Effect of insulin on human skeletal muscle mitochondrial ATP production, protein synthesis, and mRNA transcripts

Abstract

Mitochondria are the primary site of skeletal muscle fuel metabolism and ATP production. Although insulin is a major regulator of fuel metabolism, its effect on mitochondrial ATP production is not known. Here we report increases in vastus lateralis muscle mitochondrial ATP production capacity (32-42%) in healthy humans (P < 0.01) i.v. infused with insulin (1.5 milliunits/kg of fat-free mass per min) while clamping glucose, amino acids, glucagon, and growth hormone. Increased ATP production occurred in association with increased mRNA levels from both mitochondrial (NADH dehydrogenase subunit IV) and nuclear [cytochrome c oxidase (COX) subunit IV] genes (164-180%) encoding mitochondrial proteins (P < 0.05). In addition, muscle mitochondrial protein synthesis, and COX and citrate synthase enzyme activities were increased by insulin (P < 0.05). Further studies demonstrated no effect of low to high insulin levels on muscle mitochondrial ATP production for people with type 2 diabetes mellitus, whereas matched nondiabetic controls increased 16-26% (P < 0.02) when four different substrate combinations were used. In conclusion, insulin stimulates mitochondrial oxidative phosphorylation in skeletal muscle along with synthesis of gene transcripts and mitochondrial protein in human subjects. Skeletal muscle of type 2 diabetic patients has a reduced capacity to increase ATP production with high insulin levels.

Fig. 1.

Plasma insulin (A), glucose (B), and leucine (C) concentrations during 8 h of high-dose insulin, low-dose insulin, or saline infusion. Values are means ± SD. Significant differences by ANOVA are indicated in text.

Fig. 2.

Vastus lateralis muscle citrate synthase activity and COX activity after 8 h of saline, low-dose insulin, and high-dose insulin infusion. Values of these mitochondrial enzyme activities are expressed as a percentage of baseline (mean ± SEM). *, Statistically significant difference from the saline condition (P < 0.01); †, significantly different from the low insulin condition (P < 0.01).

Fig. 3.

Vastus lateralis muscle mitochondrial ATP production rates after 4 h (A) and 8 h (B) of saline, low-dose insulin, and high-dose insulin infusion. Values are expressed as a percentage of preinfusion baseline (mean ± SEM). Measurements were made in the presence of five different substrate combinations, TA, GM, PM, PCM, or SR. *, Significantly different (P < 0.05) from the saline values; †, significantly different from the low insulin values at 4 h; ‡, significant difference from the saline values and low insulin values at 8 h (P < 0.01).

Fig. 4.

Mitochondrial protein FSR for high-dose insulin and saline conditions in the same subjects, and low-dose insulin infusion in a separate group of subjects. Values are mean ± SEM. *, Significantly different from the saline value (P < 0.05); †, significantly different from the low insulin value (P < 0.05).

Fig. 5.

ND IV (A), COX III (B), and COX IV (C) mRNA transcript levels normalized to 28S rRNA at baseline, 4 and 8 h of high-dose insulin, low-dose insulin, or saline. Values are mean ± SEM. *, Significant increase above baseline for the insulin condition compared with the saline condition (P < 0.05).

Fig. 6.

(A) Vastus lateralis muscle mitochondrial ATP production rates after 7 h of high-dose insulin infusion. Values (mean ± SEM) are expressed as a percentage change from the low-dose insulin condition. Measurements were made in the presence of six different substrate combinations (KG, 10 mMα-ketoglutarate). (B) Insulin concentrations sustained during 7 h of insulin infusion (mean ± SD). *, Significant difference between the low-dose and high-dose insulin conditions (P < 0.02).