Arginine and small intestinal atrophy in parenterally-fed neonatal piglets

Piglets that are fed via total parenteral nutrition (TPN) experience profound gut atrophy and rapid decline in splanchnic blood flow compared to enterally-fed piglets. Sixty percent of total nutrient intake as enteral feeding is required to sustain “normal” small intestinal metabolism and morphology, which is likely not feasible with sick neonates. Whether the potential benefit of complete or partial enteral nutrition is related to single or multiple nutrients is not yet fully understood. However, the luminal presence of specific nutritive factors such as amino acids may act as trophic factors for intestinal recovery during TPN or enteral re-feeding. Arginine is a good candidate to investigate potential trophic effects as it is involved in many important metabolic pathways such as nitric oxide synthesis, ureagenesis, polyamine synthesis and protein synthesis. TPN-induced gut atrophy reduces intestinal de novo synthesis of arginine in neonates which may then limit arginine availability. We hypothesized that delivery of a high amount of dietary arginine alone into the gut could induce partial recovery from TPN-induced gut atrophy in neonatal piglets, while improving superior mesenteric artery (SMA) blood flow. We also hypothesized that enteral delivery of low amounts of arginine would result in attenuated responses compared to the high enteral arginine. Thus, our objectives were to assess the effect of route of intake and dietary concentration of arginine alone on tissue protein synthesis, SMA blood flow and gut morphology in TPN-fed neonatal Yucatan miniature piglets with small-intestinal atrophy. Twenty-four piglets (14 - 17 d old) were fed complete TPN, containing 1.0 g arginine/kg/d until study day 4 to induce atrophy, then switched to arginine-free TPN and randomized to receive a continuous intragastric infusion of either low arginine (0.6 g/kg/d) (IG-L Arg) (n = 6) or high arginine (1.6 g/kg/d) (IG-H Arg) (n = 6). A third group was randomized to receive high arginine intravenously (1.6 g/kg/d in TPN) (IV-H Arg) (n = 6). A group of sow-fed littermates (SF Reference) (n = 6) was also included. On study day 7, crypt cell proliferation and tissue specific protein synthesis rates were measured. This thesis demonstrated for the first time that the delivery of arginine alone intragastrically, irrespective of the amount provided, stimulated hepatic protein synthesis (P = 0.01) compared to intravenous delivery of arginine. SMA blood flow declined continuously during the period when all animals were receiving the same complete TPN. It further decreased following the initiation of the test diets and reached a plateau approximately 48 hours after the test diets were initiated. At steady state, SMA blood flow was significantly different among all treatment groups (P = 0.002). IV-H Arg treatment had the smallest reduction in blood flow (22% lower than baseline). The provision of high arginine enterally attenuated some of the reduction of blood flow, whereas the IG-L Arg piglets demonstrated the greatest reduction, at ~40% lower than baseline at study end. In piglets fed solely by TPN, some of the plasma indispensable amino acids were very high compared to the IG-H Arg piglets. Remarkably, the provision of only arginine into the gut resulted in a plasma profile of some of these amino acids that was very similar to sow-reared piglets. In spite of experiencing a greater amelioration of the reduction of SMA blood flow, the small intestinal morphological characteristics were not significantly improved compared to the other treatment groups. Although IG-H Arg was not as effective as IV-H Arg in sustaining SMA blood flow, morphological outcomes were not significantly worse than in the IV-H Arg piglets; indeed, the small intestine was significantly longer in the IG-H Arg piglets compared to IV-H Arg group, and all other outcomes were similar among treatment groups. These results suggest that IG delivery of arginine was likely beneficial as a trophic factor in gut-atrophied neonatal piglets

Arginine metabolism and creatine biosynthesis in Yucatan miniature piglets

Creatine and creatine phosphate are amino acid-derived compounds that are necessary to meet short-term energy requirements. Creatine is a potent neuromodulator such that it is critical for neurological development in neonates. Neonates receive creatine in mother's milk. However, up to 77% of the daily creatine requirement must be synthesized endogenously. Creatine synthesis involves the conversion of arginine to guanidinoacetic acid (GAA) via L-arginine:glycine amidinotransferase (AGAT). The subsequent conversion of GAA to creatine requires methionine and guanidinoacetate N-methyltransferase (GAMT). Following preterm birth, total parenteral nutrition (PN) is often required as a means of nutritional support. However, creatine is not a component of pediatric PN. In this situation, the entire creatine requirement must be met by de novo synthesis which consequently must create considerable demand for the amino acid precursors. Poor arginine status is common in neonates during PN, and this may compromise optimal creatine accretion. In this thesis, the capacity of the neonatal piglet to synthesize creatine was addressed, particularly when dietary creatine was not supplied. The data from first experiment demonstrated that PN support with creatine led to greater creatine concentrations in plasma and tissues, suggesting that neonates receiving PN may not be able to sustain optimal creatine accretion. The second major objective of this thesis was to investigate whether the low creatine accretion in piglets fed creatine-free diets was due to limited enzyme capacity or limited substrate availability. Using a multiple isotope tracer method, we determined that inadequate dietary arginine and methionine, not enzyme capacity, limits GAA and creatine biosynthesis. Lastly, because we measured high AGAT activity in kidney and pancreas, we investigated how the plasma concentrations of precursor amino acids (arginine/citrulline) and creatine affected the release of GAA and creatine from these organs as well as from the intestine. We determined both arginine and citrulline can provide precursors for renal GAA synthesis. However, in the pancreas and intestine, citrulline cannot provide arginine to serve as a precursor for GAA. Overall, the results of this thesis clearly demonstrate that dietary concentrations of arginine and methionine are important mediators of creatine synthesis; the absence of dietary creatine, such as during PN feeding, must be taken into consideration when determining dietary requirements of neonates

Antidepressant Activity of Pharmacological and Genetic Deactivation of the Small-Conductance Calcium-Activated Potassium Channel Subtype-3

Funding and Disclosure This research was supported by awards from the Neuroscience Catalyst program (Toronto) (FRB and JNN), the Canadian Institutes of Health Research (FRB and JN) and the National Science and Engineering Research Council of Canada (FRB). M.N. was additionally supported by a CAMH Discovery Fund Post-doctoral Fellowship. Conflict of Interest: None declared. Acknowledgments We thank J. Li, U. Mumtaz, S. Khan, S. Sivaruban, M. Billyard, E. Hauck, D. Oleinichenko, Michael Coombs and Lucas Francis Fowler for technical assistance at different stages of the work.Peer reviewedPostprin