Arginine, molecular formula: C6H 14N4O2, relative molecular weight: 174.2, is white crystal or crystalline powder. There are two isomers of arginine in nature: D- Arg and L- Arg, which mainly exist in animals. L- arginine (L-Arg) is a basic amino acid and one of the essential amino acids for the synthesis of animal body proteins, which has important physiological functions in animals [1]. L- arginine (L-Arg) plays an important role in many physiological activities. Arginine is traditionally defined as a non-essential amino acid, but arginine is an essential amino acid to maintain the growth and nitrogen balance of young animals. Under normal circumstances, most adult mammals can synthesize arginine by themselves, but it is not enough to meet the needs of the body, especially in the state of hunger, trauma and rapid growth. Therefore, arginine is considered as a conditionally essential amino acid [2]. Arginine is an essential amino acid in stress state and special growth stage, and L- Arg synthesized in vivo can not meet the needs of physiological metabolism [3].
Metabolism of 1 arginine
Many mammalian cells, such as intestinal cells, arterial and venous endothelial cells, macrophages, neutrophils and so on. L- arginine (L-Arg) can be deaminated by structural enzymes or induced enzymes to produce bioactive nitric oxide (NO), and the concentration of L- Arg in cell fluid is the key factor to limit the synthesis ofNO. Nitric oxide (NO) is an important endogenous active factor, which can regulate the immunity of the body, affect immune cells and immune factors expressed by them, and has certain effects on maintaining vascular tension and reducing the damage of gastrointestinal mucosa. In vivo, L- arginine (L-Arg) is the precursor of endogenous nitric oxide (NO) [4-5]. L- arginine (L-Arg) plays an important role in physiology, metabolism and nutrition. Almost all mammalian tissues use L- Arg to synthesize cytoplasmic proteins and nuclear proteins. Tong et al. [3] research shows that arginine in animals can increase nitrogen storage, play a regulatory role, control protein renewal, promote muscle protein synthesis, improve nitrogen balance and enhance immune status.
2 the nutritional role of arginine
2. 1 Improve the intrauterine growth retardation of sows.
In pig production, IUGR (intrauterine growth retardation) is not only the main reason for weak fetus, stillbirth and abnormal fetus in sows, but also the important reason for the growth retardation and the decrease of feed conversion efficiency of piglets after birth [6-7]. For intensive pigs, IUGR is mostly caused by poor placental transport ability and dysfunction of sows during pregnancy (especially in the middle and late pregnancy) [7]. During pregnancy, the maternal reproductive system and even the whole body will undergo a series of great changes to meet the needs of fetal growth and development. Fetal development is closely related to placental blood flow and nutrient supply. During normal pregnancy, the hemodynamic characteristics are that the resistance of peripheral venous system decreases and the placental blood flow increases significantly under the action of vasodilators. Therefore, interfering with placental blood flow through arginine NO signaling pathway has become an ideal way to regulate fetal nutrition supply and prevent or improve IUGR in piglets.
Nitric oxide (NO) is a kind of relaxing factor from vascular endothelium, and it is an important regulator to maintain normal pregnancy [8]. Because the placental circulation of the fetus lacks autonomic innervation, it mainly relies on vasoactive substances to maintain its hypotonic state. Therefore, a high level of nitric oxide (NO) in the body can reduce the vascular tension by weakening the sensitivity of placental blood vessels to endothelin ⅰ and thromboxane A2, and maintain the low resistance of placental-fetal blood circulation by preventing platelet adhesion and aggregation, thus ensuring the nutritional supply of the fetus [9]. Sladek et al. [10] research shows that nitric oxide (NO) plays an important role in regulating vasodilation during pregnancy, increasing placental blood flow, reducing fetal-placental circulation resistance, keeping uterus still before labor, inhibiting platelet adhesion and aggregation, and regulating blood coagulation function. Nitric oxide synthase catalyzes arginine to produce nitric oxide, which can promote the secretion of vascular endothelial growth factor and play an important role in vasodilation. Fetal nutrition mainly depends on placental vascular transport, which means that placental blood flow may be directly regulated by nitric oxide or indirectly regulated by arginine. If the content of protein in maternal diet is restricted, the levels of arginine and ornithine in maternal body will decrease, the levels of nitric oxide synthase (NOS) and nitric oxide (NO) will decrease, and the activity of ornithine decarboxylase related to polyamine production and its polyamine concentration will also decrease [1 1]. Mateo et al. [12] found that11% arginine added to the diet during the 30 ~/4d pregnancy of sows could prevent intrauterine growth restriction, thus increasing the number of live births by 22% and increasing the average number of live births from 9.37 to/kloc-0. Mateo et al. [13] showed that adding 1% arginine to the diet of primiparous sows from the 30th day of pregnancy to 2 1d postpartum can increase the expression of immune-related genes in white blood cells of sows, thus improving the immunity of primiparous sows.
2.2 Improve the composition of milk
The content of arginine in breast milk is far from meeting the demand of piglets for arginine. It is difficult for newborn piglets to supply arginine through sows, mainly because arginine metabolism in breast tissue is very complicated [14]. Wu et al. [15] added 0.4% arginine (* * * containing 0.8 1% arginine) to the traditional diet of sows, compared with the same nitrogen diet containing 0.82% alanine, it had no effect on the arginine content in 2 1d breast milk during lactation. Guo Changyi [16] showed that adding arginine to sow diet can increase the arginine concentration in plasma of lactating sows, but it has no significant effect on the arginine content in pig milk. This is mainly because the mammary glands of lactating sows contain arginase II which decomposes arginine. Breast tissue has a strong ability to absorb amino acids, and arginine is metabolized vigorously in breast tissue, producing proline, polyamines and so on. , resulting in low arginine content in milk [14].
Glutamine/glutamic acid and proline in breast milk are important precursors of citrulline and arginine produced by intestinal cells in newborn piglets in the first few days [17]. Because proline is the main precursor of arginine produced by intestinal cells of newborn piglets [18], there is an arginine-proline cycle between mother and child, that is, arginine is first converted into proline from sow breast tissue, then enters the intestine of piglets with breast milk, and then is converted into arginine again from intestinal tissue for use by piglets [14]. In addition, breast tissue contains nitric oxide synthase, which can convert arginine into NO and L- citrulline, which is the secondary pathway of arginine in breast tissue. The content of polyamines in pig milk is very rich [19], which is necessary for cell proliferation and differentiation, so polyamines may have important nutritional significance for the intestinal growth and development of newborn piglets [20].
2.3 Immunity of arginine
The regulation of arginine on immunity is mainly realized through arginase and NO pathway. Arginine can improve the nutritional status of animals, increase the quality of thymus and the number of thymus lymphocytes, promote the production of mitogens such as phytoagglutinin and concanavalin, and significantly improve the responsiveness of T lymphocytes to mitosis, thus stimulating the proliferation of T lymphocytes. Enhance the phagocytosis of macrophages and the dissolution of tumor target cells by natural killer cells; Increase the activity of spleen monocytes secreting interleukin -2(IL-2) and IL-2 receptor; Enhance T-cell-mediated type IV hypersensitivity to reduce the level of prostaglandin (pGE2), further promote the synthesis of IL-2, and finally produce IL-2 to improve the immunoregulation of T-lymphocyte-mediated direct or indirect reactions [2 1]. NO inhibits platelet aggregation and adhesion, neutrophil aggregation and adhesion, reduces endothelial cell permeability and inhibits inflammatory exudation in the early stage of acute inflammation. NO transforms guanosine triphosphate (GTp) into cyclic guanosine monophosphate (cGMp) through soluble guanosine cyclase (sGC). Most functions of NO depend on the signal transduction mechanism of cGMp.
2.4 Antagonism of Arginine
Adding arginine to the diet should follow the principle of moderation, and exceeding a certain dose will inevitably bring negative effects to the animal body. There is evidence that the absorption of arginine has antagonistic effect on tryptophan, lysine and histidine, so the effect of exogenous arginine addition is not ideal [22]. Wink et al. [23] found that excessive absorption of arginine may antagonize tryptophan, lysine and histidine, and excessive exogenous L- arginine (L-Arg) will sharply increase the content of nitric oxide (NO) in a short time, causing strong damage to the body. Moreover, the negative effects of excessive arginine (2.5% in dry matter) include diarrhea, reduced food intake, slow growth, low immunity and even death. These phenomena are directly related to the amino acid imbalance caused by excessive addition of arginine. Antagonism and toxic side effects brought by exogenous arginine have adverse effects on sows, but these effects can be avoided by regulating the synthesis of endogenous arginine. Therefore, it is necessary to further study the synthesis and regulation of endogenous arginine in order to solve the problem of arginine deficiency in sows.
3 abstract
The mechanism of improving sow's reproductive performance by adding arginine to diet may be as follows: firstly, adding arginine to diet can increase the concentration of arginine in sow's plasma, thus enhancing sow's immunity and anti-stress ability, ensuring the stability of placenta environment, increasing embryo implantation and reducing embryo death; Secondly, with the increase of arginine concentration in plasma, more arginine will be used for fetal synthesis of protein, which can ensure sufficient raw materials for fetal tissue growth; Thirdly, arginine can promote the secretion of insulin and growth hormone, which is beneficial to improve the survival rate of embryos; NO, the metabolite of arginine, is an important molecular effector and neurotransmitter in animals, which can induce the pituitary gland to release prolactin and growth hormone. L- arginine can regulate the performance of sows and has great market prospects, but its high price limits its application in practice. Therefore, it is an urgent problem to find a cheap and non-toxic arginine substitute.