Changes in intracellular free calcium activity in Xenopus eggs following imposed intracellular pH changes using weak acids and weak bases

Abstract

The aim of this work was to determine the potential relationships between rises in intracellular pH (pH i) and intracellular free calcium activity (Ca i 2+) during cell activation in Xenopus eggs. To do this, we used two weak bases, NH 4Cl and procaine, and a weak acid, CO 2, and measured Ca i 2+ variations in response to these imposed pH i variations. NH 4Cl and procaine increased Ca i 2+ in both unactivated and activated eggs. Procaine was found to alkalinize the egg cytoplasm, whereas the other weak base, NH 4Cl, acidified the egg cytoplasm. On the other hand, CO 2 was found to acidify the cytoplasm and to substantially decrease Ca i 2+, also in unactivated and activated eggs. In addition, CO 2 triggered an increase in the conductance of the plasma membrane to Cl − ions, similarly to what had been found previously with weak bases (Charbonneau, M. (1989) Cell Differ. Develop. 26, 39–52). These Cl − channels, similarly to the sperm-triggered Cl − channels during the fertilization potential, are supposed to be Ca 2+-sensitive. Therefore, the changes in Ca 2+ observed in response to CO 2 do not seem to be responsible for the opening of these Cl − channels, which would rather be triggered by an increase in Ca i[su2+] localized near the plasma membrane. We conclude therefore that weak acids and bases represent appropriate tools for studying cytosolic Ca 2+ homeostasis, but not for dissecting the complex pathways involved in signal transduction.