Variations of phospholipid methyltransferase(s) activity in the rat pituitary: estrous cycle and sex differences

We previously demonstrated a specific stimulatory action of estrogens on phosphatidylethanolamine methylation in rat pituitary membranes. To investigate the physiological relevancy of this effect, the activity of methylating enzyme(s) was evaluated during the rat estrous cycle, a period in which both endogenous ovarian steroid levels and the sensitivity of pituitary membrane receptors fluctuate. Anterior pituitary membranes (P2) were prepared from adult female rats at different stages of the estrous cycle and assayed for phospholipid methylation in the presence of S-adenosyl-[methyl-3H]methionine as a donor of 3H-methyl groups. Methylated phospholipids were separated by TLC. Formation of phosphatidyl-mono- and dimethylethanolamine and that of phosphatidylcholine increased significantly in the morning, reaching maximal values on the afternoon of proestrus; they decreased thereafter during estrus, metestrus, and diestrus. Plasma estradiol concentrations increased in late diestrus and then varied similarly with the fluctuations of phospholipid methyltransferase activity throughout the cycle. In parallel, plasma levels of LH and PRL were significantly elevated during the afternoon of proestrus, but remained low throughout the rest of the cycle. Under the same experimental conditions, phospholipid methylation in membranes prepared from mediobasal-hypothalamic structures was not affected. These data demonstrate that under physiological conditions the increased pituitary methyltransferase activity is associated with the progressive increment of plasma estradiol levels occurring shortly before proestrus and precedes the release of LH and PRL. Ovariectomy significantly decreased methyltransferase activity; however, 17 beta-estradiol treatment of ovariectomized rats for 5 days restored the enzyme activity, which was further augmented after progesterone administration. Attempting to investigate variations of pituitary methyltransferase activity in male rats, we demonstrated that the intact males showed weaker activity than that of females; orchidectomy diminished the phospholipid methylation, but adrenalectomy had no effect.