External Na+ inhibits Ca2+-ionophore activation of Xenopus eggs |
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| Institution: | 1. Helen Wills Neuroscience Institute, University of California, Berkeley, CA, United States;2. Advanced MRI Technologies, Sebastopol, CA, United States;1. Centre National de la Recherche Scientifique, Université de Strasbourg, Fédération de Médecine Translationnelle de Strasbourg, Institut des Neurosciences Cellulaires et Intégratives UPR3212, 67000 Strasbourg, France;2. Douglas Mental Health University Institute, McGill University, Montréal, Canada;3. Department of Psychiatry and Neuroscience, Université Laval, Québec, Canada;1. Department of Molecular Biology, University of Wyoming, Laramie, WY 82071, USA;1. University of Chemistry and Technology Prague, Prague, 166 28, Czechia;2. Helmholtz-Zentrum Dresden-Rossendorf, Dresden, 01328, Germany;3. TU Dresden, Dresden, 01062, Germany;4. Hiroshima University, Higashi-Hiroshima, 739-8521, Japan;1. School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China;2. Tianjin Key Lab of Biomass/Wastes Utilization, School of Environmental Science & Engineering, Tianjin University, Tianjin 300072, China;3. National-Local Joint Engineering Laboratory for Energy Conservation of Chemical Process Integration and Resources Utilization, Hebei University of Technology, Tianjin 300130, China |
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| Abstract: | In medium containing 8.25 mM NaCl, eggs of Xenopus laevis can be activated by threshold concentrations (3 to 5 × 10−8 M) of the divalent cation ionophore, A23187. Activation by threshold concentrations of A23187 is reduced substantially when the concentration of NaCl in the medium is raised to 40 mM. Ion substitution experiments with NaI, Na isethionate, and choline chloride demonstrate that the inhibitory effect is due to Na+ rather than Cl−. The inhibitory effect of 40 mM Na+ is blocked by the sodium influx inhibitor, amiloride (1 mM), and by 1 mM verapamil and 1 mM La3+. Elevation of intracellular pH (pHi) with NH4Cl markedly increased the effectiveness of threshold levels of A23187, as evidenced by hypercontraction of the cortex. Neither amiloride nor changes in extracellular Na+ concentration alter pHi, however. Changing the concentration of extracellular Ca2+ had no effect on activation by A23187, regardless of the concentration of Na+ in the extracellular medium. The effect of Na+ on ionophore-induced activation is discussed in terms of alternative hypotheses, including a sodium-calcium exchange mechanism that operates in somatic cells to maintain low intracellular concentrations of Ca2+. |
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