Effects of heat stress on carbohydrate and lipid metabolism in growing pigs

Abstract

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.

Keywords: Epinephrine challenge; glucose tolerance test; heat stress; metabolism; thyroid hormones.

Figure 1

Effects of ad libitum feed intake in constant heat stress conditions (HS; 32°C) and pair-feeding in thermoneutral conditions (PFTN; 20°C) on temporal changes in plasma (A) glucose, (B) insulin, (C) insulin to glucose ratio (Insulin:G) (D) C-peptide, (E) C-peptide to glucose ratio (C-peptide:G), (F) nonesterified fatty acids (NEFA), (G) β-hydroxybutyrate (BHB), and (H) lipopolysaccharide-binding protein (LBP). Values on day 0 represent period 1 average that was statistically used as covariate. ^a-dMeans with different superscript differ (P ≤ 0.05). ^x-zDays with different superscript differ (P ≤ 0.05).

Figure 2

Effects of ad libitum feed intake in constant heat stress conditions (HS; 32°C) and pair-feeding in thermoneutral conditions (PFTN; 20°C) on temporal changes in plasma (A) T₃, (B) T₄, (C) T₃ to T₄ ratio (T₃:T₄); and (D) hepatic 5’-deiodinase type I activity (5'D) after 8 days of environmental treatment. Values on day 0 represent period 1 average that was statistically used as covariate. ^a–cMeans with different superscript differ (P ≤ 0.05).

Figure 3

Effects of 8 days of ad libitum feed intake in constant heat stress conditions (HS; 32°C) and pair-feeding in thermoneutral conditions (PFTN; 20°C) pancreatic insulin content. (A) Representative picture of PFTN pancreas; (B) representative picture of PFTN pancreas; (C) insulin content; (D) insulin-stained area; (E) number of insulin-stained clusters; (F) insulin-stained clusters size distribution. *Represents differences with PFTN controls (P ≤ 0.05).