Blockade of Pseudopregnancy in the Rat by Treatment with Antiprogesterone Serum

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Abstract
The role of peripheral progesterone [P] in the initiation of pseudopregnancy (psp) was investigated. Antiprogesterone serum was generated in a rabbit immunized with progesterone-11.alpha.-hemisuccinate coupled with ovine serum albumin and injected i.v. once or twice into cervically stimulated female rats. The uterine cervical stimulation was performed on proestrus (day 0) at 1900 h. The antiserum was injected at various times between days 0-2. Serum prolactin (PRL) and P concentrations were determined by respective radioimmunoassays from days 1-5. The amount of "free" P was calculated by subtracting the amount of P bound to the previously injected P antibody from the total amount of P. Antiprogesterone serum injections (1 ml) at 0700 h on days 1 and 2 prevented the initiation of psp in 9 of 10 rats, but the other combination of 2 injections was ineffective. In psp-blocked rats, nocturnal PRL surges decreased gradually in magnitude and were absent on day 5, while diurnal surges were absent from day 1. Free P stayed at low levels between days 3-5. Another group of rats received a single antiprogesterone serum injection. Only the treatment at 0700 or 0800 h on day 1 was effective in preventing psp initiation in 5 of 15 rats. Only those rats which underwent psp in spite of this treatment were used to measure changes in levels of free P. Free P concentrations were lowered on days 2 and 3 and started to rise on Day 4 in contrast to normal psp rats, whose 1st significant rise was observed on day 3. The nocturnal surge was detectable throughout the period examined, but the magnitude on day 2, 1 day after the antiprogesterone treatment, was suppressed. Apparently the magnitude of the nocturnal PRL surge is attenuated if circulating P is curtailed at 0700 h of the previous morning and the occurrence of the diurnal PRL surge is highly susceptible to the peripheral P level. Antiprogesterone treatment at 0700 h on day 1 or 2 will trigger the following sequential events: attenuation of the nocturnal surge, preclusion of the diurnal surge, impairment of P secretion and interruption of psp.

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