Activation of frog (Xenopus laevis) eggs by inositol trisphosphate. I. Characterization of Ca2+ release from intracellular stores.

Abstract

Iontophoresis of inositol 1, 4, 5-trisphosphate into frog (Xenopus laevis) eggs activated early developmental events such as membrane depolarization, cortical contraction, cortical granule exocytosis, and abortive cleavage furrow formation (pseudocleavage). Inositol 1,4-bisphosphate also triggered these events, but only at doses -100-fold higher, whereas no level of fructose-I, 6-bisphosphate tested activated eggs. Using Ca2+-selective microelectrodes, we observed that activating doses of inositol 1, 4, 5-trisphosphate triggered a Ca 2+ release from intracellular stores that was indistinguishable from that previously observed at fertilization (Busa, W. B., and R. Nuccitelli, 1985r J. Cell Biol., 100:1325-1329), whereas subthreshold doses triggered only a localized Ca 2÷ release at the site of injection. The subthreshold IP3 response could be distinguished from the major Ca 2÷ release at activation with respect to their dose-response characteristics, relative timing, sensitivity to external Ca 2+ levels, additivity, and behavior in the activated egg, suggesting that the Xenopus egg may possess two functionally distinct Ca 2+ pools mobilized by different effectors. In light of these differences, we suggest a model for intracellular Ca 2+ mobilization by sperm-egg interaction.A transient increase in intracellular free Ca 2÷ concentration ([Ca2*]i) ~ accompanies fertilization in both invertebrate (8, 9, 23) and vertebrate eggs (4,6,16,21), and considerable evidence suggests that such Ca 2+ pulses provide the primary regulatory signal that triggers the initiation of development in the zygote (13,18). The means by which sperm-egg interaction triggers intracellular Ca 2+ release in the egg is unknown, but Ca2+-induced Ca 2÷ release has been suggested to be involved (18,22).Recently, considerable evidence has accumulated that implicates the turnover of inositol lipids in the mobilization of intracellular Ca 2÷ seen in a variety of cell types during hormonal stimulation. In response to hormones such as acetylcholine, vasopressin, or platelet-derived growth factor, responsive cells such as pancreatic acinar cells, hepatocytes, or cultured fibroblasts, respectively, undergo a pronounced increase in the hydrolysis of the plasma membrane lipid, phosphatidylinositol-4, 5-bisphosphate (PIP2), releasing into the cytosol the water-soluble product, inositol-1, 4, 5-trisphosphate (IP3), which, in turn, can trigger Ca z÷ release from intracellular stores in these and other cells (see references 1 t Abbrevmtions used in this paper.[Ca2÷]i, intracellular free Ca 2÷ concentration; IP2, inositol-l, 4-bisphosphate; IP~, inositol-l, 4, 5-trisphosphate; pCa, negative log of free Ca 2+ concentration; PIP2, phosphatidylinositol-4, 5-bisphosphate. and 2 for review). That hydrolysis of PIP2, yielding IP3, may also be the trigger for Ca 2÷ release in eggs at fertilization is suggested by recent observations on sea urchin eggs. Fertilization of these eggs triggers a rapid turnover of PIP2 (24), and microinjection of IP3 into the unfert...

Dissociation of store release from transmembrane influx of calcium in human neutrophils

“…The storage and handling of intracellular Ca 2+ in these cells may thus be similar. In some cell types, a second Ca -~+ store whose release is regulated by cytosolic free Ca v concentration also exists [12,[17][18][19]. However, here the release of Ca 2+ from the store remained loealised, suggesting the absence of Ca~+-induced Ca -~÷ release sites in neutrophils.…”

Analysis of the mechanism by which calcium negatively regulates the tyrosine phosphorylation cascade associated with sperm capacitation

“…Furthermore, such changes have been suggested to regulate [Ca 2+ ]i levels and consequently, tyrosine phosphorylation levels. In other biological systems, changes in [pH]i have been shown to regulate various cellular functions including Ca 2+ homeostasis and the status of ion channels (Busa et al, 1985), gene expression (Isfort et al, 1993) and cell death (Reynolds et al, 1996). In our hands, spermatozoa incubated in Ca 2+ -depleted medium also displayed cytosolic alkalinization, which rose simultaneously with tyrosine phosphorylation.…”

Oscillatory chloride current evoked by temperature jumps during muscarinic and serotonergic activation in Xenopus oocyte.

“…The released calcium, in turn, activates the membrane chloride channels. This calcium-mobilizing activity of InsP3 has been well documented in a variety of permeabilized cells (Berridge & Irvine, 1984), and has recently been demonstrated also in intact oocytes (Busa Ferguson, Joseph, Williamson & Nuccitelli, 1985;Parker & Miledi, 1986).…”

“…These changes can be mimicked by intracellular injection of InsP3 (Whitaker & Irvine, 1984;Busa et al 1985;Slack, Bell & Benos, 1986), suggesting that they involve processes similar to those which give rise to the oscillatory chloride current. Furthermore, cyclic oscillations in intracellular calcium have been observed in mouse oocytes following activation by sperm (Cuthbertson & Cobbold, 1985), so it is likely that the mechanism responsible for generating the oscillations is important during fertilization.…”