Myofibrillar protein synthesis in young and old men

Abstract

We tested the hypothesis that healthy older men (> 60 yr old) have a slower rate of myofibrillar protein synthesis than young men (< 35 yr old). Myofibrillar protein synthesis was determined by the in vivo incorporation of L-[1-13C]leucine into myofibrillar proteins obtained by muscle biopsy. Subjects were eight young (21-31 yr) and eight older (62-81 yr) men, all healthy and moderately active. There was no significant difference in the mean height and weight of the two age groups, but the older group had 12% less lean body mass (determined by 40K counting) and 21% less muscle mass (estimated by urinary creatinine excretion). Upper leg strength was approximately one-third lower in the older subjects according to isokinetic dynamometry. The fractional rate of myofibrillar protein synthesis was 28% slower in the older group (0.039 +/- 0.009 vs. 0.054 +/- 0.010 %/h, mean +/- SD, P < 0.01). Total myofibrillar protein synthesis, estimated as total myofibrillar mass (from creatinine excretion) times the fractional synthesis rate, was 44% slower in the older group (1.4 vs. 2.5 g/h, P < 0.001). Whole body protein synthesis, assessed as the difference between leucine disappearance rate and leucine oxidation, was marginally slower (8%, P = 0.10) in the older group, but not when the data were adjusted for lean body mass. Myofibrillar protein synthesis was a smaller fraction of whole body protein synthesis in the older group (12 vs. 19%). Reduced myofibrillar protein synthesis may be an important mechanism of the muscle atrophy associated with aging.

Muscle Wasting in Aged, Sarcopenic Rats Is Associated with Enhanced Activity of the Ubiquitin Proteasome Pathway

“…Transcriptional profiling (59,60) and mass spectrometry (31) of muscles from aged rodents showed a reduced expression of myofibrillar components, which presumably reflects decreased protein synthesis in aged muscle (83)(84)(85). The finding of increased content of proteasomes and other UPS components (e.g.…”

Assessment of biomarkers of protein anabolism in skeletal muscle during the life span of the rat: sarcopenia despite elevated protein synthesis

“…Although this preconditioning would be expected to reduce variability between subjects due to differences in preexperimental activity, its success relies on the assumption that both young and old subjects have the same sensitivity to bed rest and fixed protein intake. In such studies, elderly subjects exhibited a reduced rate of both protein synthesis and protein degradation compared with young subjects (2,39). Consequently, it has been proposed that the observed reduction in the protein synthetic rate with age is the primary stimulus behind age-related muscle loss and that reduced protein degradation is a compensatory response by the muscle in response to reduced anabolic activity.…”

Amino acid metabolism and regulatory effects in aging

“…Since muscle protein degradation has been consistently reported to remain essentially unchanged with advancing age [12][13][14][15][16], there has been an emphasis on studies examining the influence of age on muscle protein synthesis in the basal (postabsorptive) and fed (post-prandial) state. While some researchers have reported a decrease in basal muscle protein synthesis rate with age [17,18], others [14,19] could not confirm those findings in older individuals exhibiting a reduction in muscle mass. The reasons for these discrepancies are still unclear, but it is likely that differences in the health, nutritional status and physical activity level of the different older cohorts enrolled in the various studies [14,19] may have played a significant role.…”