TY - JOUR
T1 - Timing of the initial muscle biopsy does not affect the measured muscle protein fractional synthesis rate during basal, postabsorptive conditions
AU - Smith, Gordon I.
AU - Villareal, Dennis T.
AU - Lambert, Charles P.
AU - Reeds, Dominic N.
AU - Selma Mohammed, B.
AU - Mittendorfer, Bettina
PY - 2010/2
Y1 - 2010/2
N2 - The muscle protein fractional synthesis rate (FSR) is determined by monitoring the incorporation of an amino acid tracer into muscle protein during a constant-rate intravenous tracer infusion. Commonly two sequential muscle biopsies are obtained some time after starting the tracer infusion. However, other protocols, including those with an initial biopsy before starting the tracer infusion to measure the background enrichment and those with only a single biopsy after several hours of tracer infusion have been used. To assess the validity of these approaches, we compared the muscle protein FSR obtained by calculating the difference in [ring 2H 5]phenylalanine and [5,5,5- 2H 3]leucine incorporation into muscle protein at ∼3.5 h after starting the tracer infusion and 1) at 60 min; 2) before starting the tracer infusion (background enrichment); 3) a population average muscle protein background enrichment; and 4) by measuring the tracer incorporation into muscle protein at ∼3.5 h assuming essentially no background enrichment. Irrespective of the tracer used, the muscle protein FSR calculated from the difference in the muscle protein labeling several hours after starting the tracer infusion and either the labeling at 60 min or the background enrichment were not different (e.g., 0.049 ± 0.007%/h vs. 0.049 ± 0.007%/h, respectively, with [ 2H 5] phenylalanine; P = 0.99). However, omitting the initial biopsy and assuming no background enrichment yielded average FSR values that were ∼15% (with [ 2H 5]phenylalanine) to 80% (with [ 2H 3]leucine) greater (P ≤ 0.059); using a population average background enrichment reduced the difference to ∼3% (P = 0.76) and 22% (P = 0.52) with 2H 5]phenylalanine and [ 2H 3]Ieucine, respectively. We conclude that during basal, postabsorptive conditions, valid muscle protein FSR values can be obtained irrespective of the timing of the initial biopsy so long as the protein labeling in two sequential biopsies is measured whereas the single biopsy approach should be avoided.
AB - The muscle protein fractional synthesis rate (FSR) is determined by monitoring the incorporation of an amino acid tracer into muscle protein during a constant-rate intravenous tracer infusion. Commonly two sequential muscle biopsies are obtained some time after starting the tracer infusion. However, other protocols, including those with an initial biopsy before starting the tracer infusion to measure the background enrichment and those with only a single biopsy after several hours of tracer infusion have been used. To assess the validity of these approaches, we compared the muscle protein FSR obtained by calculating the difference in [ring 2H 5]phenylalanine and [5,5,5- 2H 3]leucine incorporation into muscle protein at ∼3.5 h after starting the tracer infusion and 1) at 60 min; 2) before starting the tracer infusion (background enrichment); 3) a population average muscle protein background enrichment; and 4) by measuring the tracer incorporation into muscle protein at ∼3.5 h assuming essentially no background enrichment. Irrespective of the tracer used, the muscle protein FSR calculated from the difference in the muscle protein labeling several hours after starting the tracer infusion and either the labeling at 60 min or the background enrichment were not different (e.g., 0.049 ± 0.007%/h vs. 0.049 ± 0.007%/h, respectively, with [ 2H 5] phenylalanine; P = 0.99). However, omitting the initial biopsy and assuming no background enrichment yielded average FSR values that were ∼15% (with [ 2H 5]phenylalanine) to 80% (with [ 2H 3]leucine) greater (P ≤ 0.059); using a population average background enrichment reduced the difference to ∼3% (P = 0.76) and 22% (P = 0.52) with 2H 5]phenylalanine and [ 2H 3]Ieucine, respectively. We conclude that during basal, postabsorptive conditions, valid muscle protein FSR values can be obtained irrespective of the timing of the initial biopsy so long as the protein labeling in two sequential biopsies is measured whereas the single biopsy approach should be avoided.
KW - Amino acid
KW - Muscle protein turnover
UR - http://www.scopus.com/inward/record.url?scp=75749157004&partnerID=8YFLogxK
U2 - 10.1152/japplphysiol.00957.2009
DO - 10.1152/japplphysiol.00957.2009
M3 - Article
C2 - 19940095
AN - SCOPUS:75749157004
SN - 8750-7587
VL - 108
SP - 363
EP - 368
JO - Journal of Applied Physiology
JF - Journal of Applied Physiology
IS - 2
ER -