Impact of food deprivation on ova transport 47 from about 20 h after the onset of oestrus and until ovulation had occurred. The time of ovu- lation was set as the time when all large follicles had collapsed. Blood sampling A few days before the second oestrus after weaning was expected (days 15-18), a perma- nent silastic tubing was inserted into the right jugular vein (Rodriguez-Martinez et al. 1983). The silastic tubing was passed subcutaneously to the back and exteriorized through the skin where it was connected to a cannula to make blood collection easy and free from stress for the sows. The tubings were ﬂushed twice daily with heparinized saline (25 IE/ml) until the blood collection began. Blood collection in heparinized tubes started 12 h before expected ovulation in the second oes- trus after weaning and was performed every second hour until the sows were slaughtered. The heparinized tubes were immediately centri- fuged at a speed of 3000 rpm for 10 min and the collected plasma was frozen and stored at –20 °C. At 10 am and 6 pm blood was also col- lected in tubes without additive. Before storage, and in a period of 30 to 60 min after sampling, the tubes were centrifuged twice at the same speed and duration as above. Slaughter and recovery of the ova The sows were slaughtered at approximately 48 h after ovulation and the genital tract was im- mediately recovered. After the corpora lutea had been counted in each ovary, the uterine horns on both sides were separated from the uterine body and ﬂushed separately with 100 ml of 37 °C warm phosphate buffered saline (PBS). The length of the isthmus was estimated by the thickness of the oviductal wall, after which, the isthmic part of the oviduct was di- vided into 3 equally long segments. The isthmic segments were ﬂushed separately with 40 ml of PBS. Most of the ﬂushing ﬂuid was ﬁltered away before the remaining volume was exam- ined under a light microscope and the ova present in each part were counted. Hormone assays P r o s t a g l a n d i n m e t a b o l i t e . The main in- itial blood plasma metabolite of prostaglandin F2α, 15-keto-13,14-dihydro-PGF2α (15-ketodi- hydro-PGF2α), was analysed by radioimmu- noassay as described previously by Kunavong- krit et al. (1983). The relative cross-reactions of the antibody were 16% with 15-keto-PGF2α, 4% with 13,14-dihydro-PGF2α and 0.4% with PGF2α. The intra-assay coefﬁcients of variation ranged between 3.4% and 7.6% for different ranges of the standard curve and the inter-assay coefﬁcient of variation was approximately 14%.The practical limit of sensitivity for the assay, analysing 0.2 ml of plasma, was 60 pmol/l. P r og e s t e r o n e . The concentration of proges- terone (P4) in peripheral blood plasma was de- termined using an enzyme immunoassay (Amerlite, Kodak Clinical Diagnostics Ltd., Amersham, England). The kit was used accord- ing to the manufacturer’s instructions with the following modiﬁcations: to increase the sensi- tivity of the P4 assay, both the assay reagent (rabbit anti-progesterone) and the conjugate re- agent (HPR-labelled progesterone) were di- luted 1:2 with saline solution. To produce a new standard curve, standard B (plasma containing 2 nmol/l of P4) was diluted with standard A (hu- man 0-plasma) to 1.0, 0.5 and 0.25 nmol/l, which resulted in a standard curve with follow- ing concentrations: 0, 0.25, 0.5, 1.0, 2.0 (stan- dard B) and 8.0 (standard C). Serial dilutions of porcine plasma produced displacement curves parallel to the standard curve. The intra-assay coefﬁcients of variation calculated from 10 as- says were below 10% for concentrations between 0.25 and 8 nmol/l. The inter-assay co- Acta vet. scand. vol. 42 no. 1, 2001