The organization of the cortical endoplasmic reticulum in Xenopus eggs depends on intracellular pH: artefact of fixation or not?

Phosphate induced developmental arrest of hamster two-cell embryos is associated with disrupted ionic homeostasis

Culture of hamster embryos with 0.35 mM inorganic phosphate results in developmental arrest at the 2-cell stage. These arrested 2-cell embryos were found to have significantly elevated levels of both intracellular pH and intracellular free calcium. Culture of 2-cell embryos with both glucose and phosphate did not further alter intracellular ionic homeostasis. Developmental arrest of 2-cell embryos was dependent on the concentration of phosphate used. Culture with 1.25 microM phosphate did not alter development, while concentrations of 2.5 microM and 5.0 microM resulted in a percentage of embryos arresting development at the 2-cell stage. Analysis of intracellular levels of pH and calcium after culture with different phosphate concentrations revealed a significant negative correlation between intracellular calcium levels and development beyond the 2-cell stage. There was no correlation between the increase in intracellular pH and embryo development in the presence of phosphate. The increase in intracellular calcium levels after culture with phosphate appears to be derived from intracellular pools, as preventing the influx of extracellular calcium did not alter development beyond the 2-cell stage. Therefore, it is apparent that a disruption in ionic homeostasis is associated with developmental arrest of hamster embryos cultured with phosphate.

Intracellular pH changes during zona drilling

OBJECTIVE

To investigate whether zona drilling of oocytes, a technique used to locally disrupt the zona pellucida, causes important changes in intracellular pH.

DESIGN AND METHODS

In a first set of experiments, intracellular pH was measured during zona drilling of mouse oocytes, using pH-sensitive microelectrodes. In a second set of experiments human oocytes that failed to fertilize were used to measure intracellular pH during drilling. In these cells intracellular pH was measured using microfluorimetry with the pH-sensitive dye 2',7'-bis-carboxyethyl-5(6)-carboxyfluorescein.

RESULTS

In mouse oocytes intracellular pH dropped from 7.25 +/- 0.02 (mean +/- SEM) to 7.09 +/- 0.03 during zona drilling, followed by a recovery to pH 7.17 +/- 0.02 after 4 minutes (n = 20). In human oocytes, intracellular pH dropped over 0.36 +/- 0.10 pH units during drilling, followed by a recovery that was complete within 4 minutes (n = 14).

CONCLUSION

Zona drilling is associated with a significant cytoplasmic acidification both in mouse and human oocytes. This effect is perhaps related to the high incidence of cytoplasmic degeneration after zona drilling of human oocytes.

Inositol 1,4,5-trisphosphate receptors in Xenopus laevis oocytes: localization and modulation by Ca2+

Inositol 1,4,5-trisphosphate receptors (InsP3R) in Xenopus laevis oocytes were localized and their regulation by Ca2+ was investigated. Antibodies raised against the C-terminal region of the mouse cerebellar InsP3R (cAb) cross-reacted with a 255 kD protein in Western blots of Xenopus microsomal membranes. Immunolocalization of this protein in cryosections of oocytes revealed diffuse staining of the cytoplasm, intense staining of the sub-plasma membrane region of the animal hemisphere, and punctate staining in association with the germinal vesicle. In the presence of 40 microM free Ca2+, isolated oocyte membranes exhibited a high affinity binding site for Ins 1,4,5-P3 (KD = 5nM) and a binding capacity of 450 fmol/mg protein. The specific binding capacity of oocyte membranes for [3H]-Ins 1,4,5-P3 increased as the level of free Ca2+ present in binding assays was raised from < 0.1 nM to 4.0 microM, with an apparent EC50 of 60 nM. Increasing the concentration of free Ba2+ failed to facilitate [3H]-Ins1,4,5-P3 binding. Other inositol phosphates competed for Ins1,4,5-P3 binding sites with approximate IC50 values of: Ins1,3,4,5-P4 = 79 nM, Ins2,4,5-P3 = 455 nM and L-Ins1,4,5-P3 = 20 microM. In addition, 150 micrograms/ml (approximately 12 microM) heparin displaced 50% of bound [3H]-Ins1,4,5-P3, whereas caffeine (10 mM) had little effect. Functional reconstitution of solubilized InsP3Rs into lipid bilayers revealed that Ca2+ was a necessary co-agonist for activation of the InsP3R. When InsP3 (5 microM) and Ca2+ (5 microM) were applied together, conductance steps were observed. InsP3 or Ca2+ alone had little effect. These results suggest that the subcellular organization of InsP3Rs and the facilitation of InsP3 binding and channel opening by Ca2+ contribute to the Ins1,4,5-P3-mediated Ca2+ spikes, waves, and oscillations observed in Xenopus oocytes.