Role of 5'AMP-activated protein kinase in glycogen synthase activity and glucose utilization: insights from patients with McArdle's disease

Abstract

It has been suggested that 5'AMP-activated protein kinase (AMPK) is involved in the regulation of glucose and glycogen metabolism in skeletal muscle. We used patients with chronic high muscle glycogen stores and deficient glycogenolysis (McArdle's disease) as a model to address this issue. Six McArdle patients were compared with control subjects during exercise. Muscle alpha2AMPK activity increased in McArdle patients (from 1.3 +/- 0.2 to 1.9 +/- 0.2 pmol min(-1) mg(-1), P = 0.05) but not in control subjects (from 1.0 +/- 0.1 to 1.3 +/- 0.3 pmol min(-1) mg(-1)). Exercise-induced phosphorylation of the in vivo AMPK substrate acetyl CoA carboxylase (ACCbeta; Ser(221)) was higher (P < 0.01) in McArdle patients than in control subjects (18 +/- 3 vs. 10 +/- 1 arbitrary units). Exercise-induced whole-body glucose utilization was also higher in McArdle patients than in control subjects (P < 0.05). No correlation between individual AMPK or ACCbeta values and glucose utilization was observed. Glycogen synthase (GS) activity was decreased in McArdle patients from 11 +/- 1.3 to 5 +/- 1.2 % (P < 0.05) and increased in control subjects from 19 +/- 1.6 to 23 +/- 2.3 % (P < 0.05) in response to exercise. This was not associated with activity changes of GS kinase 3 or protein phosphatase 1, but the changes in GS activity could be due to changes in activity of AMPK or protein kinase A (PKA) as a negative correlation between either ACCbeta phosphorylation (Ser(221)) or plasma adrenaline and GS activity was observed. These findings suggest that GS activity is increased by glycogen breakdown and decreased by AMPK and possibly PKA activation and that the resultant GS activity depends on the relative strengths of the various stimuli. Furthermore, AMPK may be involved in the regulation of glucose utilization during exercise in humans, although the lack of correlation between individual AMPK activity or ACCbeta phosphorylation (Ser(221)) values and individual glucose utilization during exercise implies that AMPK may not be an essential regulator.

Figure 1. Effect of exercise on the concentration of glycogen in skeletal muscle (A) and blood lactate (B) in McArdle patients (▪, •) and control subjects (□, ○)

Please refer to text for detailed statistical analysis. Data are presented as mean ± s.e.m. of n = 6 subjects in each group. *P < 0.01versus corresponding control value. †P < 0.01versus rest value (0 min).

Figure 2. Whole-body rate of glucose disappearance (glucose utilization) in McArdle patients (•) and control subjects (○) before and during 20 min of exercise

*P < 0.05versus controls (main effect). †P < 0.001versus rest (-5 min) (main effect). Data are presented as means ± s.e.m. of n = 6 subjects in each group.

Figure 3. Effect of exercise on glycogen synthase activity expressed as fractional velocity in skeletal muscle from McArdle patients (▪) and healthy control subjects (□)

*P < 0.05 and **P < 0.001versus corresponding control value. †P < 0.05 and ††P < 0.01versus corresponding rest value. Data are presented as means ± s.e.m. of n = 6 subjects in each group.

Figure 4. Activity of α1 (A) and α2 (B) isoforms of the 5′AMP-activated protein kinase (AMPK) in skeletal muscle of McArdle patients (▪) and control subjects (□)

†P = 0.05versus corresponding rest value. Data are presented as mean ± s.e.m. of n = 6 subjects in each group.

Figure 5. Phosphorylation of ACCβ (Ser²²¹) (A) and total ACCβ protein (B) in skeletal muscle of McArdle patients (▪) and healthy control subjects (□)

Representative Western blots are shown. *P < 0.01versus corresponding control value. †P < 0.01 and ††P < 0.001versus corresponding rest value. §P < 0.05versus control subjects (main effect). Data are presented as mean ± s.e.m. of n = 6 subjects in each group.