No major sex differences in muscle protein synthesis rates in the postabsorptive state and during hyperinsulinemia-hyperaminoacidemia in middle-aged adults

Abstract

Men have more muscle than women, but most studies evaluating sex differences in muscle protein metabolism have been unable to discern sexual dimorphism in basal muscle protein turnover rates in young and middle-aged adults. We hypothesized that the anabolic response to nutritional stimuli (i.e., amino acids and insulin) would be greater in young/middle-aged men than women. We therefore measured the rates of muscle protein synthesis (MPS) in 16 healthy individuals [8 men and 8 women, matched for age (mean +/- SE: 37.7 +/- 1.5 yr) and body mass index (25.2 +/- 0.7 kg/m2)] after an overnight fast (plasma insulin approximately 5 microU/ml and plasma phenylalanine approximately 60 microM) and during a hyperinsulinemic-hyperaminoacidemic-euglycemic clamp (plasma insulin approximately 28 microU/ml; plasma phenylalanine approximately 110 microM; plasma glucose approximately 5.4 mM). The rates of MPS were not different between men and women (ANOVA main effect for sex; P = 0.49). During the clamp, the rate of MPS increased by approximately 50% (P = 0.003) with no difference in the increases from basal values between men and women (+0.019 +/- 0.004 vs. +0.018 +/- 0.010%/h, respectively; P = 0.93). There were also no differences between men and women in the basal concentrations of muscle phosphorylated Akt(Ser473), Akt(Thr308), mTOR(Ser2448), and p70s6k(Thr389) or in the hyperinsulinemia-hyperaminoacidemia-induced increases in phosphorylation of those signaling elements (P > or = 0.25). We conclude that there are no major differences in the rate of MPS and its intracellular control during basal conditions and during hyperinsulinemia-hyperaminoacidema between young and middle-aged adult men and women.

Figures

Fig. 1.

Mixed skeletal muscle protein fractional synthesis rate (FSR) during basal postabsorptive conditions and during the hyperinsulinemic-hyperaminoacidemic-euglycemic clamp procedure in men and women. Data are means ± SE. ANOVA revealed a significant main effect for feeding (P = 0.003) but no effect of sex (P = 0.49) and no interaction (P = 0.93).

Fig. 2.

Average concentrations (arbitrary units) and representative blots (top: protein of interest; bottom: GADPH) of phosphorylated p70s6kThr389 (A), mTORSer2448 (B), AktSer473 (C), and AktThr308 (D) during basal postabsorptive conditions and during the hyperinsulinemic-hyperaminoacidemic-euglycemic clamp procedure in men and women. Values are means ± SE.*ANOVA revealed a significant main effect for amino acid/insulin infusion (P ' 0.05) but no effect of sex and no interaction.

Fig. 3.

Average concentrations (arbitrary units) and representative blots (top: protein of interest; bottom: GADPH) of phosphorylated Erk 1/Erk 2 MAPKTyr202/204 (top) and eukaryotic elongation factor 2 (eEF2Thr56; bottom) in muscle of men and women during basal postabsorptive conditions and during the hyperinsulinemic-hyperaminoacidemic-euglycemic clamp procedure. Values are means ± SE. ANOVA revealed a trend for a main effect of sex (Erk 1/Erk 2 MAPK, P = 0.051; eEF2, P = 0.11) but no effect of amino acid/insulin infusion and no interaction.