Progesterone-induced second messengers at the onset of meiotic maturation in the amphibian oocyte: Interrelationships between phospholipid N-methylation, calcium and diacylglycerol release, and inositol phospholipid turnover

The steady-state turnover in phospholipid N-methylation, 1,2-diacylglycerol and inositol phospholipids in prophase-arrested Rana pipiens oocytes was compared with changes occurring in these pathways immediately following progesterone induction of the first meiotic division. Oocytes were preincubated with [ 3H- methyl]methionine, [ 3H]glycerol, [ 3H] myo-inositol or [ 3H]arachidonic acid. Ca 2+ efflux was measured in oocytes preloaded with 45Ca 2+. Membrane phospholipids and cytosolic levels of radiolabeled 1,2-diacylglycerol (DAG), inositol bis- (InsP 2), tris- (InsP 3), and tetrakisphosphate (InsP 4) were monitored immediately following induction with progesterone. A transient increase in both N-methylation of ethanolamine phospholipids and in [ 3H]DAG coincides with a release of 45Ca 2+ from the oocyte surface during the first minute. At least 80% of the total phospholipid N-methylation is associated with the plasma membrane. 45Ca 2+ and [ 3H]DAG release occur prior to a rise in intracellular lnsP 3, the latter beginning 2–3 min after exposure to the hormone and reaching a maximum by 15–30 min. Progesterone induces rapid and successive changes in ethanolamine, choline, and inositol-containing phospholipids, which represent three of the four major phospholipid classes found in membranes. The maintenance of higher levels of DAG and InsP 3 during the first 90 min might be expected to sustain the previously observed increase in protein kinase C activity.