Effect of high ambient temperature and genotype on thermoregulatory responses and gene expression in various tissues in growing pigs

Abstract Text: Heat stress (HS) in pig production is a critical problem in temperate countries during summer but also in tropical areas where pig production dramatically increased over the past two decades. Physiological and metabolic adjustments resulting from the thermoregulatory responses to HS have negative consequences on pig productivity and health. The objectives of this experiment were to improve knowledge on the mechanisms underlining heat tolerance by comparing thermoregulatory responses in three different pig genotypes (G). This experiment was performed on a total of 36 castrated male pigs: one genotype sensitive to HS [Large White (LW)], one tolerant to HS [Créole (CR)], and a cross between LW and CR pigs. Pigs were housed at 24°C for 10 days (d) and thereafter at 32°C for 5 d. The temperature (T) transition from 24 to 32°C occurred on d 0 at a constant rate of 2°C/h. Pigs had free access to water and feed. Body T and respiratory rate were measured on d-7, -4 and -3 at 24°C, and on d 1, 2, 3, 4 and 5 at 32°C. Eighteen pigs (6 per G) were slaughtered on d -2 (before the T transition) and on d 5. At slaughter, blood and three tissues [backfat (BF), longissimus dorsi (LD), liver (LI)] were collected. A transcriptome analysis was performed on blood and tissues using a semi-custom 60 K microarray (GPL16524). Blood hormones and metabolites were also measured. The T×G interaction was not significant (P > 0.05) for feed intake and thermoregulatory responses (except for skin T). Whatever the G, HS had a significant negative effect on voluntary feed intake (-200 g/d on average). The T increase resulted in an increased rectal and skin T and respiratory rate within the first 24-72 h of exposure, and in a subsequent recovery phase characterized by a rapid decrease of these thermoregulatory responses. Plasma glucose, lactate, glycerol, albumin concentrations were significantly reduced under HS but creatine concentration increased at 32°C. Thyroid hormone (T3 and T4) concentrations were reduced at 32°C. Irrespective to the G, a considerable number of differentially expressed genes (FDR < 0.05) were found by comparing blood (13,807), LD (6,261), BF (5,236) and LI (435) samples collected at 24°C with those collected at 32°C. A first functional analysis revealed that most of the significantly differentially expressed genes especially in BF and LD were involved in the regulation of the oxidative phosphorylation pathway

Keywords: heat stress, pig, genotype, thermoregulatory responses, gene expression.