بررسی پاسخ هورمون‎های تیروییدی و فاکتور رشد شبه انسولینی – I به محدودیت خوراکی و دوره انتقال در گوسفندان دنبه‎دار

Abstract
Introduction: During the transition period, the lack of sufficient feed supply and the increase in energy required for fetal growth and the initiation of lactation are associated with changes in blood metabolites and hormones in dairy ruminants (van Dorland et al. 2009; Zarin et al. 2017; Ahmadpour et al. 2019). Thyroid hormones play an important role in circadian rhythms even in animals such as dairy cows (Bateman et al. 1984). The concentration of these hormones in dairy cattle is influenced by various factors such as the transition period, parturition, ambient temperature, feed restriction and diet components (Todini 2007; Todini et al. 2007; Fiore et al. 2015). Insulin-like growth factor-1 (IGF-I) is a peptide hormone with 70 amino acids. This hormone is one of the factors affecting the growth and development of breast tissue, which is done through the differentiation of breast epithelial cells and its maintenance (Flint and Vernon, 1998). In order to better understand the mechanism of these metabolic adjustments, the present study was conducted with the aim of investigating the consequences of feed restriction before and after parturition on the concentration of thyroid hormones involved in basal metabolism as well as insulin-like growth factor-1. Our hypothesis in this experiment was that feed restriction in the pre- and post-partum period as well as the physiological phase change from pregnancy to lactation has an effect on the concentration of these hormones in fat-tailed sheep.
Materials and Methods: To examine the response of thyroid hormones and insulin-like growth factor 1 in sheep subjected to feed restriction during parturition, 20 pregnant ewes with a mean age of 40.8 to 6.2 months and weighing 56 to 1 .8 kg (mean standard error) selected. The animals were randomly divided into two control (control; n=10) and feed restriction (restriction; n=10) groups. The control group had free access to feed from the beginning of the period to parturition (-5 to -1). The restriction group was fed complete at weeks -5 and -1 and 50, 65 and 80% of recommended diets at weeks -4, -3 and -2, respectively. After parturition, the same feeding program was repeated for five weeks (weeks 1 to 5). Blood samples were taken weekly. The plasma was isolated and the concentration of the desired hormones was measured using the ELISA method. The data analysis was based on the SAS mixed model.
Results and discussion: In the prepartume, the concentration of T3 and T4 hormones was not affected by feed restriction. The concentration of T3 hormone in the prepartume period in the control group remained unchanged, but in the restriction group only in the fifth week of the experiment (-5) it showed a significant increase compared to the other weeks of the experiment and also the fifth week in the control group (P<0.05). In the postpartum period, dietary restriction increased the concentration of T3 hormone in the restriction group, especially in the second to fifth weeks after delivery compared to the control group (P<0.05). The interaction effect of oral treatments on test weeks was also significant in the postpartum period for T3 hormone (P<0.01). The concentration of T4 hormone in the postpartum period was affected by food restriction and showed a higher concentration than the control group (P<0.01). showed (P<0.05). It seems that sheep with tails during the time of food restriction by regulating the basic metabolism of the body through increasing the activity. The thyroid gland and the increase in the concentration of hormones secreted from it keep the basic metabolism high to provide energy and nutrients needed by different tissues. This idea can be justified by the increase in lipolysis of reserve tissues (Puchi et al. 2000; Sinha et al. 2018), no decrease in blood glucose, no change in immunoglobulin content, and no difference between the components and compositions of milk in animals of the food restriction group(Zarin et al. 2021; Nouri et al. 2023). About 70% of ewes' mammary growth occurs in the last four weeks of pregnancy (Mellor and Murray, 1985), which can be affected during insufficient feeding.
In the prenatal period, the concentration of IGF-I showed a decreasing trend from the start of the experiment (week-5) to the time of delivery in both treatment groups, so that in the control group, it was lower from week-3 to week-1 than when the experiment started. (P<0.05). In the restriction group, this significant decrease started from the second week and showed a lower concentration (-5) until the last week compared to the first week of the experiment (P<0.05). Feed restriction caused the concentration of this hormone to be higher in the third week (-3) and fifth week of the experiment (-1) compared to the control group (P<0.05). The interaction effect of nutritional treatments and test time was statistically significant (P<0.01). In the postpartum period in the control group, no significant difference was observed between the weeks of the experiment, but in the restriction group, the second to fourth weeks where the 50%, 65%, and 80% restrictions were applied respectively, the concentration of IGF-I compared to the week The animals in the group with free access to feed (weeks 1 and 5) were less (P<0.05). Feed restriction caused a significant difference in IGF-I concentration in the fourth week of the experiment compared to the control group (P<0.05). Postpartum plasma IGF-I concentrations decreased in both groups, and this decrease was greater in the feed restriction group. The findings of the present study in the postnatal period are consistent with previous reports of low IGF-I levels in feed-restricted animals (Anderson et al. 2004; Banchero et al. 2006; Radcliffe et al. 2006). According to the obtained results, it is concluded that the increase in the concentration of this hormone in the pre-natal period due to the decrease in the availability of food for mammary development is a kind of homeostasis specific to long-tailed sheep to provide the minimum needs of newborns for their survival, so that the immunoglobulin content of colostrum and even lamb blood plasma was not significantly different between the two groups (Nouri et al. 2023).
Conclusion: According to the obtained results, it seems that the fat-tailed sheep during feed restriction through increasing the activity of the thyroid gland and increasing the concentration of the hormones secreted from it keeps the basic metabolism high to provide the energy and nutrients needed by the tissue are different. The decrease in the concentration of IGF-I in the feed restriction group after parturition also indicated the decrease in the development of mammary gland following the feed restriction, the results of which were confirmed in previous reports. But the increase in the concentration of this hormone in the prepartume period due to the decrease in the availability of edible substances in a way indicates the special ability of fat-tailed sheep to provide the minimum needs of their newborn for survival, so that the immunoglobulin content of the colostrum and even the blood plasma of the lambs are different between the two groups. These findings provide important new information regarding the relationship between thyroid hormones and nutritional status during feed restriction around parturition, suggesting that these hormones play an important role in providing energy requirements through the mobilization of body stores.