The effect of decreasing oestrogen secretion on the oviducal migration of embryos was investigated in pregnant rats. The reduction of oestradiol production was achieved by administration of the aromatase inhibitor 4-hydroxy-4-androstene-3,17-dione (4-OH-A) at various times after coitus.
When 4-OH-A was administered from days 2 to 5, nearly half the embryos were retained in the oviducts at midday on day 5 of pregnancy, in contrast with control animals in which all embryos were transferred to the uterus. Shorter treatments were less effective. The rate of secretion of oestradiol from the ovary on days 2–5 of pregnancy in control rats was low in the morning and high in the afternoon. Treatment with 4-OH-A from days 2 to 5 reduced the secretory surges of oestradiol in the afternoon by 77% without significantly changing the progesterone output. Systemic testosterone levels were significantly increased by this treatment.
To assess whether changes in the transport of ova were due to an increase in testosterone concentrations the influence of exogenous testosterone on embryo transport and oestradiol production was tested. Testosterone administered by subdermal implants from days 2 or 3 to day 5 disturbed embryo transport in a manner similar to that of 4-OH-A. The longest period of testosterone administration decreased ovarian oestradiol production by 82% without changing the secretion of progesterone.
Since ovarian oestradiol production in pregnant rats was reduced similarly by treatment with 4-OH-A and testosterone implants, and since exogenous oestradiol given s.c. on days 2, 3 and 4 counteracted the blocking effect of 4-OH-A and testosterone on embryo transport, the delay in oviducal transport of the embryos can be accounted for by decreased availability of oestradiol rather than increased testosterone production. This interpretation supports a crucial role for endogenous oestradiol in timing the passage of embryos to the uterus in the pregnant rat.
J. Endocr. (1988) 118,93–100