Timing of the initial muscle biopsy does not affect the measured muscle protein fractional synthesis rate during basal, postabsorptive conditions

Abstract

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-(2)H(5)]phenylalanine and [5,5,5-(2)H(3)]leucine incorporation into muscle protein at approximately 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 approximately 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 [(2)H(5)]phenylalanine; P = 0.99). However, omitting the initial biopsy and assuming no background enrichment yielded average FSR values that were approximately 15% (with [(2)H(5)]phenylalanine) to 80% (with [(2)H(3)]leucine) greater (P < or = 0.059); using a population average background enrichment reduced the difference to approximately 3% (P = 0.76) and 22% (P = 0.52) with [(2)H(5)]phenylalanine and [(2)H(3)]leucine, 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.

Fig. 1.

Plasma leucine and α-ketoisocaproic acid (KIC) tracer-to-tracee ratio (TTR; top) and the ratio between the α-KIC and leucine labeling in plasma (bottom) during a primed, constant infusion of [5,5,5-²H₃]leucine. Values are means ± SE. *Value significantly different from all other corresponding values, P < 0.05.

Fig. 2.

Muscle protein phenylalanine labeling [tracer-to-tracee ratio (TTR)] measured immediately before and 210 min after the start of a primed constant [ring-²H₅]phenylalanine infusion or at 60 min and 240 min after the start of the tracer infusion. Values are means ± SE. There were no differences in either the slopes (P = 0.99) or the intercepts (P = 0.98) of the two lines. The intercept (0.000022 ± 0.000012) was significantly different from zero (P = 0.05) but similar to the average background TTR (0.000018) from a previously studied cohort (P = 0.75).

Fig. 3.

Muscle protein fractional synthesis rate (FSR) during basal, postabsorptive conditions assessed by collecting muscle biopsies 1) immediately before and 210 min after the start of a primed constant [ring-²H₅]phenylalanine infusion or 2) at 60 min and 240 min after the start of the tracer infusion, 3) omitting the first biopsy and assuming zero background labeling, and 4) omitting the first biopsy and using a population average background enrichment. Values are means ± SE. *Value different (P = 0.059) from values obtained by measuring the labeling of muscle protein in two sequential biopsies.

Fig. 4.

Muscle protein FSR during basal, postabsorptive conditions assessed by collecting muscle biopsies 1) immediately before and 210 min after the start of a primed constant or [5,5,5-²H₃]leucine infusion or 2) at 60 min and 240 min after the start of the tracer infusion, 3) omitting the first biopsy and assuming zero background labeling, and 4) omitting the first biopsy and using a population average background enrichment. Values are means ± SEM. *Value significantly different (P < 0.05) from values obtained by measuring the labeling of muscle protein in two sequential biopsies.