Effectiveness of different estrogen pulses in plasma for accelerating ovum transport and their relation to estradiol levels in the rat oviduct

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.