Elevations of circulating estradiol (E2) levels due to administration of exogenous E2 accelerate embryo transport through the genital tract in pregnant rats. This study relates oviductal embryo transport to tissue E2 levels associated with blood E2 oscillations of differing profiles. Plasma E2 pulses differing in rate of increase, amplitude, and duration were achieved through various schedules of iv and sc E2 administration. Rats on the first day of pregnancy received a total dose of 5 μg 17β-E2 by short (10–15 min) or long (200–300 min) term iv infusions. Some animals were used to monitor blood and tissue levels of E2 (oviduct and diaphragm). Others were killed 24 h after treatment to assess number of embryos recovered. Fast iv infusions caused brief, high amplitude (>1000 pg/ml) E2 oscillations which were ineffective in eliciting accelerated embryo transport. The longer iv infusions produced lower but sustained elevations of circulating E2 levels comparable to those achieved by sc administration and were associated with accelerated embryo transport. The oviductal E2 concentration during and after a short iv infusion was never lower than that associated with a sc injection. The lack of response to a brief, high amplitude increase in circulating E2, therefore, could not be accounted for by decreased tissue content of the hormone. These results indicate that when the total mass of E2 administered is kept constant, the magnitude of embryo transport acceleration is positively correlated with the duration and negatively correlated with the amplitude and/or slope of increase in circulating estrogen. Since different tissue content of E2 does not account for the response or no response observed it follows that the geometry of E2 oscillations in plasma has a signal value for the target cells which acts independently from the bioavailability of the hormone.
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