EFFECTS OF OESTRADIOL AND PROSTAGLANDIN F2α ON THE TIMING OF PARTURITION IN THE RAT
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Summary.
Groups of rats were treated with oestradiol or PGF2α, or both, at various times between Days 19 and 21 of pregnancy and the times of onset of parturition were recorded.
On Day 20, oestradiol (10 μg/rat) appeared to delay the onset of parturition in those rats which would otherwise have littered in the early part of the normal range, so that all the rats went into labour in the later part of that range. On Day 21, oestradiol (10 μg/rat) advanced the time of littering in about half the rats, but delayed parturition in about 30%.
Prostaglandin F2α (4×25 μg) on Day 19 or Day 20 caused all the rats to litter prematurely, but PGF2α (2×25 μg) on Day 21 advanced parturition only in rats which normally produced late litters. When oestradiol (10 μg/rat) was given on Day 20 or 21, following treatment with PGF2α (4×25 μg) on Day 19 or Day 20, the spread of times of onset of parturition was reduced.
It is postulated that (a) the final changes in preparation for parturition in rats are initiated at different times in different animals between late evening on Day 20 and early evening on Day 21, about 45 hr before parturition; (b) these changes, once started, probably take comparable times in different animals. The spread of times of parturition normally seen is thus attributed to the variation in the time at which the preparations begin and not in the length of time required for their completion; and (c) the first phase of the preparations involves prostaglandin-mediated luteolysis, and that this is followed by a final oestrogen-dependent phase which begins when progesterone concentration falls below a critical level and that of oestrogen attains a critical level.
Oestradiol on Day 20 may have a direct or indirect luteotrophic action in rats close to the onset of luteolysis at the time of dosing, thus causing retardation of luteolysis and delay of littering.
The advance of littering in many rats after oestradiol treatment on Day 21 is attributed to acceleration of the final phase of preparations for parturition in those rats at advanced stages of luteolysis at the time of dosing. Rats in which parturition was delayed by oestradiol treatment on Day 21 are believed to be those in which luteolysis had not begun and in which oestradiol exerted a luteotrophic effect.
The effects of exogenous PGF2α are all attributed to its luteolytic activity. An anti-inflammatory agent, fenclozic acid (50 mg/kg), given close to the time of parturition, neither delayed nor disturbed normal labour. This suggests that active prostaglandin biosynthesis at the time of parturition is not essential for the parturient activity of the uterus.
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Online ISSN: 1741-7899
Print ISSN: 1470-1626
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