Free calcium changes associated with hormone action in amphibian oocytes

Ca²⁺-sensitive electrodes and the photoproteins obelin and aequorin were used with the oocytes of the anuran Xenopus laevis and the urodeles Ambystoma mexicanum and Pleurodeles waltlii in order to detect any changes in internal free Ca²⁺ which might result from progesterone or agonist stimulation. A dramatic Ca²⁺ surge was recorded: from 0.7 × 10^?6M in the unstimulated oocyte to 7 × 10^?6M after stimulation but before germinal vesicle breakdown (GVBD). Ca²⁺ efflux was also measured, but it accounted for less than 0.2% of the internal Ca²⁺ transient; this efflux did not take place in the absence of external calcium. The Ca²⁺ surge and maturation in response to progesterone, p-hydroxymethylenesulfonate (PHMPS), or Mn²⁺ occurred normally even when divalent cations were absent from the external medium. In contrast, external divalent cations were necessary for the induction of meiosis and a Ca²⁺ transient by the K⁺ ionophore valinomycin. HCO₃^? also triggers meiosis and causes Ca²⁺ release, but the release occurs with quite different kinetics. Incompletely grown or seasonally dormant oocytes as well as 10 mM theophilline- or procaine-treated oocytes neither release Ca²⁺ nor respond to the hormone. We conclude that intracellular released Ca²⁺ is likely to be the major “second messenger” following hormone stimulation in amphibian oocytes as in starfish.