Current understanding of insulin dysregulation and its relationship with carbohydrate and protein metabolism in horses

Insulin dysregulation (ID) is a common metabolic disorder in horses, characterized by hyperinsulinemia and/or peripheral insulin resistance. The critical role of hyperinsulinemia in endocrinopathic laminitis has driven research into the insulinotropic effects of dietary nutrients and the reciprocal impact of ID on nutrient metabolism. The relationship between ID and carbohydrate metabolism has been extensively studied; however, the effects of ID on protein metabolism in horses remain largely unexplored. This review begins with an overview of the importance of insulin in the regulation of muscle protein synthesis and degradation and then examines the current understanding of the interplay between ID and protein and carbohydrate metabolism in horses. Horses with ID exhibit altered resting plasma amino acid concentrations and shifts in postprandial amino acid dynamics. Recent work illustrated that ID horses had higher levels of plasma amino acids following a protein meal and delayed postprandial clearance from the blood compared to non-ID horses. The postprandial muscle synthetic response does not seem to be diminished in ID horses, but alterations in key cellular signaling molecules have been reported. ID horses display a pronounced hyperinsulinemic response following the consumption of feeds providing a range of protein, non-structural carbohydrate, starch and water-soluble carbohydrate intakes. Recent studies have shown that ID horses have an increased postprandial incretin response, contributing to the observed hyperinsulinemia. To minimize the postprandial insulin response, thresholds for carbohydrate consumption have recently been proposed. Similar thresholds should be established for protein to aid in the refinement of nutritional strategies to manage ID horses.

Influence of mTOR-regulated anabolic pathways on equine skeletal muscle health

Skeletal muscle is a highly dynamic organ that is essential for locomotion as well as endocrine regulation in all populations of horses. However, despite the importance of adequate muscle development and maintenance, the mechanisms underlying protein anabolism in horses on different diets, exercise programs, and at different life stages remain obscure. Mechanistic target of rapamycin (mTOR) is a key component of the protein synthesis pathway and is regulated by biological factors such as insulin and amino acid availability. Providing a diet ample in vital amino acids, such as leucine and glutamine, is essential in activating sensory pathways that recruit mTOR to the lysosome and assist in the translation of important downstream targets. When the diet is well balanced, mitochondrial biogenesis and protein synthesis are activated in response to increased exercise bouts in the performing athlete. It is important to note that the mTOR kinase pathways are multi-faceted and very complex, with several binding partners and targets that lead to specific functions in protein turnover of the cell, and ultimately, the capacity to maintain or grow muscle mass. Further, these pathways are likely altered across the lifespan, with an emphasis of growth in young horses while decreases in musculature with aged horses appears to be attributable to degradation or other regulators of protein synthesis rather than alterations in the mTOR pathway. Previous work has begun to pinpoint ways in which the mTOR pathway is influenced by diet, exercise, and age; however, future research is warranted to quantify the functional outcomes related to changes in mTOR. Promisingly, this could provide direction on appropriate management techniques to support skeletal muscle growth and maximize athletic potential in differing equine populations.

Nutritional Influences on Skeletal Muscle and Muscular Disease

Skeletal muscle comprises 40% to 55% of mature body weight in horses, and its mass is determined largely by rates of muscle protein synthesis. In order to support exercise, appropriate energy sources are essential: glucose can support both anaerobic and aerobic exercise, whereas fat can only be metabolized aerobically. Following exercise, ingestion of nonfiber carbohydrates and protein can aid muscle growth and recovery. Muscle glycogen replenishment is slow in horses, regardless of dietary interventions. Several heritable muscle disorders, including type 1 and 2 polysaccharide storage myopathy and recurrent exertional rhabdomyolysis, can be managed in part by restricting dietary nonstructural carbohydrate intake.

Pathways regulating equine skeletal muscle protein synthesis respond in a dose-dependent manner to graded levels of protein intake

Activation of the mechanistic target of rapamycin (mTOR)-controlled anabolic signaling pathways in skeletal muscle of rodents and humans is responsive to the level of dietary protein supply, with maximal activation and rates of protein synthesis achieved with 0.2 to 0.4 g protein/kg body weight (BW). In horses, few data are available on the required level of dietary protein to maximize protein synthesis for maintenance and growth of skeletal muscle. To evaluate the effect of dietary protein level on muscle mTOR pathway activation, five mares received different amounts of a protein supplement that provided 0, 0.06, 0.125, 0.25, or 0.5 g of crude protein (CP)/kg BW per meal in a 5 × 5 Latin square design. On each sample day, horses were fasted overnight and were fed only their protein meal the following morning. A preprandial (0 min) and postprandial (90 min) blood sample was collected and a gluteus medius muscle sample was obtained 90 min after feeding the protein meal. Blood samples were analyzed for glucose, insulin, and amino acid concentrations. Activation of mTOR pathway components (mTOR and ribosomal protein S6 [rpS6]) in the muscle samples was measured by Western immunoblot analysis. Postprandial plasma glucose (P = 0.007) and insulin (P = 0.09) showed a quadratic increase, while total essential amino acid (P < 0.0001) concentrations increased linearly with the graded intake of the protein supplement. Activation of mTOR (P = 0.02) and its downstream target, rpS6 (P = 0.0008), increased quadratically and linearly in relation to the level of protein intake, respectively. Comparisons of individual doses showed no differences (P > 0.05) between the 0.25 and 0.5 g of protein intake for either mTOR or rpS6 activation, indicating that protein synthesis may have reached near maximal capacity around 0.25 g CP/kg BW. This is the first study to show that the activation of muscle protein synthetic pathways in horses is dose-dependent on the level of protein intake. Consumption of a moderate dose of high-quality protein resulted in near maximal muscle mTOR pathway activation in mature, sedentary horses.

Metabolomics in equine sport and exercise

metabolomics is the high-throughput, multiparametric identification and classification of hundreds of low molecular weight metabolites in a biological sample. Ultimately, metabolites are the downstream readouts of cellular signalling, transcriptomic and proteomic changes that can provide a comprehensive view of tissue and organismal phenotype. The popularity of metabolomics in human sport and exercise has been gaining over the past decade and has provided important insights into the energetic demands and mechanistic underpinnings of exercise and training. To the contrary, metabolomics in the field of equine exercise physiology is lagging despite the horse's superior aerobic and muscular capabilities, as well as its prominence in competitive sport. As such, this narrative review aims to describe metabolomics, its routine implementation, the various analytical methods applied and the state of its use in the equine athlete. Sufficient attention will be paid to methodological considerations, as well as gaps in the equine literature, particularly with regard to the skeletal muscle metabolome. Finally, there will be a brief discussion of the future directions and barriers to metabolomics use in the athletic horse. A thorough understanding of the metabolomics changes that occur in the equine athlete with exercise will undoubtedly help to improve horse management and health across the lifespan.

A high protein meal affects plasma insulin concentrations and amino acid metabolism in horses with equine metabolic syndrome

Equine metabolic syndrome (EMS) is characterized by an abnormal insulin response to a glycemic challenge but despite the known insulinotropic effects of certain amino acids, there is a paucity of data evaluating the impact of dietary protein on insulin dynamics in these horses. The objective was therefore to assess insulin and amino acid responses following intake of a high protein meal in healthy horses and those with EMS. Six mature horses diagnosed with EMS and six age-matched control horses without EMS were used. Horses were fed 2g/kg body mass (BM) of a high protein pellet (31% crude protein) at time 0 and 30min, for a total of 4g/kg BM, following an overnight fast. Blood samples collected during a 4h period were analysed for plasma glucose, insulin, amino acids and urea concentrations. Glucose concentrations were not different between groups (P=0.2). Horses with EMS had a 9-fold greater insulinemic response to the consumption of a high protein meal compared with controls (P=0.046). Post-prandial levels of histidine, citrulline, tyrosine, valine, methionine, isoleucine, leucine and ornithine were higher in horses with EMS (P<0.05). Baseline urea nitrogen concentrations were not significantly different between groups (P=0.1). Knowing that certain amino acids are insulin secretagogues, these results illustrate that consumption of a high protein meal caused a hyperinsulinemic response and affected amino acid dynamics in horses with EMS. These findings suggest that dietary protein content should be taken into consideration in the management of horses with insulin dysregulation.

Effects of dietary amino acid supplementation on measures of whole-body and muscle protein metabolism in aged horses

The objective of this study was to examine markers of whole-body and muscle protein metabolism in aged horses fed a diet typical for North American aged horses, supplemented with amino acids. In a replicated Latin square design, six aged horses (20 ± 1.1 years) were studied while receiving each of three isocaloric, isonitrogenous diets, a control treatment concentrate (CON; 100 mg/kg^-1 BW day^-1 lysine, 84 mg kg^-1  day^-1 threonine, 51 mg kg^-1  day^-1 methionine), LYS/THR (134 mg kg^-1 BW day^-1 lysine, 110 mg kg^-1 BW day^-1 threonine, 52 mg kg^-1 BW day^-1 methionine) and LYS/THR/MET (132 mg kg^-1 BW day^-1 lysine, 112 mg kg^-1 BW day^-1 threonine, 62 mg kg^-1 BW day^-1 methionine). In each 15-days period, urine and faeces were collected for assessment of nitrogen balance. Blood samples were collected before and after feeding for analysis of plasma urea nitrogen (PUN), glucose, insulin and plasma amino acid concentrations. Skeletal muscle samples were collected for measurement of proteins associated with muscle protein synthesis and degradation, and horses underwent stable isotope infusion procedures for comparison of differences in whole-body rates of protein synthesis and degradation. There was no effect of treatment on relative abundance of proteins involved in protein synthesis, nitrogen retention or phenylalanine kinetics. PUN concentrations tended to be higher for LYS/THR (p = 0.054) and were higher for LYS/THR/MET (p = 0.0056) than for CON. Atrogin-1 abundance tended to be higher in the post-absorptive state for the CON treatment (p = 0.07), indicating that amino acid supplementation resulted in less muscle protein degradation when horses were in the post-absorptive state. However, lack of differences in nitrogen retention and phenylalanine kinetics indicated that whole-body protein metabolism was not improved, and higher PUN concentrations in the supplemented diets suggest that the supplemented amino acids may have been catabolized. Amino acid availability was not limiting protein synthesis in the sedentary aged horses in this study when fed the CON diet.

Pituitary pars intermedia dysfunction does not necessarily impair insulin sensitivity in old horses

Equine pituitary pars intermedia dysfunction (PPID) has been associated with reduced insulin sensitivity in comparison with younger adult horses; however, the difference in insulin sensitivity between horses with PPID and aged-matched controls has not been well characterized. The objective of the study was to determine if aged horses with PPID had reduced insulin sensitivity and alterations in the insulin-mediated signaling pathways in the skeletal muscle when compared with healthy aged horses. Isoglycemic hyperinsulinemic clamp procedures were conducted in 12 horses that were classified as either PPID (n = 6; age: 25.0 ± 2.5 yr; mean ± standard deviation) or non-PPID, aged-matched controls (control) (n = 6; age: 25.7 ± 2.0 yr). Blood samples were taken before and during the clamp procedures to measure plasma glucose, insulin, and amino acid concentrations, and 2 muscle biopsies were collected from the gluteus medius muscle, one in the basal state and the second at the end of the clamp procedure (insulin-stimulated state). Plasma insulin concentrations increased ∼9-fold during the clamp compared with basal conditions (P < 0.001) in both groups. During the last 30 min of the clamp, the rate of glucose infusion required to maintain isoglycemia in horses with PPID was similar to that in the control horses (P = 0.67). The plasma concentrations of most indispensible amino acids were lower in the insulin-stimulated state than the basal state (P < 0.05). PPID status did not have an effect on the activation of factors associated with protein synthesis and breakdown; however, factors associated with protein synthesis had increased phosphorylation in the insulin-stimulated state, compared with basal. The results from this study provide evidence that PPID is not always associated with impairments in insulin sensitivity.

Whole-body phenylalanine kinetics and skeletal muscle protein signaling in horses with pituitary pars intermedia dysfunction

OBJECTIVE

To compare whole-body phenylalanine kinetics and the abundance of factors in signaling pathways associated with skeletal muscle protein synthesis and protein breakdown between horses with pituitary pars intermedia dysfunction (PPID) and age-matched control horses without PPID.

ANIMALS

12 aged horses (6 horses with PPID and 6 control horses; mean age, 25.0 and 25.7 years, respectively).

PROCEDURES

Plasma glucose, insulin, and amino acids concentrations were determined before and 90 minutes after feeding. Gluteal muscle biopsy samples were obtained from horses 90 minutes after feeding, and the abundance and activation of factors involved in signaling pathways of muscle protein synthesis and breakdown were determined. The next day, horses received a priming dose and 2 hours of a constant rate infusion of (13)C sodium bicarbonate followed by a priming dose and 4 hours of a constant rate infusion of 1-(13)C phenylalanine IV; whole-body protein synthesis was determined.

RESULTS

Plasma glucose and insulin concentrations were higher after feeding than they were before feeding for both groups of horses; however, no significant postprandial increase in plasma amino acids concentrations was detected for either group. Phenylalanine flux, oxidation, release from protein breakdown, and nonoxidative disposal were not significantly different between groups. No significant effect of PPID status was detected on the abundance or activation of positive or negative regulators of protein synthesis or positive regulators of protein breakdown.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of this study suggested that whole-body phenylalanine kinetics and the postprandial activation of signaling pathways that regulate protein synthesis and breakdown in muscles were not affected by PPID status alone in aged horses.

Insulin infusion stimulates whole-body protein synthesis and activates the upstream and downstream effectors of mechanistic target of rapamycin signaling in the gluteus medius muscle of mature horses

Little is known about the role insulin plays in regulating whole-body and muscle protein metabolism in horses. The objective of this study was to determine the effects of graded rates of insulin infusion on plasma amino acid concentrations and the activation of factors in the mechanistic target of rapamycin signaling pathway in the skeletal muscle of horses. Isoglycemic, hyperinsulinemic clamp procedures were conducted in 8 mature, thoroughbred mares receiving 4 rates of insulin infusion: 0 mU · kg(-1) · min(-1) (CON), 1.2 mU · kg(-1) · min(-1) (LOWINS), 3 mU · kg(-1) · min(-1) (MEDINS), and 6 mU · kg(-1) · min(-1) (HIGHINS). Blood samples were taken throughout the clamp procedures to measure plasma amino acid concentrations, and a biopsy from the gluteus medius muscle was collected at the end of the 2-h clamp to measure phosphorylation of protein kinase B, eukaryotic initiation factor 4E-binding protein 1, and riboprotein S6. Plasma concentrations of most of the essential amino acids decreased (P < 0.05) after 120 min of insulin infusion in horses receiving the LOWINS, MEDINS, and HIGHINS treatments, with the largest decreases occurring in horses receiving the MEDINS and HIGHINS treatments. Phosphorylation of protein kinase B, 4E-binding protein 1, and riboprotein S6 increased with all 3 rates of insulin infusion (P > 0.05), relative to CON, with maximum phosphorylation achieved with MEDINS and HIGHINS treatments. These results indicate that insulin stimulates whole-body and muscle protein synthesis in mature horses.

Effect of gluteus medius muscle sample collection depth on postprandial mammalian target of rapamycin signaling in mature Thoroughbred mares

OBJECTIVE

To determine the effect of biopsy collection depth on the postprandial activation of mammalian target of rapamycin (mTOR) signaling factors, particularly protein kinase B, ribosomal protein S6 kinase, ribosomal protein S6, and eukaryotic initiation factor 4E binding protein 1 in middle-aged horses.

ANIMALS

6 healthy Thoroughbred mares (mean ± SD age, 13.4 ± 3.4 years).

PROCEDURES

Horses were fed a high-protein feed at 3 g/kg. Sixty minutes after horses were fed, the percutaneous needle biopsy technique was used to collect biopsy specimens from the gluteus medius muscle at 6, 8, and 10 cm below the surface of the skin. Muscle specimens were analyzed for the activation of upstream and downstream mTOR signaling factors, myosin heavy chain (MHC) isoform composition, and amino acid concentrations.

RESULTS

A 21% increase in MHC IIA isoform expression and a 21% decrease in MHC IIX isoform expression were identified as biopsy depth increased from 8 to 10 cm below the surface of the skin; however, no significant change was evident in the degree of MHC I expression with muscle depth. Biopsy depth had no significant effect on the phosphorylation of any of the mTOR signaling factors evaluated.

CONCLUSIONS AND CLINICAL RELEVANCE

Postprandial mTOR signaling could be compared between middle-aged horses when biopsy specimens were collected between 6 and 10 cm below the surface of the skin. Optimization of muscle biopsy techniques for evaluating mTOR signaling in horses will facilitate the design of future investigations into the factors that regulate muscle mass in horses.