Roles of Calcium and pH in Activation of Eggs of the Medaka Fish, Oryzias latipes

• Published in: The Journal of cell biology Vol. 97; no. 3; pp. 669 - 678
• Main Author: Gilkey, John C.
• Format: Journal Article
• Language: English
• Published: New York, NY Rockefeller University Press 01.09.1983; The Rockefeller University Press
• Subjects: Ammonium Chloride - pharmacology; Animals; Biological and medical sciences; Buffers; Calcium; Calcium - physiology; Carbon Dioxide - pharmacology; Cell membranes; Cytoplasm; Cytoplasm - physiology; Early stages. Segmentation. Gastrulation. Neurulation; Eggs; Embryology: invertebrates and vertebrates. Teratology; Female; Fertilization; Fishes - physiology; Fundamental and applied biological sciences. Psychology; Hydrogen; Hydrogen-Ion Concentration; Ova; Ovum - physiology; Protein synthesis; Sea urchins; Freshwater environment; Vertebrata; Calcium; Pisces; Cell microenvironment; Egg activation; PH; Intracellular; Fecundation; Inorganic element
• ISSN: 0021-9525; 1540-8140
• DOI: 10.1083/jcb.97.3.669

Unfertilized eggs of the medaka fish (Oryzias latipes) were injected with pH-buffered calcium buffers. Medaka egg activation is accompanied by a transient increase in cytoplasmic free calcium (Gilkey, J. C., L. F. Jaffe, E. B. Ridgway, and G. T. Reynolds, 1978, J. Cell Biol., 76:448-466). The calcium buffer injections demonstrated that (a) the threshold free calcium required to elicit the calcium transient and activate the egg is between 1.7 and 5.1 μM at pH 7.0, well below the 30 μM reached during the transient, and (b) buffers which hold free calcium below threshold prevent activation of the buffered region in subsequently fertilized eggs. Therefore an increase in free calcium is necessary and sufficient to elicit the calcium transient, and the calcium transient is necessary to activate the egg. Further, these results are additional proof that the calcium transient is initiated and propagated through the cytoplasm by a mechanism of calcium-stimulated calcium release. Finally, a normal calcium transient must propagate through the entire cytoplasm to ensure normal development. Unfertilized eggs were injected with pH buffers to produce short-term, localized changes in cytoplasmic pH. The eggs were then fertilized at various times after injection. In other experiments, unfertilized and fertilized eggs were exposed to media containing either NH4Cl or CO2to produce longer term, global changes in cytoplasmic pH. These treatments neither activated the eggs nor interfered with the normal development of fertilized eggs, suggesting that even if a natural change in cytoplasmic pH is induced by activation, it has no role in medaka egg development. The injected pH buffers altered the rate of propagation of the calcium transient through the cytoplasm, suggesting that the threshold free calcium required to trigger calcium-stimulated calcium release might be pH dependent. The results of injection of pH-buffered calcium buffers support this conjecture: for a tenfold increase in hydrogen ion concentration, free calcium must also be raised tenfold to elicit the calcium transient.