Age-related deficit in load-induced skeletal muscle growth

Abstract

The growth response of ankle flexor and extensor muscles to two models of increased loading, functional overload (FO) and hind-limb reloading following hind-limb suspension, was measured by wet weight in Fisher 344-Brown Norway rats at ages ranging from 6 to 30 months. In response to FO, there was a 40% decrease in absolute growth of the plantaris beginning in middle age. Interestingly, the growth response to FO of 30-month old rats maintained on a 40% calorie-restricted diet improved by more than twofold relative to 30-month old rats on a normal chow diet. Recovery of muscle mass upon reloading following disuse was significantly impaired (reduced 7-16%) in predominantly fast, but not slow, muscles of 30-month relative to 9-month old rats. Initial investigation of the Akt signaling pathway following FO suggests a reduction or delay in activation of Akt and its downstream targets in response to increased loading in old rats.

Figure 1.

The absolute difference in mass (expressed in milligrams) between the plantaris of the control and functional overload groups was calculated and plotted for each age group. Bars are mean ± standard error. Asterisk denotes a significant difference (p < .001) from the 9-mo age group.

Figure 2.

Histograms representing the extent of atrophy in the plantaris (PL), medial gastrocnemius (MG), tibialis anterior (TA), and extensor digitorum longus (EDL) muscles of 9-mo (open bars) and 30-mo (solid bars) old Fisher 344–Brown Norway F1 hybrid rats following 14 days of reloading after a 14-day unloading period. The amount of atrophy is expressed as the % difference between the control and the reloaded muscles. * denotes a significant difference from the control at the p < .001 level. # denotes a significant difference from control at the p = .02 level.

Figure 3.

Akt and eIF2Bϵ expressions in male 6-, 18-, 26-, and 30-mo old Fisher 344–Brown Norway F1 hybrid rats following 14 days of functional overload (FO). (A) Western blots of phosphorylated (Ser-473) and native Akt in the plantaris (PL) of control (C) or 14-day FO rats. Each lane represents 200 μg of total protein extracted from a pool of three PL muscles. (B) Histogram of Akt activation, calculated as level of phosphorylation per amount of protein, in control (white) and FO (black) PL muscles of rats at different ages. Bars represent mean ± standard error of six rats. (C) Western blots of eIF2ϵ in the PL of control (C) or 14-day FO rats. Each lane represents 200 μg of total protein extracted from a pool of three PL muscles. The numbers below the FO lanes represent the mean fold increase in protein expression relative to control.

Figure 4.

Western blots (WB) of 4E-BP1, eIF4G, and eIF4E following immunoprecipitation (i.p.) of eIF4E from plantaris (PL) homogenates. eIF4E was immunoprecipitated from muscle homogenates using a monoclonal anti-eIF2E antibody. Each lane represents a pool of three muscles. The first lane represents positive controls (pc) for each of the antibodies. PL muscles were taken from the control (Con) or 14-day overloaded (FO) rats. Functional overload was performed in male Fisher 344–Brown Norway F1 hybrid rats age 6 and 26 mos.

Figure 5.

Akt and eIF2Bϵ expressions in male 6- and 26-mo old Fisher 344–Brown Norway F1 hybrid rats following 7 days of functional overload (FO). (A) Western blots of phosphorylated (Ser-473) and native Akt in the plantaris (PL) of control (C) or 7-day FO rats aged 6 and 26 mos. Each lane represents 200 μg of total protein extracted from a single PL muscle. Histogram of Akt activation, calculated as level of phosphorylation per amount of protein, in control (white) and FO (black) PL muscles of rats aged 6 and 26 mos. Bars represent mean ± standard error of six rats. (B) Western blot of eIF2ϵ in the PL of control (C) or 7-day FO rats aged 6 and 26 mos. Each lane represents 200 μg of total protein extracted from a single PL muscle. Histogram of eIF2ϵ protein expression, calculated as a relative optical density, in control (white) and FO (black) PL muscles of rats aged 6 and 26 mos. Bars represent mean ± standard error of six rats.