Protein kinase C, a pivotal regulator of hamster egg activation, functions after elevation of intracellular free calcium

We investigated the signal transduction pathways that mediate activation of Syrian hamster eggs. Under conditions in which the concentration of intracellular free calcium ([Ca²⁺]₁) is clamped low, activation of protein kinase C (PKC) can induce second polar body formation, reformation of the nuclear envelope, and decondensation of chromatin, as well as golgi reformation. However, calcium is necessary for normal transition from meiotic metaphase II to anaphase II. Conversely, under conditions in which the level of PKC activity is clamped low, induction of a rise in [Ca²⁺]₁ using the calcium ionophore A23187, does not induce egg activation. These results strongly suggest that PKC acts after the calcium signal as a proximal inducer of egg activation. This suggestion is supported by the kinetics of egg activation; PKC stimulators activate the eggs at a significantly enhanced rate (P < 0.01) compared with activation by calcium ionophore. We show here that PKC stimulators induce emission of the second polar body, but that subsequently, with longer culture, the emitted polar body is absorbed. Our results suggest that the rise in [Ca²⁺]₁ serves two functions, to activate PKC and to induce the transition from metaphase II to anaphase II. PKC, once activated, mediates several other events of egg activation.