The effects of aging on markers of satellite cell activity after three sequential bouts of resistance exercise

Abstract

Background. Satellite cell content in human skeletal muscle has shown to be reduced in older versus younger populations. Further, the ability of older satellite cells to proliferate following resistance exercise has been shown to be impaired in older individuals; an effect which seemingly limits the hypertrophic response to resistance exercise with aging. Previous in vitro and in vivo research has demonstrated that genes involved in satellite cell proliferation, myoblast differentiation, and insulin-like growth factor (IGF)-1 signaling are differentially expressed in older versus younger muscle. However, no evidence to date exists examining how multiple bouts of resistance training affect the genetic and proteomic expression of these indices between younger and older age groups.

Purpose. Therefore, the purposes of this study were to: 1) assess baseline markers indicative of satellite cell activity between younger versus older males, 2) examine if three sequential bouts of resistance exercise affects markers of satellite cell activity in these populations, and 3) examine if the potential age-dependent decrements in satellite cell proliferation and/or differentiation were related to decrements in myogenic genes and/or a decrement in muscle IGF-1 protein expression in older individuals at baseline and throughout the resistance training intervention.

Methods. Ten younger (age: 18-25 y) and 10 older (age: 60-75 y) males were recruited for this study and were instructed to complete 3 lower body workouts (M, W, F) consisting of 9 sets of lower-body exercises with 10 repetitions per set at an intensity of 80% of each individual\'s one repetition maximum. Percutaneous muscle biopsies were collected prior to the exercise intervention (T1), 48 hours following workout 1 (T2), 48 hours following workout 2 (T3), and 24 hours following workout 3 (T4). A fraction (~20 mg) of muscle tissue was rinsed of excess blood, connective, and adipose tissue and was homogenized in order to assess the concentrations (denoted by [__]) of muscle [DNA], [total protein], and [myofibrillar protein]. A second fraction of muscle (~30 mg) was also homogenized, total RNA was precipitated out the homogenate and mRNA from the total RNA pool was reverse transcribed into cDNA for gene expression analysis. Finally, a third fraction of muscle (~25 mg) was pre-rinsed and homogenized to obtain total protein for subsequent Western blotting. Semi-quantitative real-time reverse transcriptase polymerase chain reaction (RT-PCR) was performed to quantitate baseline and changes in the mRNA expression levels of CYCLIN D1, cyclin dependent kinase (CDK)4, CDK2, P21CIP1, P27KIP1, MYOD, MGF, IGF-IEA, and embryonic myosin heavy chain (MHCEMB) with the arithmetic mean of beta-2 microglobulin (B2M) and 28S rRNA (28S) being used as an internal control. Muscle homogenates were also assayed for the protein contents of proliferating cell nuclear antigen (PCNA), hepatic growth factor receptor (c-met), and muscle IGF-1.

Results. In partial agreement with my initial hypotheses, repeated bouts of conventional resistance training increased muscle [c-met], a protein marker indicative of satellite cell quantity, in young males 48 hours following the first bout (p < 0.01) and decreased this marker in young males 24 hours following the third bout (p < 0.05), whereas this value did not change within the old group. Furthermore, MYOD gene expression modestly increased at T2 (p < 0.05) and MHCEMB gene expression modestly increased (p < 0.05) at T4 relative to baseline expression values in the younger males. Interestingly, MGF expression increased at T2-4 in the older group relative to baseline values (p < 0.05), albeit muscle IGF-1 peptide levels remained stable throughout the intervention in both age groups. Other findings from this study indicate that repeated training bouts: 1) do not alter the expression of genes indicative of satellite cell activity (i.e., CDK2, CDK4, CYCLIN D1, MYOD, P27KIP1, P21CIP1) and/or the expression of IGF-1EA up to 24-48 hours following exercise in younger or older males, 2) do not increase markers of satellite cell proliferation (i.e., muscle [PCNA], and [DNA]) in either age group at the sampled time points, and 3) do not alter [myofibrillar protein] or [total protein] in either age group at the sampled time points. In regards to baseline physiological parameters, these data illustrate that the CYCLIN D1 gene is expressed more highly in older versus younger males (p < 0.05), whereas all other genes and markers of satellite cell activity were similar between age groups.

Conclusions. The findings from this study suggest: 1) both age groups retain proliferative characteristics during resting, unexercised states given the similarities between these groups in regards to muscle [PCNA], muscle [c-met], CDK2, CDK4, as well as an increased CYCLIN D1 mRNA expression in older individuals, 2) three consecutive bouts of resistance exercise seem to significantly increase a marker indicative of satellite cell quantity concomitant with the increased expression of a select few myogenic genes in younger participants only (i.e., MYOD at T2 and MHCEMB at T4), and 3) repeated exercise bouts facilitated a summation effect on MGF expression only in older individuals which is contrary to preliminary research examining this gene in these populations. In summary, these data indicate that more exercise bouts may be needed to stimulate satellite cell activity during the initial stages of resistance training in older populations which possibly explains why there are impairments in muscle hypertrophy in older adults, albeit future research should employ immunohistochemistry in order to confirm these preliminary findings.