Heat stress (HS) jeopardizes human and animal health and reduces animal agriculture productivity; however, its pathophysiology is not well understood. Study objectives were to evaluate the direct effects of HS on carbohydrate and lipid metabolism. Female pigs (57 ± 5 kg body weight) were subjected to two experimental periods. During period 1, all pigs remained in thermoneutral conditions (TN; 20°C) and were ad libitum fed. During period 2, pigs were exposed to: (1) constant HS conditions (32°C) and fed ad libitum (n = 7), or (2) TN conditions and pair-fed (PFTN; n = 10) to minimize the confounding effects of dissimilar feed intake. All pigs received an intravenous glucose tolerance test (GTT) and an epinephrine challenge (EC) in period 1, and during the early and late phases of period 2. After 8 days of environmental exposure, all pigs were killed and tissue samples were collected. Despite a similar reduction in feed intake (39%), HS pigs tended to have decreased circulating nonesterified fatty acids (NEFA; 20%) and a blunted NEFA response (71%) to the EC compared to PFTN pigs. During early exposure, HS increased basal circulating C-peptide (55%) and decreased the insulinogenic index (45%) in response to the GTT. Heat-stressed pigs had a reduced T3 to T4 ratio (56%) and hepatic 5′-deiodinase activity (58%). After 8 days, HS decreased or tended to decrease the expression of genes involved in oxidative phosphorylation in liver and skeletal muscle, and ATGL in adipose tissue. In summary, HS markedly alters both lipid and carbohydrate metabolism independently of nutrient intake.

Growing ruminants under extended dietary restriction exhibit compensatory growth upon ad libitum feeding, which is associated with increased feed efficiency, lower basal energy requirements, and changes in circulating concentrations of metabolic hormones. To identify mechanisms contributing to these physiological changes, 8-month-old steers were fed either ad libitum (control; n = 6) or 60-70% of intake of control animals (feed-restricted; n = 6) for a period of 12 weeks. All steers were fed ad libitum for the remaining 8 weeks of experimentation (realimentation). Liver was biopsied at days -14, +1, and +14 relative to realimentation for gene expression analysis by microarray hybridization. During early realimentation, feed-restricted steers exhibited greater rates of gain and feed efficiency than controls and an increase in expression of genes functioning in cellular metabolism, cholesterol biosynthesis, oxidative phosphorylation, glycolysis, and gluconeogenesis. Gene expression changes during feed restriction were similar to those reported in mice, indicating similar effects of caloric restriction across species. Based on expression of genes involved in cell division and growth and upregulation of genes encoding mitochondrial complex proteins in early realimentation, it was concluded that reduced hepatic size and increased mitochondrial function may contribute to improved feed efficiency observed during compensatory growth.

Intrinsic in the equation for successful animal production is the efficiency of nutrient use for assimilation into useful animal-derived products. However, when young growing animals encounter various stressors that activate the proinflammatory response (PR), the biochemical effects of the resulting cascade of PR mediators [cytokines, prostaglandin and prosta-cyclin derivatives, nitric oxide (NO), superoxide anion (O2(.-)), etc.] override the regulatory signals normally ascribed to anabolic tissue accretion and growth. The efficiency of energy and nutrient use will proportionally decrease for growth rate due to the redirection of nutrient use to support immune defense processes. These proinflammatory events can develop in association with infectious disease but also are apparent in and a part of the natural response to birth, parturition, and weaning. If growth patterns are tracked during the PR, growth deficits are often apparent. Some growth deficits are relatively transient in duration, whereas others are quite long lasting, persisting although traditional clinical markers of PR are no longer evident. Recent evidence indicates that the PR cascades initiated by cytokines like tumor necrosis factor-alpha play a major role in these growth deficits. Perturbations in mitochondrial energetics and NO and O2(.-) interactions further affect metabolic balance. Free radicals and reactive nitrogen intermediates interact with select molecular targets in proteins (i.e., enzymes, histone proteins, and signal transduction proteins), causing the nitration and nitrosylation of select amino acids. If these posttranslational modifications occur in proteins associated with control points critical in metabolic stability, the resulting altered protein structure blocks its functionality. Attenuation of these overt posttranslational protein modifications at their site of production offers a strategy to minimize their detrimental impact while preserving needed cytokine, NO, and O2(.-) functions.