Effects of jugular infusions of isoleucine, leucine, methionine, threonine, and other amino acids on insulin and glucagon concentrations, mammalian target of rapamycin (mTOR) signaling, and lactational performance in goats

摘要

The supply and profile of absorbed AA may affectmilk protein synthesis through hormonal changes andmammalian target of rapamycin (mTOR) signalingpathways; and Ile, Leu, Met, and Thr (ILMT) are the4 AA that have been reported to have the greatesteffect on mammary mTOR signaling. The extent towhich ILMT and the other remaining AA (RAA) differin their effects on milk protein synthesis needs to besystematically investigated. In this study, 5 lactatinggoats, averaging 120 ± 10 d in milk, fitted with jugularvein and carotid artery catheters, were fasted for 24 h,followed by intravenous infusions of a mixture containingAA and glucose for 8 h in a 5 × 5 Latin squaredesign. The AA mixtures were formulated according tothe profile of casein. The amounts of AA infused werecalculated based on supplies of AA when metabolizableprotein (MP) was at requirement (MR). Treatmentswere an infusate containing glucose without AA(NTAA); an infusate containing 3 × the MR of Ile, Leu,Met and Thr (3F0R); and infusates containing 3F0Rplus 1, 2, or 3 × MR of RAA (3F1R, 3F2R, and 3F3R,respectively) according to amounts provided when fedto meet MP requirements for maintenance and lactationfor each goat. Milk, arterial blood, and mammarytissue samples were collected immediately after haltingthe infusion. Relative to NTAA, supplementation ofILMT tended to increase milk protein production andplasma glucose concentrations, and increased milk andlactose production, but had no effects on production orcontent of milk fat. Graded supplementation of RAAtended to quadratically affect production of milk andlactose. Arterial glucose and glucagon concentrationsdecreased linearly, and plasma insulin concentrationsdecreased quadratically with increased RAA. Mammaryp70-S6K1 phosphorylation was decreased by additionof ILMT compared with NTAA but increasedlinearly with increased RAA infusion. Furthermore,EIF4EBP1 gene expression was much lower for 3Ftreatedgoats than for the NTAA treatment. BothMTOR and RPS6KB1 gene expressions were decreasedquadratically with increased RAA supply. These resultssuggested that short-term milk protein yield tended tobe increased by elevated ILMT availability, and thistrend was not explained by variations in mammarymTOR signaling or pancreatic hormone secretions,whereas graded increase of RAA in combination withILMT appeared to regulate the efficiency of conversionof glucose to lactose in a manner not involving milkprotein production.