Plasma Levels of 13,14-Dihydro-15-keto Prostaglandin F2α in Relation to Oviposition and Ovulation in the Domestic Hen (Gallus domesticus)1

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Abstract
This study describes a specific radioimmunoassay with a sensitivity of 5-10 pg for the determination of 13,14-dihydro-15-keto prostaglandin F,2α (PGFM) in plasma of laying hens in relation to oviposition (OP) and ovulation (OV). The biological activity of PGFM both in vivo and in vitro was also compared with that of PGF2α. In addition, evidence is presented that plasma PGFM concentrations are directly correlated (r = 0.968, P2α and that PGFM disappears from the circulation with a t½ of about 8 min. Serially sampled plasma PCFM levels rose from 267 pg/ml 1 h before midsequence OP to 1747 pg/ml at OP, and returned to lower levels (546 pg/ml) by 1 h post-OP. Hens laying the terminal egg of a sequence (CtOP), which is not immediately followed by OV, displayed an increase in PGFM plasma levels from 347 pg/ml 1 h before OP to 1086 pg/ml at CtOP, returning to basal values 1 h post-CtOP. When OP was induced by arginine vasotocin (AVT, 0.2 µg, i.v.) 2-5 h before expected OP, plasma PGFM levels doubled at the time of expected spontaneous OP. No significant increase in either PGF or PGFM levels was noted in response to AVT-induced premature OP. Plasma PGFM also showed a peak at about the time of the first OV of the next sequence. Both PGFM and PGF2α stimulate uterine contractility in vitro and promote OP in vivo, but PGFM is ∼3 orders of magnitude less potent than PGF2α. These results suggest that 1) PGF2α is metabolized to PGFM, and PGFM plasma levels reflect relative changes in PGF2α levels; 2) in the fowl, as in other species, the biological activity of PGFM is less than 1% of that of PGF2α; and 3) plasma PGFM peaks at OP are not solely the result of increased uterine activity or AVT-stimulated PG production. Rather, PG synthesis and release may represent the primary event responsible for expulsion of the egg from the shell gland.