Hypothalamic regulation of energy homoeostasis when consuming diets of different energy concentrations: comparison between Tibetan and Small-tailed Han sheep.

Seasonal energy intake of Tibetan sheep on the harsh Qinghai-Tibetan Plateau (QTP) fluctuates greatly and is often well below maintenance requirements. The aim of this study was to gain insight into how the hypothalamus regulates energy homoeostasis in Tibetan sheep. We compared Tibetan and Small-tailed Han sheep (n 24 of each breed), which were each allocated randomly into four groups and offered one of four diets that differed in digestible energy densities: 8·21, 9·33, 10·45 and 11·57 MJ/kg DM. Sheep were weighed every 2 weeks, and it was assumed that the change in body weight (BW) reflected the change in energy balance. The arcuate nucleus of the hypothalamus in Tibetan sheep had greater protein expressions of neuropeptide Y (NPY) and agouti-related peptide (AgRP) when in negative energy balance, but lesser protein expressions of proopiomelanocortin (POMC) and cocaine and amphetamine-regulated transcript (CART) when in positive energy balance than Small-tailed Han sheep. As a result, Tibetan sheep had a lesser BW loss when in negative energy balance and stored more energy and gained more BW when in positive energy balance than Small-tailed Han sheep with the same dietary intake. Moreover, in the hypothalamic adenosine monophosphate-activated protein kinase (AMPK) regulation pathway, Tibetan sheep had greater adenosine monophosphate-activated protein kinase-α 2 protein expression than Small-tailed Han sheep, which supported the premise of a better ability to regulate energy homoeostasis and better growth performance. These differences in the hypothalamic NPY/AgRP, POMC/CART and AMPK pathways between breeds conferred an advantage to the Tibetan over Small-tailed Han sheep to cope with low energy intake on the harsh QTP.

Comparison between Tibetan and Small-tailed Han sheep in adipocyte phenotype, lipid metabolism and energy homoeostasis regulation of adipose tissues when consuming diets of different energy levels.

This study aimed to gain insight into how adipose tissue of Tibetan sheep regulates energy homoeostasis to cope with low energy intake under the harsh environment of the Qinghai-Tibetan Plateau (QTP). We compared Tibetan and Small-tailed Han sheep (n 24 of each breed), all wethers and 1·5 years of age, which were each divided randomly into four groups and offered diets of different digestible energy (DE) densities: 8·21, 9·33, 10·45 and 11·57 MJ DE/kg DM. When the sheep lost body mass and were assumed to be in negative energy balance: (1) adipocyte diameter in subcutaneous adipose tissue was smaller and decreased to a greater extent in Tibetan than in Small-tailed Han sheep, but the opposite occurred in the visceral adipose tissue; (2) Tibetan sheep showed higher insulin receptor mRNA expression and lower concentrations of catabolic hormones than Small-tailed Han sheep and (3) Tibetan sheep had lower capacity for glucose and fatty acid uptake than Small-tailed Han sheep. Moreover, Tibetan sheep had lower AMPKα mRNA expression but higher mammalian target of rapamycin mRNA expression in the adipocytes than Small-tailed Han sheep. We concluded that Tibetan sheep had lower catabolism but higher anabolism in adipose tissue and reduced the capacity for glucose and fatty acid uptake to a greater extent than Small-tailed Han sheep to maintain energy homoeostasis when in negative energy balance. These responses provide Tibetan sheep with a high ability to cope with low energy intake and with the harsh environment of the QTP.

Tibetan sheep have a high capacity to absorb and to regulate metabolism of SCFA in the rumen epithelium to adapt to low energy intake.

The nutritional intake of Tibetan sheep on the harsh Qinghai-Tibetan Plateau is often under maintenance requirements, especially during the long, cold winter. However, they have adapted well and even thrive under these conditions. The aim of the present study was to gain insight into how the rumen epithelium of Tibetan sheep has adapted to the consumption of low-energy-level diets. For this purpose, we compared Tibetan and small-tailed Han sheep (n 24 of each breed, all wethers and 1·5 years of age), which were divided randomly into one of four groups and offered ad libitum diets of different digestible energy (DE) densities: 8·21, 9·33, 10·45 and 11·57 MJ DE/kg DM. The Tibetan sheep had higher rumen concentrations of total SCFA, acetate, butyrate and iso-acids but lower concentrations of propionate than small-tailed Han sheep. The Tibetan sheep had higher absorption capability of SCFA due to the greater absorption surface area and higher mRNA expression of the SCFA absorption relative genes than small-tailed Han sheep. For the metabolism of SCFA in the rumen epithelium, the small-tailed Han sheep showed higher utilisation of the ketogenesis pathway than Tibetan sheep; however, Tibetan sheep had greater regulation capacity in SCFA metabolism pathways. These differences between breeds allowed the Tibetan sheep to have greater capability of absorbing SCFA and better capacity to regulate the metabolism of SCFA, which would allow them to cope with low energy intake better than small-tailed Han sheep.

Effect of dietary energy on digestibilities, rumen fermentation, urinary purine derivatives and serum metabolites in Tibetan and small-tailed Han sheep.

Tibetan sheep are indigenous to the Qinghai-Tibetan Plateau, graze the grassland all year round without supplementation and are well-adapted to the harsh conditions. Small-tailed Han sheep were introduced to the plateau and are raised mainly in feedlots. Based on their different backgrounds, we hypothesized that the ability to cope with poor diets would be better in Tibetan than in Han sheep. To test our prediction, we examined the effect of dietary energy on apparent digestibilities, rumen fermentation, urinary purine derivatives and serum metabolites by using a 4 × 4 Latin square design in each sheep breed. Four diets were formulated to be low in crude protein (~7%) but to differ in metabolizable energy concentration. Average daily gain was greater in Tibetan than in Han sheep (p < 0.01) and increased linearly with an increase in energy intake (p < 0.001). The digestibilities of dry matter, organic matter, gross energy, and neutral and acid detergent fibres were greater in Tibetan than in Han sheep (p < 0.05). Ruminal pH was lower (p < 0.05), while volatile fatty acids (VFAs), urea-N, ammonia-N and soluble protein-N concentrations were higher (p < 0.05) in Tibetan than in Han sheep. As a molar proportion of total VFA, acetate decreased (p < 0.001) with an increase in dietary energy whereas propionate and butyrate increased (p < 0.05). Urinary purine derivative excretion was greater in Tibetan than in Han sheep (p < 0.01), as was microbial nitrogen production; both parameters increased with dietary energy (p < 0.01). Serum concentrations of glucose, insulin and insulin-like growth factor-1 increased (p < 0.05) as energy level increased, while non-esterified fatty acids and growth hormone decreased (p < 0.05). It was concluded that Tibetan sheep were better able to cope with low-protein, low-energy diets and, consequently, our prediction was supported.

Bacterial communities related to 3-nitro-1-propionic acid degradation in the rumen of grazing ruminants in the Qinghai-Tibetan Plateau.

The objectives of this current study were to characterize the overall rumen bacterial community in grazing yak and two sheep species (Tibetan and Small Tail Han sheep) reared in the unique environmental conditions of the Qinghai-Tibetan Plateau, as well as the bacterial community associated with the detoxification of a phytotoxin, 3-nitro-1-propionic acid (NPA), during in vitro culture with 4.2 mM NPA. Using 16S rRNA gene high-throughput sequencing, it was found that the yak rumen harbored populations showing a higher bacterial diversity compared to Tibetan sheep. The rumen bacterial community in the three ruminant species differed from each other. PICRUSt analysis identified that the pathway involved in nitrogen metabolism was enriched in Tibetan sheep while that related to fatty acid biosynthesis was over-represented in the yak. The methane metabolism pathway was dominant in bacterial populations from the Small Tail Han sheep. Comparisons between freshly collected rumen fluid and populations subjected to consecutive 72 h batch cultures revealed substantial decreases in alpha diversity in populations cultured with NPA. Moreover, the relative abundances of some bacterial taxa changed significantly, with increased abundance of Proteobacteria and Actinobacteria. In addition, the overall community structure of the bacterial population in the freshly collected ruminal fluid was clearly different than that within populations observed in the 72 h batch cultures likely due to the impact of NPA treatments and the more restrictive growth conditions of the culture medium. In regard to PICRUSt analysis, the methane metabolism pathway became scarce in Tibetan and Small Tail Han sheep, whereas the energy and carbohydrate metabolic pathways such as nitrogen metabolism, ABC transporters and glycolysis/gluconeogenesis were found to be maintained across all populations. Results from the present study provide new information on the bacterial and functional composition within ruminal populations adapted to three economically important grazing ruminant species prominent on the Qinghai-Tibetan Plateau. The results further reveal that effects of NPA treatment on community structure can have an impact not only the metabolism of NPA but on other digestive functions as well.