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

Pituitary Pars Intermedia Dysfunction (PPID) in Horses

Substantial morbidity results from pituitary pars intermedia dysfunction (PPID) which is often underestimated by owners and veterinarians. Clinical signs, pathophysiology, diagnostic tests, and treatment protocols of this condition are reviewed. The importance of improved recognition of early clinical signs and diagnosis are highlighted, as initiation of treatment will result in improved quality of life. Future research should be targeted at improving the accuracy of the diagnosis of PPID, as basal adrenocorticotropic hormone (ACTH) concentration can lack sensitivity and thyrotropin releasing hormone (TRH) used to assess ACTH response to TRH stimulation is not commercially available as a sterile registered product in many countries. The relationship between PPID and insulin dysregulation and its association with laminitis, as well as additional management practices and long-term responses to treatment with pergolide also require further investigation.

Markers of muscle atrophy and impact of treatment with pergolide in horses with pituitary pars intermedia dysfunction and muscle atrophy

Pituitary pars intermedia dysfunction (PPID) is a common endocrine disorder of aged horses, with muscle atrophy as one of the clinical signs. We sought to compare muscle mass and regulation of skeletal muscle proteolysis between horses with PPID and muscle atrophy to older horses without PPID, and to assess the impact of treatment with pergolide (dopaminergic agonist) on PPID horses. We hypothesized that PPID-associated muscle atrophy is a result of increased proteolysis, and that markers of muscle atrophy and proteolysis would improve over time with pergolide treatment. Markers of muscle atrophy, adiposity, insulin regulation, skeletal muscle composition, and proteolysis (muscle atrophy F- box/atrogin 1 [MAFbx1], muscle RING finger 1 [MuRF1], Bcl2/adenovirus EIV 19kD interacting protein 3 [Bnip3], and microtubule-associated light chain 3 [LC3]) were compared between PPID and control horses. PPID horses were treated for 12 weeks with either pergolide or placebo. Dose of pergolide was adjusted based upon monthly measurement of adrenocorticotropin, and markers of muscle atrophy, adiposity, insulin regulation, skeletal muscle composition, and proteolysis were compared after 12 weeks of treatment. Horses with PPID exhibited increased transcript abundance of MuRF1 (P= 0.04) compared to control. However, no difference was observed in transcript abundance of markers of proteolysis with treatment (P ≥ 0.25). Pergolide treated horses lost weight (P = 0.02) and improved fasting insulin (P = 0.02), while placebo treated horses gained weight and rump fat thickness (P = 0.02). Findings from this study suggest that treatment with pergolide may promote weight loss and improve insulin regulation in horses with PPID, but does not impact muscle mass or markers of muscle proteolysis.

Nutritional Considerations When Dealing with an Obese Adult Equine

Equine obesity is common, reducing quality of life and requiring dietary energy restriction. Equine obesity is identified using subjective body condition scoring. Considerations are given for life stage and health status when managing obese equines. Every effort should be made to maximize feeding duration, and minimize time spent without feed while meeting all essential nutrient requirements. Limiting total daily dry matter intake to 2% of current bodyweight per day of a low caloric, forage-based diet may result in adequate body weight loss. Weight loss and weight management plans should be monitored for success and potential gastrointestinal, metabolic, and/or behavioral complications.

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.

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.