Progesterone-induced changes in the phosphoryl potential during the meiotic divisions in amphibian oocytes: Role of Na/K-ATPase

Progesterone initiates an increase in phosphoryl potential during the first meiotic division, resulting in the accumulation of high energy protein phosphate by second metaphase arrest. 31P-NMR, with saturation transfer, demonstrates that the phosphocreatine level rises ~2 fold and that the "pseudo" first order rate constant for the creatine kinase reaction falls to ~20% of the control by the onset of nuclear membrane breakdown. 32PO4 pulse-labeling reveals a net increase in phosphorylation of yolk protein phosvitin during this period. The increased yolk protein phosphorylation coincides with internalization of membrane Na/K-ATPase and membrane depolarizatioThese results indicate that progesterone binding to the catalytic subunit of the Na-pump diverts ATP from cation regulation at the plasma membrane to storage of high energy phosphate in yolk protein. Phosvitin serves as a major energy source during fertilization and early cleavage stages and is also a storage site for cations (e.g. Na+, K+, Ca2+, Fe2+/3+) essential for embryonic development.The amphibian ovarian oocyte is blocked in first meiotic prophase until a transient rise in gonadotropin stimulates its surrounding follicle cells to release progesterone [1], which binds to the first external loop of the catalytic subunit of the Na//K-ATPase at the oocyte surface to reinitiate the meiotic divisions [2,3]. The oocytes then complete one and one-half meiotic divisions, are released from the ovarian follicle, and become blocked at second meiotic metaphase. Sperm penetration results in the completion of meiosis followed by a period of rapid mitoses characteristic of the developing blastula. Rana pipiens ovaries contain a single population of mature oocytes that respond to gonadotropin, in contrast to the multiple growth stages seen in Xenopus laevis when maintained under laboratory conditions [4].Our previous studies with Rana pipiens oocytes showed that, in vivo, gonadotropin induces phosphorylation of the yolk pr