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 13C sodium bicarbonate followed by a priming dose and 4 hours of a constant rate infusion of 1-13C 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.