Collection of LutealTissue
The estrous cycles of 18 Holstein-Friesian heifers were synchronized using progesterone-releasing intravaginal devices (PRID; Sanofi Animal Health, Watford, UK) for 12 days and an injection of 500 ^g cloprostenol, a synthetic prostaglandin F2 (PGF2; Estrumate; Schering Plough, Hertfordshire, UK) 2 days before PRID removal. The CL were collected at slaughter on Days 15-20 of the estrous cycle (estrus = Day 0). Luteal tissue was snap frozen in dry ice/ isopentane and then stored at -70°C. Blood samples (7 ml) were collected once daily for 1-2 days prior to slaughter, and at slaughter, into lithium heparin-treated (143 U/tube) blood sample tubes (Vacutainer; Becton Dickinson, Oxford, UK). The plasma was collected after centrifugation at 1000 X g for 30 min and stored at -20°C prior to analysis of progesterone and 15-keto-13,14-dihydro-PGF2a (PGFM, the stable metabolite of PGF2a) content. buy yasmin online
Progesterone Assay and PGFM Assay
Plasma progesterone concentrations were measured using an RIA described by Corrie et al., as modified by Law et al.. All samples were measured in a single assay with an intraassay coefficient of variation of 13.9%. The assay sensitivity was 0.1 ng/ml, and values of less than 1 ng/ml were considered to indicate that functional luteo-lysis had taken place.