Protein kinase C acts downstream of calcium at entry into the first mitotic interphase of Xenopus laevis

Transit into interphase of the first mitotic cell cycle in amphibian eggs is a process referred to as activation and is accompanied by an increase in intracellular free calcium ([Ca²⁺]_i), which may be transduced into cytoplasmic events characteristic of interphase by protein kinase C (PKC). To investigate the respective roles of [Ca²⁺]_i and PKC in Xenopus laevis egg activation, the calcium signal was blocked by microinjection of the calcium chelator BAPTA, or the activity of PKC was blocked by PKC inhibitors sphingosine or H7. Eggs were then challenged for activation by treatment with either calcium ionophore A23187 or the PKC activator PMA. BAPTA prevented cortical contraction, cortical granule exocytosis, and cleavage furrow formation in eggs challenged with A23187 but not with PMA. In contrast, sphingosine and H7 inhibited cortical granule exocytosis, cortical contraction, and cleavage furrow formation in eggs challenged with either A23187 or PMA. Measurement of egg [Ca²⁺]_i with calcium-sensitive electrodes demonstrated that PMA treatment does not increase egg [Ca²⁺]_i in BAPTA-injected eggs. Further, PMA does not increase [Ca²⁺]_i in eggs that have not been injected with BAPTA. These results show that PKC acts downstream of the [Ca²⁺]_i increase to induce cytoplasmic events of the first Xenopus mitotic cell cycle.