Subcellular Ca2+ dynamics inside and around the nucleus of immature hamster oocytes were analysed with confocal Ca2+ imaging. 2. The ratio value between emission intensity of two injected fluorescent Ca2+ indicators, Calcium Green and Fura Red, was almost uniform over the entire oocyte, suggesting that nucleoplasmic Ca2+ concentration ([Ca2+]n) is comparable to cytoplasmic Ca2+ concentration ([Ca2+]c) at the resting state. 3. When Ca2+ was iontophoretically injected into the nucleoplasm or the perinuclear cytoplasm, it diffused across the nuclear envelope (NE), and perinuclear [Ca2+]c and [Ca2+]n reached the same level within 2 s, although the NE worked as a weak but detectable barrier for Ca2+ diffusion. 4. Inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release from the NE through the inner membrane was not detected, even when a large amount of IP3 was delivered in close proximity to the inner nuclear membrane. 5. When an oocyte was uniformly stimulated by photolysis of caged IP3, a Ca2+ rise was initiated in the perinuclear cytoplasm. The [Ca2+]n rise was always delayed with respect to, but rapidly equilibrated with, the [Ca2+]c rise. 6. Clusters of the endoplasmic reticulum were located in the perinuclear cytoplasm and served as the trigger zone of IP3-induced Ca2+ release. 7. The results indicate that the [Ca2+]n rise occurs as the consequence of the influx of Ca2+ which was released in the perinuclear cytoplasm, not Ca2+ release from NE to the nucleoplasm.
