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. Ca2+ efflux was measured in oocytes preloaded with 45Ca2+. Membrane phospholipids and cytosolic levels of radiolabeled 1,2-diacylglycerol (DAG), inositol bis- (InsP2), tris- (InsP3), and tetrakisphosphate (InsP4) 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 45Ca2+ from the oocyte surface during the first minute. At least 80% of the total phospholipid N-methylation is associated with the plasma membrane. 45Ca2+ and [3H]DAG release occur prior to a rise in intracellular InsP3, 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 InsP3 during the first 90 min might be expected to sustain the previously observed increase in protein kinase C activity.