Skeletal muscle signaling associated with impaired glucose tolerance in spinal cord-injured men and the effects of contractile activity

Abstract

The mechanisms underlying poor glucose tolerance in persons with spinal cord injury (SCI), along with its improvement after several weeks of neuromuscular electrical stimulation-induced resistance exercise (NMES-RE) training, remain unclear, but presumably involve the affected skeletal musculature. We, therefore, investigated skeletal muscle signaling pathways associated with glucose transporter 4 (GLUT-4) translocation at rest and shortly after a single bout of NMES-RE in SCI (n = 12) vs. able-bodied (AB, n = 12) men. Subjects completed an oral glucose tolerance test during visit 1 and ≈90 NMES-RE isometric contractions of the quadriceps during visit 2. Muscle biopsies were collected before, and 10 and 60 min after, NMES-RE. We assessed transcript levels of GLUT-4 by quantitative PCR and protein levels of GLUT-4 and phosphorylated- and total AMP-activated protein kinase (AMPK)-α, CaMKII, Akt, and AS160 by immunoblotting. Impaired glucose tolerance in SCI was confirmed by higher (P < 0.05) plasma glucose concentrations than AB at all time points after glucose ingestion, despite equivalent insulin responses to the glucose load. GLUT-4 protein content was lower (P < 0.05) in SCI vs. AB at baseline. Main group effects revealed higher phosphorylation in SCI of AMPK-α, CaMKII, and Akt (P < 0.05), and Akt phosphorylation increased robustly (P < 0.05) following NMES-RE in SCI only. In SCI, low skeletal muscle GLUT-4 protein concentration may, in part, explain poor glucose tolerance, whereas heightened phosphorylation of relevant signaling proteins (AMPK-α, CaMKII) suggests a compensatory effort. Finally, it is encouraging to find (based on Akt) that SCI muscle remains both sensitive and responsive to mechanical loading (NMES-RE) even ≈22 yr after injury.

Keywords: glucose uptake signaling; neuromuscular electrical stimulation; resistance exercise; skeletal muscle; spinal cord injury.

Fig. 1.

Mean plasma glucose (A) and insulin (B) concentrations at fasting and 60, 90, and 120 min after 75-g oral glucose challenge. AB, able-bodied; SCI, spinal cord injury. Values are means ± SE. Time × group interaction, main effect of time and group for mean plasma glucose concentration (P < 0.05); main effect of time for mean plasma insulin concentration (P < 0.05). *P < 0.05, different from AB. #P < 0.05, different from baseline within group.

Fig. 2.

Whole body insulin sensitivity from the data obtained by oral glucose tolerance test (OGTT). 10,000/√(FPG FPI) × (G × I) (38), where FPG is fasting plasma glucose, FPI is fasting plasma insulin, G is mean plasma glucose concentration during OGTT, and I is mean plasma insulin concentration during OGTT.

Fig. 3.

Phosphorylated protein levels of AMP-activated protein kinase (AMPK)-α before and 10 and 60 min after neuromuscular electrical stimulation (NMES)-induced resistance exercise in individuals with and without SCI. AU, arbitrary units. Values are means ± SE. Main effect of group (P < 0.05). *P < 0.05, different from AB.

Fig. 4.

Phosphorylated protein levels of Ca⁺²/calmodulin-dependent protein kinase (CaMK) II before and 10 and 60 min after NMES-induced resistance exercise in individuals with SCI and AB individuals. Values are means ± SE. Main effect of group (P < 0.05). *P < 0.05, different from AB.

Fig. 5.

Phosphorylated protein levels of Akt before and 10 and 60 min after NMES-induced resistance exercise in individuals with SCI and AB individuals. Values are means ± SE. Time × group interaction, main effect of time and group (P < 0.05); *P < 0.05, different from AB. #P < 0.05, different from baseline within group.

Fig. 6.

Protein levels of glucose transporter (GLUT)-4 before and 10 and 60 min after NMES-induced resistance exercise in individuals with SCI and AB individuals. Values are means ± SE. Main effect of group (P < 0.05). *P < 0.05, different from AB.

Fig. 7.

Muscle fiber distribution in individuals with SCI and AB individuals. Values are means ± SE. *P < 0.05, different from AB.

Fig. 8.

Phosphorylated protein levels of p44-42 extracellular signal-regulated kinase (ERK)-1/2 before and 10 and 60 min after NMES-induced resistance exercise in individuals with SCI and AB individuals. Values are means ± SE. Time × group interaction, main effect of time and group (P < 0.05). *P < 0.05, different from AB. #P < 0.05, different from baseline within group.