Effect Of Dietary Energy Level On Urea Recycling And UT-B Expression In Tibetan Sheep

Tibetan sheep is an indigenous herbivore that inhabited on the Qinghai-Tibetan Plateau.They are grazing on the rangeland all year round and without any supplement.The aim of this study was to explore the effects of low nitrogen diet under different energy levels on urea recycling and the relative expression of UT-B(urea transporter B)in the Tibetan sheep(in contrast to Small tailed Han sheep),which is in order to clarify the influence of dietary energy regulation on nitrogen metabolism and reveal the physiological mechanism of Tibetan sheep adapted to low nitrogen stress,and then provide theoretical basis and technical support for Tibetan sheep supplementary of alpine ruminants during the cold season.The study consisted of the following two experiments.The main results were as followed:1.Effects of different dietary energy level on nitrogen utilization and urea kinetics in Tibetan sheep and Small tailed Han sheepAn experiment was conducted to characterize nitrogen utilization efficiency and urea kinetics in Tibetan sheep and Small tailed Han sheep fed 4 different digestible energy levels diets(8.23,9.31,10.45 and 11.57 MJ/kg,respectively)with similar crude protein content(7 %).Ten healthy wethers(5 Tibetan sheep and 5 Small tailed Han sheep)with similar body weight(47.7±2.46 kg;46.2±3.42 kg)were selected in 2 concurrent 4×4 Latin square designs.The results were obtained as following:(1)Nitrogen intake of Tibetan sheep and Small tailed Han sheep was similar(P>0.05),while the digestible nitrogen intake was higher in Tibetan sheep than that in Small tailed Han sheep(P<0.05).The fecal nitrogen excretion of Tibetan sheep was significantly lower than that of Small tailed Han sheep(P<0.05),and the excretion of Tibetan sheep was linear decreased with the increase of dietary energy level,while the fecal nitrogen excretion of Small tailed Han sheep was relatively stable(linear interaction,P<0.05).The urinary nitrogen excretion,urinary urea-N excretion were decreased linearly as dietary energy increase(P<0.001),while there was no significant difference among varieties(P>0.05).Nitrogen balance and nitrogen deposition ratewere increased linearly(P<0.001),while nitrogen deposition rate and nitrogen balance in Tibetan sheep were outstandingly higher than in Small tailed Han sheep(P <0.05).(2)Urea-N entry rate(UER),gastrointestinal tract entry rate(GER),return to ornithine cycle(ROC),urea-N used for anabolism(UUA)in the Tibetan sheep were higher than that in Small tailed Han sheep(P<0.05).With the improvement of dietary energy level,UER,GER and UUA decreased linearly(P<0.05).The concentrations of urea-N in the plasma and salivary decreased with the improvement of dietary energy level(P<0.05),while all of them were higher in Tibetan sheep(P<0.05).As the dietary energy increase,the urea-N reabsorption rate by the kindey increased linearly(P<0.05),and the renal urea-N reabsorption rate in Tibetan sheep was a little high than Small tailed Han sheep(P<0.05).2.Effects of different dietary energy level on UT-B expression in Tibetan sheep and Small tailed Han sheepHealthy and 1.5-year-old castrated Tibetan sheep(n=24)and Small tailed Han sheep(n=24)with similar body weight(48.5±1.89 kg;49.2±2.21 kg)were selected,and then divided into 4groups(LE,MLE,MHE and HE).The dietary energy level is same as that of experiment 1.The effects of dietary energy levels on UT-B mRNA and protein expression were studied using real-time fluorescence quantitative PCR(qRT-PCR)and Western blot.The results showed that:(1)UT-B mRNA was expressed in the parotid gland,rumen and kidney tissues of Tibetan sheep and Small tailed Han sheep.Immunoblotting confirmed the presence of 38 kDa(probable N-glycosylated form of UT-B)protein band in the parotid gland,rumen and kidney tissues.(2)As the dietary energy increase,UT-B mRNA and protein expression in the parotid gland decreased linearly(P<0.05),and the medulla UT-B protein expression in Tibetan sheep was linearly increased(linear interaction,P<0.05).Generally,dietary energy supplementation can significantly improve the nitrogen retention and nitrogen utilization efficiency of sheep.Compared with Small tailed Han sheep,Tibetan sheep showed higher UER,GER and renal urea-N reabsorption rate,perhaps the different regulation of UT-B expression in related tissues caused more effective urea recycling mechanism in Tibetan sheep.