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J. T. Lanman and Rosemarie B. Thau

Summary. When fetuses and placentas were removed on Day 18 in normal pregnant rabbits, plasma progesterone levels declined rapidly to non-pregnant values within 48 h. This decline was largely prevented if only fetectomy was performed, leaving the placentas in situ, but not when the fetal components of the placentas were also removed. These results suggested that ovarian progesterone production was dependent upon trophic influences emanating from the fetal portions of the placentas. Ovarian progesterone production was maintained by an extract of rabbit pituitaries for at least 72 h after removal of fetuses and placentas.

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Ovariectomized pregnant rabbits given sufficient progesterone maintained their pregnancies well but, after kindling, only a low percentage of young were found alive. Addition of oestradiol to the treatment regimen did not alter the effectiveness of progesterone in preventing abortion but increased the percentage of young found alive after parturition. The beneficial effect of oestrogen appeared to be related to a reduced incidence of prolonged pregnancies and prolonged labours, and suggested a rôle for oestrogen in the successful termination of pregnancy.

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Rosemarie B. Thau, J. T. Lanman and A. Brinson

Summary. The metabolic clearance rate (MCR) and production rate (PR) of progesterone were measured in rhesus monkeys both before and during intravenous infusion of progesterone at rates which approximately doubled or tripled the peripheral plasma levels. The monkeys were normally pregnant or fetectomized and were studied during the second half of pregnancy. Raising the peripheral plasma levels did not significantly alter the MCR or the PR of progesterone. We conclude that peripheral progesterone levels are not the factor which controls the PR of progesterone in rhesus monkeys.