Arginine and Insulin / Growth Hormone:
Quote:
Dietary Arginine Supplementation Enhances the Growth of Milk-Fed Young Pigs
ABSTRACT
This study was conducted to determine the effect of dietary arginine supplementation on the growth of artificially reared piglets. The pigs were removed from sows to a nursery facility and assigned randomly to 1 of the 3 treatments representing diets supplemented with 0, 0.2, or 0.4% L-arginine (on the basis of milk replacer powder). Each milk feeder was assigned to 1 dietary treatment. Fresh liquid milk replacer (18.6% dry matter) was provided daily (0800 h) to piglets. Body weights of piglets were measured and jugular venous blood samples were obtained for metabolite analysis at d 7, 14, and 21 of age. Food intake did not differ between control and arginine-supplemented piglets [66.7 vs. 69.5 g dry matter/(kg body wt d)]. Compared with control piglets, dietary supplementation with 0.2 and 0.4% L-arginine dose dependently increased (P 0.05) plasma concentrations of arginine by 30 and 61%, and decreased (P 0.05) plasma concentrations of ammonia by 20 and 35%, and those of urea by 19 and 33%, respectively. Dietary supplementation with 0.4% L-arginine also increased plasma concentrations of insulin and growth hormone by 24–27% in piglets, compared with controls.
Between 7 and 21 d of age, the supplementation of 0.2 and 0.4% L-arginine to piglets enhanced (P 0.05) average daily weight gain by 28 and 66%, and body weight by 15 and 32%, respectively, compared with control piglets.
Plasma concentrations of orotate and glucose did not differ between control and arginine-supplemented piglets.
Compared with control pigs, dietary supplementation with 0.4% L-arginine increased plasma concentrations of insulin and growth hormone by 24–27%. Dietary supplementation with 0.2% L-arginine did not affect plasma concentrations of insulin and growth hormone during the 2-wk experimental period.
Arginine serves as the most abundant nitrogen carrier in body proteins (17) and takes part in multiple metabolic pathways (18). In addition, as the physiologic nitrogenous precursor for the synthesis of nitric oxide (the endothelium-derived relaxing factor, a neurotransmitter, a mediator of immune response, and a signaling molecule), arginine plays an important role in whole-body homeostasis (21).
Paradoxically, in neonatal pigs, intestinal synthesis of citrulline and arginine from glutamine/glutamate and proline (abundant amino acids in sow’s milk) decreases markedly within the first 3 wk of life via unknown mechanisms (8,14).
Intriguingly, a substantial decrease in arginine availability occurs around the time (d 8 of life) (9) when suckling piglets start to exhibit submaximal growth (1).
Plasma concentrations of arginine and ammonia are sensitive indicators of arginine nutritional status in neonatal pigs (22) and human infants (23,24). As an allosteric activator of N-acetylglutamate synthase, which synthesizes N-acetylglutamate, an activator of carbamoylphosphate synthase I, arginine
plays a crucial role in ammonia detoxification via the hepatic urea cycle (25). Thus, a hallmark of arginine deficiency is an elevated concentration of plasma ammonia in mammals (22– 24). Consistent with this, plasma concentrations of ammonia were greater in 14- and 21-d-old control pigs than in 7-d-old
pigs, but were decreased by dietary supplementation with 0.2 and 0.4% L-arginine.
Although milk-fed piglets continue to grow, this does not necessarily mean that their growth is maximal, as exemplified by submaximal growth and impaired nitric oxide synthesis in arginine-deficient young rats (28).
There is experimental evidence supporting the view that a decrease in arginine availability may limit maximal growth in young pigs. For example, Leibholz (11) reported that in early weaned piglets, supplementing 0.2 and 0.4% L-arginine to a milk-protein–based powder diet (containing 19.2% crude protein) numerically improved weight gain between d 7 and 14 of life compared with control piglets. However, that study involved a small number of piglets (n 4/treatment group) and the data were not subjected to statistical analysis (11).
Importantly, dietary supplementation with 0.2 and 0.4% L-arginine dose dependently increased plasma concentrations of arginine and the body weight gain of piglets. It is noteworthy that plasma concentrations of lysine and histidine were not affected by dietary supplementation with 0.2 and 0.4% L-arginine, suggesting a lack of either an antagonism or an imbalance among the basic amino acids.
The ratios of lysine:arginine:histidine (g:g:g) in the milk replacer diets supplemented with 0, 0.2 and 0.4% L-arginine were:
1:0.346:0.221
1:0.449:0.221
1:0.552:0.221
which would not be expected to impair intestinal absorption of lysine or histidine (30). Thus, supplementing the liquid milk replacer with 0.2 and 0.4% L-arginine ensured its continuous availability to piglets without affecting lysine and histidine utilization. Because dietary intake of all nutrients (including protein, fat, carbohydrates, vitamins, and minerals), except for arginine, did not differ between control and arginine-supplemented piglets, the enhanced growth of piglets in response to arginine supplementation was attributed to the increase in arginine availability.
Arginine is a potent stimulator of the secretion of insulin by pancreatic -cells and of growth hormone by the anterior pituitary gland in mammals (33), including young pigs (34).
Through an increase in arginine availability as well as the concurrent increases in plasma concentrations of anabolic hormones, dietary arginine supplementation improved the efficiency of nutrient utilization for enhancing tissue protein synthesis and growth performance.
Although arginine was identified 50 years ago as an essential amino acid for young pigs (35), and it displays remarkable metabolic and regulatory versatility (36,37), it is surprising that little is known about its quantitative requirements in neonates (including milk-fed piglets) (38).
In conclusion, dietary supplementation with 0.2 and 0.4% L-arginine to milk-fed young piglets dose dependently increases plasma concentration of arginine, decreases plasma concentration of ammonia, and promotes growth performance. Importantly, increasing arginine availability holds great promise as a potent method for enhancing neonatal piglet growth.
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