AMP kinase activation with AICAR further increases fatty acid oxidation and blunts triacylglycerol hydrolysis in contracting rat soleus muscle

Abstract

Muscle contraction increases glucose uptake and fatty acid (FA) metabolism in isolated rat skeletal muscle, due at least in part to an increase in AMP-activated kinase activity (AMPK). However, the extent to which AMPK plays a role in the regulation of substrate utilization during contraction is not fully understood. We examined the acute effects of 5-aminoimidazole-4-carboxamide riboside (AICAR; 2 mm), a pharmacological activator of AMPK, on FA metabolism and glucose oxidation during high intensity tetanic contraction in isolated rat soleus muscle strips. Muscle strips were exposed to two different FA concentrations (low fatty acid, LFA, 0.2 mm; high fatty acid, HFA, 1 mm) to examine the role that FA availability may play in both exogenous and endogenous FA metabolism with contraction and AICAR. Synergistic increases in AMPK alpha2 activity (+45%; P<0.05) were observed after 30 min of contraction with AICAR, which further increased exogenous FA oxidation (LFA: +71%, P<0.05; HFA: +46%, P<0.05) regardless of FA availability. While there were no changes in triacylglycerol (TAG) esterification, AICAR did increase the ratio of FA partitioned to oxidation relative to TAG esterification (LFA: +65%, P<0.05). AICAR significantly blunted endogenous TAG hydrolysis (LFA: -294%, P<0.001; HFA: -117%, P<0.05), but had no effect on endogenous oxidation rates, suggesting a better matching between TAG hydrolysis and subsequent oxidative needs of the muscle. There was no effect of AICAR on the already elevated rates of glucose oxidation during contraction. These results suggest that FA metabolism is very sensitive to AMPK alpha2 stimulation during contraction.

Figure 1. Time course for AMPK α1 (A) and α2 (B) activity in the presence or absence of AICAR in isolated contracting rat soleus muscle

Values are means ± s.e.m., pmol min⁻¹ (mg protein)⁻¹, n = 5–8 per group. *Significantly different from 30 min contraction alone (P < 0.05). **Significantly different from 0 and 10 min of same condition. †Treatment effect of AICAR significantly different from No AICAR (P < 0.05).

Figure 2. Effect of AICAR on fatty acid oxidation and triacylglycerol (TAG) esterification in isolated contracting rat soleus muscle

Effect of AICAR on fatty acid oxidation in low fatty acid (0.2 mm, LFA: A) or high fatty acid (1 mm, HFA: B) modified KHB, on TAG esterification (LFA: C; HFA: D) and on oxidation: TAG esterification ratio (LFA: E; HFA: F). Values are means ± s.e.m., nmol (g wet wt)⁻¹, n = 7 per group. *Significantly different from No AICAR (P < 0.05).

Figure 3. Effect of AICAR on TAG hydrolysis and endogenous fatty acid oxidation in isolated contracting rat soleus muscle

Effect of AICAR on TAG hydrolysis and endogenous fatty acid oxidation in low fatty acid (0.2 mm, LFA: A) or high fatty acid (1 mm, HFA: B) modified KHB. Values are means ± s.e.m., nmol (g wet wt)⁻¹, n = 7 per group. *Significantly different from No AICAR (P < 0.05).

Figure 4. Effect of AICAR on glucose oxidation in isolated contracting (last 30 min) rat soleus muscle

There was no significant effect of AICAR on glucose oxidation in low fatty acid (0.2 mm, LFA) or high fatty acid (1 mm, HFA) modified KHB. Values are means ± s.e.m., n = 6–8 per group.