Glucose turnover and recycling in unrestrained and unanesthetized 48-h-old fasting or post-absorptive newborn pigs

Developmental changes in hepatic glucose metabolism in a newborn piglet model: A comparative analysis for suckling period and early weaning period

The liver glucose metabolism, supplying sufficient energy for glucose-dependent tissues, is important in suckling or weaned animals, although there are few studies with piglet model. To better understand the development of glucose metabolism in the piglets during suckling period and early weaning period, we determined the hepatic glycogen content, and investigated the relative protein expression of key enzymes of glucogenesis (GNG) and mRNA levels of some glucose metabolism-related genes. During suckling period, the protein level of G6Pase in the liver of suckling piglets progressively declined with day of age compared with that of newborn piglets (at 1 day of age), whereas the PEPCK level stabilized until day 21 of age, indicating that hepatic GNG capacity gradually weakened in suckling piglets. The synthesis of hepatic glycogen, which was consistent with the fluctuation of glycolytic key genes PFKL and PKLR that gradually decreased after birth and was more or less steady during latter suckling period, although both the mRNA levels of GCK and key glucose transporter GLUT2 presented uptrend in suckling piglets. However, early weaning significantly suppressed the hepatic GNG in the weaned piglets, especially at d 3-5 of weaning period, then gradually recovered at d 7 of weaning period. Meanwhile, PFKL, PKLR and GLUT2 showed the similar trend during weaning period. On the contrast, the hepatic glycogen reached the maximum value when the G6Pase and PEPCK protein expression were at the lowest level, although the GCK level maintained increasing through 7 days of weaning period. Altogether, our study provides evidence that hepatic GNG and glycolysis in newborn piglets were more active than other days during suckling period, and early weaning could significantly suppressed glucose metabolism in liver, but this inhibition would progressively recover at day 7 after weaning.

Effect of Provision of Feed and Water during Transport on the Welfare of Weaned Pigs

Simple Summary

Transportation is a complex stressor, which has the potential to negatively impact the health and welfare of weaned pigs. Transport duration and withdrawal from feed and water are two factors that could potentially adversely affect the welfare of pigs transported at weaning. In this study, the effect of a 32 h transport period and the provision of feed and water on the welfare of weaned pigs was investigated using a multi-disciplinary approach. Body weight decreased in weaned pigs over time and this response was exacerbated by exposing pigs to a 32 h transport period and withdrawing feed and water. The greatest changes in body weight loss were observed after 8 h of transport or weaning. Furthermore, the neutrophil to lymphocyte ratio (N:L) stress measure was elevated in pigs in response to an 8 h transport period or 8 h after weaning alone. With the exception of weaned pigs provided with feed and water, transported and weaned pigs continued to be different from control pigs until 16 h after weaning or exposure to a 16 h transport period. These findings suggest that pigs experience an acute stress response due to transport and weaning, but these two stressors do not appear to be additive. Overall, transportation had a negative effect on performance, physiology and behavior of weaned and transported pigs, especially if not provided with feed and water for more than 24 h.

Abstract

Transportation is a complex stressor made up of factors including weaning itself and withdrawal from feed and water. Therefore, transportation has the potential to negatively impact the health and welfare of weaned pigs. Pigs were transported for 32 h and measures of performance, physiology, and behavior were taken to assess piglet welfare. Treatment groups included pigs not weaned or transported (CON), weaned pigs provided with feed and water (WEAN+), weaned pigs not provided with feed and water (WEAN−), weaned and transported pigs provided with feed and water (TRANS+), and weaned and transported pigs not provided with feed and water (TRANS−). Body weight loss was different among treatments (p < 0.01). CON pigs had a 6.5% ± 0.45% gain in body weight after 32 h. WEAN+, WEAN−, TRANS+, and TRANS− groups all had a loss in body weight of 5.9% ± 0.45%, 7.8% ± 0.45%, 6.5% ± 0.45% and 9.1% ± 0.46%, respectively. The N:L was greater in all weaned pigs at 8 h compared to CON pigs (p < 0.01). WEAN− and transported pigs had significantly higher N:L than CON pigs from 8 h through 16 h, however, all treatment groups were similar to CON pigs after 16 h irrespective of provision of feed and water. Blood glucose levels were lower in transported and/or weaned pigs than CON pigs after 16 h irrespective of the provision of feed and water. TRANS+ females had higher creatine kinase (CK) levels than males (p < 0.05). After a 16 h transport period, TRANS− pigs had higher total plasma protein (TP) levels than all other treatment groups (p < 0.05). Significant changes in behavior were observed during and after transportation, which could also be indicative of stress. Overall, transportation and weaning had a negative effect on performance, physiology and behavior (both during and post-weaning) of pigs, especially when feed and water was not provided. Transporting pigs without feed and water for more than 24 h was a welfare concern as indicated by changes in body weight and physiology measures of stress.

Dietary energy source affects glucose kinetics in trained Arabian geldings at rest and during endurance exercise

Advances in modeling and tracer techniques provide new perspective into glucose utilization and potential consequences to health or exercise performance. This study used stable isotope and compartmental modeling to evaluate how adaptation to a feed high in sugar and starch (SS) compared with a feed high in fat and fiber (FF) affects glucose kinetics at rest and during exercise in horses. Six trained Arabians adapted to each feed underwent similar tests at rest and while running approximately 4 m/s on a treadmill. For both tests, horses received 100 micromol/kg body weight [6,6-(2)H]glucose through a venous catheter. Circulating tracer glucose was described for 150 min by exponential decay curves and compartmental analysis. All parameters of glucose transfer increased with exercise (P < or = 0.004). Compared with FF horses, SS horses had higher circulating glucose (P = 0.022) and fractional glucose transfer rates (min(-1)) at rest (P = 0.055). Exercise increased glucose irreversible loss (mmol/min) more in SS horses (P = 0.037). Total glucose transfer during exercise tended to be greater in SS horses (0.027 +/- 0.002 mmol/min) compared with FF horses (0.023 +/- 0.002 mmol/min) (P = 0.109). This study characterized the effect of diet on glucose kinetics in resting and exercising horses using new modeling methods. Horses adapted to a fat-supplemented feed utilized less glucose during low-intensity exercise. Fat supplementation in horses may therefore promote greater flexibility in the selection of substrate to meet energy demands for optimal health and performance.

Acetogenesis does not replace ketogenesis in fasting piglets infused with hexanoate

The current studies were performed to better understand the physiological relevance of acetate in the poorly ketogenic piglet and to determine if endogenous acetogenesis rises with increased mitochondrial fatty acid beta-oxidation, analogous to ketogenesis. Plasma acetate concentration values in newborn, fasted, or suckled piglets (230-343 microM) were at least 10-fold higher than the ketone bodies, a pattern opposite to that in 24- to 48-h suckled rats (77-175 microM). Employing continuous infusion techniques with sodium [3H]acetate tracer in fasting approximately 40-h-old piglets, acetate rate of appearance (Ra) was found to be 34 +/- 4 micromol . min-1 . kg body wt-1. This basal Ra was double that observed in animals coinfused with sodium [1-14C]hexanoate (P < 0.001), despite active oxidation of the latter as determined by 14CO2 production. Active acetogenesis in vivo and relatively abundant acetate in piglet blood are consistent with the hypothesis that acetate plays an important physiological role in piglets. However, the negative impact of hexanoate oxidation upon acetate Ra and the lack of significant changes in circulating acetate in newborn, suckled, and fasted piglets draws into question the extent of analogy between acetogenesis and ketogenesis in vivo.

Effect of low-fat colostrum on fat accretion and lipogenic enzyme activities in adipose tissue in the 1-day-old pig

Fat accretion, body fatty acid (FA) composition and adipose tissue lipogenic enzyme activities were determined in 1-day-old piglets fed during the first day of life sow colostrum that was either normal (control) or low in fat and high in lactose, galactose or glucose as the sole source of carbohydrate. Malic enzyme activity did not change during the first postnatal day while increases (P < 0.01) were found for glucose-6-phosphate dehydrogenase and acetyl-CoA-carboxylase. However, fat accretion was closely dependent on the amount of ingested fat and FA composition of piglets indicates a shift toward that of colostrum, the extent of which was dependent on the amount of ingested fat. Plasma glucose and insulin levels were the highest (P < 0.01) in piglets fed the colostrum high in glucose and plasma free fatty acid (FFA) levels were the lowest (P < 0.01) in piglets fed the low-fat, high-carbohydrate colostra. Liver weight and glycogen concentration were higher (P < 0.01) in piglets fed the colostra low in fat and high in carbohydrate than in those on the control colostrum. It is suggested that synthesis of fatty acids from carbohydrate is negligible in the newborn pig even in the presence of high plasma insulin and low plasma FFA levels. The metabolic fate of carbohydrate is discussed.

Relationship between hepatic fatty acid oxidation and gluconeogenesis in the fasting neonatal pig

Hepatocytes were isolated from sixteen fasting neonatal pigs and used in two experiments: (1) to determine the effect of various factors on the ability for hepatic oxidation of fatty acids and (2) to clarify the relationship between fatty acid oxidation and glucose synthesis. In Expt 1, newborn pigs were either fasted from birth for 24 h or allowed to suck ad lib. for 3 d followed by a 24 h fast. In the presence of pyruvate, oxidation of octanoate (2 mM) was about 30-fold greater than oleate (1 mM) regardless of age, but glucose synthesis was not enhanced beyond that observed for pyruvate alone. Inclusion of carnitine (1 mM), glucagon (100 nM) or dibutyryl cAMP (50 microM) in the incubation media did not stimulate either fatty acid oxidation (octanoate or oleate) or glucose synthesis. Extending the period of fasting to 48 h (Expt 2) failed to enhance the fatty acid oxidative capacity or glucose synthesis rate. Likewise, the redox potential of the gluconeogenic substrate (lactate v. pyruvate) did not influence glucose synthesis regardless of the oxidative capacity exhibited for fatty acids. These data indicate that fatty acid oxidative capacity is not the first limiting factor to full expression of gluconeogenesis in hepatocytes isolated from fasted newborn pigs.

A longitudinal study of starvation in piglets and the introduction of a modified liver biopsy technique

The effect of starvation on blood glucose, muscle glycogen and liver glycogen concentration was measured in a group of newborn piglets. Liver biopsies were obtained by using a modified version of the Menghini technique. No difference in length of survival time was observed between piglets that received water and those that did not. Piglets with higher birth weight survived longer. No relationship was found between initial liver glycogen content and survival time. Moreover, we concluded that plasma glucose levels are not reliable indicators of length of survival time. The interrelated changes in liver glycogen, muscle glycogen, and plasmaglucose concentrations found in this study correspond with those reported elsewhere. Moreover, the number of experimental animals needed for the study was markedly reduced. We conclude that the liver biopsy technique is valuable in longitudinal hypoglycaemia studies of piglets.

Relationship of substrate level to turnover rate in fasted adult and newborn dogs

Glucose turnover, clearance and response to insulin were determined in fasted newborn and adult dogs. Fasting levels of glucose and insulin and rates of glucose turnover and clearance were not different between the two groups. Blood glucose correlated with basal glucose turnover in newborn pups but not in adult dogs. Glucose turnover was not related to fasting plasma insulin levels. Glucose clearance was an inverse function of blood glucose levels among newborn but not adult dogs. Glucose clearance and blood glucose levels were not related to insulin concentrations. In response to euglycemic hyperinsulinemia, glucose metabolism increased 4-fold among adults but only 1.7-fold in pups. Hyperglycemic hyperinsulinemia increased glucose metabolism in both groups but to a much greater extent in the pups. Euglycemic hyperinsulinemia increased the metabolic clearance rate of glucose 4.2-fold among adults but only 1.8-fold in newborn dogs. In response to hyperglycemic hyperinsulinemia glucose clearance rates were now similar. Despite euglycemic hyperinsulinemia, the newborn dog had an attenuated response to insulin, demonstrating lower rates of glucose metabolism and glucose clearance. The response to the hyperglycemic stimuli suggests that maximal glucose uptake was not achieved during hyperinsulinemia alone. This response supports the concept of glucose-mediated regulation of glucose disposal in newborn animals.

A comparison in neonatal piglets of model-dependent and model-independent methods for measuring glucose turnover

Steady state glucose turnover was measured in 53 conscious, unrestrained neonatal piglets by two different methods using 3H-labelled glucose as the tracer. In the first, model-dependent method, the plasma tracer concentrations following a spike injection of tracer were fitted in various different ways and the turnover was calculated from the area under the fitted curve extrapolated to infinity. Both exponential curves and negative power functions of time were fitted by non-linear least squares. In the second, model-independent method, tracer was infused continuously and turnover calculated from its equilibrium concentration. Conditions during 34 of the 53 experiments failed various tests of the steady state and the data were therefore discarded. In the remaining 19, glucose turnover measured by the different methods agreed fairly well. Although there was little difference between the results obtained by a two-exponential curve fit, by those obtained when the fast component was ignored, by a single exponential curve fit, and by the results from fitting a negative power function of time, the conventional two-exponential curve is to be preferred. There was a suggestion that at high turnover rates, the injection result underestimated the infusion result, assuming the latter to represent the reference method. This might have been due to extrapolation errors in the injection technique, either in the first few minutes following injection, or in the interval after the last sample. However, the notional volumes of distribution of the tracer suggest that the terminal phase of tracer elimination is in fact slightly faster than that predicted on the basis of samples taken during the first 120 min post-injection.

Insulin resistance during suckling period in rats

Glucose metabolism was studied in 13- to 15-day-old suckling rats and 28- to 30-day-old rats weaned at 19 days on a high-carbohydrate, low-fat diet. The glucose turnover rate in the basal state was similar in suckling and weaned rats (14 mg X min-1 X kg-1). Glucose infusion (20 mg X min-1 X kg-1) produced a moderate hyperglycemia in weaned rats (150 mg/dl), whereas the suckling rats developed a high hyperglycemia (280 mg/dl), despite a large increase of plasma insulin concentration. The effect of insulin on glucose kinetics was then assessed by the euglycemic-hyperinsulinemic clamp technique. The plasma insulin levels reached were 736 +/- 87 microU/ml in the suckling and 444 +/- 34 microU/ml in the weaned rats, despite similar insulin infusion rates. Hepatic glucose production was suppressed by 90% in the weaned rats, whereas it remained at 40% of basal value in the suckling rats. The increase of glucose utilization above basal was fourfold higher in the weaned than in the suckling rats. Thus an insulin resistance state is present in the suckling rats and disappears after weaning onto a high-carbohydrate diet.

Effect of intragastric triglyceride administration on glucose homeostasis in newborn pigs

The fasting hypoglycemia (1.78 +/- 0.29 mmol/l) which develops in 48-h-old pigs is partially reversed (3.85 +/- 0.55 mmol/l) after gastric administration of long-chain triglycerides (LCT). The increase in blood glucose induced by LCT feeding was not secondary to a decreased glucose utilization because glucose disappearance rate increased in LCT-fed piglets but resulted from a twofold increase in glucose appearance. By using the crossover-plot technique, the stimulation of hepatic gluconeogenesis induced by LCT feeding has been localized at 1) the level of pyruvate carboxylase owing to the twofold increase in hepatic acetyl-CoA concentration and 2) the level of glyceraldehyde-3-phosphate dehydrogenase secondary to the increase in reducing equivalents (NADH), which displaces this equilibrium reaction in the direction of gluconeogenesis. As blood lactate, pyruvate, and alanine concentrations increased after LCT feeding, the possible effects of LCT on pyruvate dehydrogenase in peripheral tissues are discussed. These data demonstrate that fatty acids stimulate hepatic gluconeogenesis in 48-h-old fasting piglets and underline the role of fat provision in the regulation of glucose homeostasis during the neonatal period in the pig.