells in cytoplasmic maturation, the effects of cumulus cells during maturation on the rise in [Ca²⁺]i and on the rate of activation of porcine mature oocytes induced by IP3 injection were investigated. The immature porcine oocytes were divided into three groups: COCs (intact cumulus-oocyte complexes), DOs (oocytes denuded of their cumulus cells), Co-culture (DOs attached to separated cumulus cells). These groups of immature oocytes were cultured in NCSU23 46 h for maturation. To examine the function of cumulus cells, two groups of immature oocytes were also prepared: DOs + pyruvate (DOs put into NCSU23 with pyruvate) and COCs-glucose free (COCs put into NCSU23 without glucose). The mature oocytes from each group were loaded with Ca²⁺ indicator fluorescent dye Fura2-AM, and then were irradiated by 340 nm and 360 nm of ultraviolet immediately after the injection of IP3. The intensities of emission light caused by the irradiation of 340 nm and 360 nm ultraviolet were recorded as E340 and E360. Since coupling of Ca²⁺ and the dye intensifies E340, but does not change E360, the level of [Ca²⁺]i was shown as R (ratio = E340/E360) in this study. Activation rate was calculated by counting the number of the oocytes that formed pronuclei by injection of IP3. ANOVA and Student's t-test were used in this study. Transient rise in [Ca²⁺]i was observed in the mature oocytes from every group. The peak R of the rise in [Ca²⁺]i of the mature oocytes derived from COCs, Dos, and Co-culture and induced by IP3 were 7.2, 4.0, and 6.9, respectively. The R of DOs was significantly lower than those of the others (P < 0.05). Also, the activation rate of the mature oocytes from DOs was significantly lower than those from COCs and Co-culture (31, 66, and 66%). The mature oocytes from DOs + pyruvate showed the same level of peak R compared with those from COCs (7.4 and 6.3), but COCs-glucose free showed a slight but significantly lower peak R compared with the mature oocytes from COCs (6.0 and 7.4, P < 0.05). In conclusion, cumulus cells appeared to support the rise in [Ca²⁺]i of porcine oocytes induced by IP3 during maturation and the following activation. Moreover, a function of cumulus cells supposedly produces pyruvate by metabolizing glucose and provides it to oocytes during maturation for promoting the cytoplasmic maturation.

A part of this study was supported by a Grant-in-Aid for the 21st Century COE Program of the Japan MEXT, and by a grant from the Wakayama Prefecture Collaboration of Regional Entities for the Advancement of Technological Excellence of the JST.