Validation of a technique for measuring muscle protein synthesis in vivo in the horse

Reasons for performing the study:

Measurement of incorporation of stable isotope-containing amino acids into muscle is the gold-standard technique for measuring muscle protein synthesis (MPS), but its use in horses has not been evaluated.

Objectives:

1) To establish a technique for measuring MPS in vivo in the horse using a labelled amino acid tracer and 
2) to determine basal MPS in the horse using this technique.

Methods:

Three mature Thoroughbred geldings received 0.1mg/kg bwt D5-phenylalanine (D5-Phe) i.v. and plasma D5-Phe enrichment was measured over 3 h. Pharmacokinetic modelling determined the rate of appearance and phenylalanine pool size, from which a loading dose and infusion rate that would achieve a steady 5–10% plasma enrichment were calculated. Subsequently, steady-state was confirmed by measuring plasma enrichment during 6 h of infusion and the effect of treadmill exercise and sedation (0.4 mg/kg bwt) were also determined. Horses subsequently received the calculated D5-Phe infusion rate and skeletal muscle biopsy samples were collected after 1 and 3 h, allowing basal MPS to be calculated. Precursor intramuscular phe labelling was measured by GC-MS and tracer phe incorporation into muscle myofibrillar protein was determined using GCpyrolysis-IRMS to calculate fractional synthetic rate.

Results:

The mean rate of whole body phenylalanine appearance in the horse was 56 µmol/kg bwt/h (s.d. 24) and mean phenylalanine pool size 41 µmol/kg bwt (s.d. 8.8). From this a priming dose of 3.3 µmol/kg bwt and an infusion rate of 4.5 µmol/kg bwt/h D5-Phenylalanine was calculated and subsequently confirmed to achieve steady state tracer enrichment from 5.6–9.1 APE in sedentary horses. Exercise caused a transitory, and sedation a more prolonged reduction in plasma enrichment (up to 20%). D5-Phe incorporation revealed a basal FSR of 0.029%/h (s.d. 0.013) in the horse.

Conclusions:

D5-Phenylalanine is a suitable amino acid tracer for measurement of MPS in the horse yielding rates of MPS in line with other large mammals