Effect of exogenous melatonin on in vivo and in vitro prostaglandin secretion in

The effect of exogenous melatonin on prostaglandin secretion was measured on Rasa Aragonesa ewes. Fourteen ewes received an 18 mg melatonin implant (Mþ) on 10 April and were compared with 13 control animals (without implants M). Twenty days later, intravaginal pessaries were inserted in all animals to induce a synchronized oestrus (day 0). On day 14, ewes were injected, i.v., with 0.5 IU oxytocin. Plasma 15-ketodihydro-PGF2a (PGFM) concentrations were measured to assess uterine secretory responsiveness to oxytocin. After euthanasia, pieces of endometrium were collected to determine progesterone content and PGE2 and PGF2a secretion in vitro, in the presence or absence of either 20 mg/ml recombinant ovine interferon-tau (roIFNt) or 1 nmol/l oxytocin in the medium. Endometrial progesterone content was similar in the two treatments (Mþ: 50:25 17:34 ng/mg tissue, M: 43:08 11:21 ng/mg tissue). Mþ ewes that responded to oxytocin had significantly higher plasma PGFM concentrations between 10 and 80 min after oxytocin administration, a higher mean PGFM peak (P < 0:001), higher plasma PGFM levels after the challenge (P < 0:05) and higher plasma progesterone concentrations (P < 0:01) than control ewes. In the in vitro experiment, Mþ and M control samples secreted similar amounts of PGE2. Th