Plasma and muscle amino acid concentrations in insulin resistant compared to normal horses in the fed and fasted state

Abstract

The effects of insulin resistance on protein and amino acid metabolism have not been documented in horses, despite knowledge that insulin stimulates the incorporation of amino acids into proteins and decreases the rate of protein degradation in skeletal muscle. The objective of this research was to investigate protein metabolism in insulin resistant (IR) horses compared to Normal controls. Preceding the study, horses were screened for fasting plasma insulin concentrations with insulin concentrations of 10.1 3.5 mU/L = Normal, and 41.2 15.9 mU/L = IR, and then underwent an oral sugar challenge (OST). Blood samples were collected before and at 60, 75, 90, 120, 180 min after administration of Karo Light Syrup (0.15 mL/kg of BW) for the determination of plasma insulin and glucose. Based on the OST, eight horses (16 + 3 yrs), 4 IR and 4 Normal, were studied while receiving Purina Strategy and a mixture of Timothy/Bermudagrass hay fed at 2% BW, a diet which met or exceeded NRC recommendations. After a week of dietary adaption, the morning meal (half the daily ration) was fed on day 7. Blood samples were taken at 0, 1, 3, 4, and 6 h post-prandial for determination of plasma amino acids. On day 8, muscle biopsies were taken and immediately frozen in liquid nitrogen for analysis of free amino acids. Glucose was analyzed using an enzymatic assay, and insulin by radioimmunoassay. Plasma free amino acids and semitendinosus muscle free amino acids were determined using reverse-phase HPLC of phenylisothiocyanate derivatives. Data were analyzed using a mixed model with repeated measures analysis of SAS, with time and group as main effects. The OST data confirmed higher insulin (P = 0.020) and a trend for higher glucose (P = 0.055) in IR vs Normal horses. There were no differences between IR and Normal for any plasma free amino acids (P > 0.15) or semitendinosus muscle free amino acids (P > 0.17). Contradictory to this study, hyperinsulinemic clamp procedures in healthy horses and pigs lowered plasma amino acid concentrations, with similar results reported in healthy and diabetic humans. Lack of variation in amino acid concentrations between IR and Normal horses suggests that insulin resistance does not affect amino acid absorption into the plasma pool or incorporation into the muscle in horses.

Acknowledgments: This study was conducted at Emory and Henry College with the support of a Virginia Horse Industry Board grant.