Relationship between ovulation rate and concentrations of insulin-like growth factor-1 in plasma during the oestrous cycle in various genotypes of sheep
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To determine whether concentrations of insulin-like growth factor-1 (IGF-1) in blood of ewes change during the oestrous cycle, oestrus was synchronized for 45 ewe lambs from four genotypes (Finn ewes selected for low ovulation rate (LF), Finn ewes selected for high ovulation rate (HF), unselected control Finn ewes (CF) and Cambridge ewes (CAM)) using progestin sponges and blood samples were taken every day from day 0 (day 0 = day of progestin sponge removal) to day 5, and then every second or third day until 3 days after the next oestrus. Ovulation rates (determined via laparoscopy) following the first oestrus were 1.3, 3.3, 2.0 and 2.1 for LF, HF, CF and CAM groups, respectively. In a second experiment, jugular and utero–ovarian venous blood samples were collected simultaneously from seven Rambouillet crossbred ewes during the mid-luteal phase of an oestrous cycle to determine whether the ovary is a major source of blood IGF-1. In the first experiment, plasma IGF-1 concentrations increased (P < 0.05) between days 0 and 3, and then decreased (P < 0.05) between days 4 and 8 in all groups. IGF-1 concentrations increased again at the subsequent oestrus. There was no significant difference in plasma IGF-1 between HF and LF ewe lambs. Overall, plasma IGF-1 was lowest (P < 0.05) in CAM and highest in CF ewe lambs at all stages. Plasma IGF-1-binding protein activity did not vary with stage of cycle or differ (P > 0.10) among genotypes. Among LF, HF and CF ewe lambs, ovulation rate was not correlated with plasma IGF-1 or IGF-1-binding protein activity. In the second experiment, serum concentrations of IGF-1 in jugular (174 ± 38 ng ml−1) and utero–ovarian (188 ± 43 ng ml−1) venous blood did not differ (P > 0.10) but were highly correlated (r = 0.96). We conclude that (i) plasma concentrations of IGF-1 increase during oestrus in cyclic ewes, (ii) plasma concentrations of IGF-1 are influenced by genotype in sheep but are not genetically associated with ovulation rate, (iii) plasma IGF-1-binding protein activity is not influenced by stage of cycle or genotype in sheep, and (iv) the ovary does not appear to be a major source of blood IGF-1.
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