Abstract
BackgroundUnderstanding the mechanisms of N utilization for lactation can lead to improved requirement estimates and increased efficiency, which modern dairy diets currently fail to maximize. The mechanistic target of rapamycin complex 1 (mTORC1) is a central hub of translation regulation, processing extra- and intra-cellular signals of nutrient availability and physiological state, such as amino acids and energy. We hypothesized that dietary amino acids regulate lactation through mTORC1, such that inhibition of mTORC1 will lead to decreased lactation performance when amino acids are not limiting. Our objectives were to assess lactation performance in lactating mice undergoing dietary and pharmacologic interventions designed to alter mTORC1 activity.MethodsFirst lactation mice (N = 18; n = 6/treatment) were fed an adequate protein diet (18% crude protein), or an isocaloric protein-restricted diet (9% crude protein) from the day after parturition until lactation day 13. A third group of mice was fed an adequate protein diet and treated with the mTORC1 inhibitor rapamycin (4 mg/kg every other day) intraperitoneally, with the first two groups treated with vehicle as control. Dams and pups were weighed daily, and feed intake was recorded every other day. Milk production was measured every other day beginning on lactation day 4 by the weigh-suckle-weigh method. Tissues were collected after fasting and refeeding.ResultsMilk production and pup weight were similarly decreased by both protein restriction and rapamycin treatment, with final production at 50% of control (P = 0.008) and final pup weight at 85% of control (P < 0.001). Mammary phosphorylation of mTORC1’s downstream targets were decreased by protein restriction and rapamycin treatment (P < 0.05), while very little effect was observed in the liver of rapamycin treated mice, and none by protein restriction.ConclusionsOverall, sufficient supply of dietary amino acids was unable to maintain lactation performance status in mice with pharmacologically reduced mammary mTORC1 activity, as evidenced by diminished pup growth and milk production, supporting the concept that mTORC1 activation rather than substrate supply is the primary route by which amino acids regulate synthesis of milk components.
Highlights
Dairy cows and other ruminant livestock are incredible converters of low-quality feeds [1] into milk, meat, and fiber, and when properly managed have an important role in sustainable food systems [2]
Live animal data Milk production for both Protein-restricted diet (PR) and Adequate protein diet (AP)-R by the weighsuckle-weigh method began to decrease relative to AP starting at LD9, and by LD11 (P = 0.008) was approximately 50% of AP (Fig. 1a)
Dam food consumption decreased for both PR and Adequate protein diet combined with rapamycin treatment (AP-R) relative to AP which consistently climbed until LD13, with PR falling below AP by LD10 (P = 0.04) and AP-R by LD12 (P = 0.002, Fig. 1c)
Summary
Dairy cows and other ruminant livestock are incredible converters of low-quality feeds [1] into milk, meat, and fiber, and when properly managed have an important role in sustainable food systems [2]. Dairy cattle pose a more complicated nutritional system than do monogastrics, as rumen protein degradability, microbial metabolism, and N recycling to the rumen come into play. It is postabsorption where lactating dairy cattle experience the largest N losses, falling behind monogastrics in N efficiency [5]. The mammary glands exhibit plasticity in response to changes in AA supply: for example, when His was restricted in lactating goats, mammary clearance of His increased by 43-fold while clearance of other AA was concomitantly decreased [9]. Our objectives were to assess lactation performance in lactating mice undergoing dietary and pharmacologic interventions designed to alter mTORC1 activity
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