AMP-activated protein kinase kinase activity and phosphorylation of AMP-activated protein kinase in contracting muscle of sedentary and endurance-trained rats

Abstract

This study was designed to examine activity of AMP-activated protein kinase kinase (AMPKK) in muscles from nontrained and endurance-trained rats. Rats were trained 5 days/wk, 2 h/day for 8 wk at a final intensity of 32 m/min up a 15% grade with 30-s sprints at 53 m/min every 10 min. Gastrocnemius muscles were stimulated in situ in trained and nontrained rats for 5 min at frequencies of 0.4/s and 1/s. Gastrocnemius LKB1 protein, a putative component of the AMPKK complex (LKB1, STRAD, and MO25), increased approximately twofold in response to training. Phosphorylation of AMP-activated protein kinase (AMPK) determined by Western blot and AMPK activity of immunoprecipitates (both isoforms) was increased at both stimulation rates in both trained and nontrained muscles. AMPKK activity was 73% lower in resuspended polyethylene glycol precipitates of muscle extracts from the trained compared with nontrained rats. AMPKK activity did not increase in either trained or nontrained muscle in response to electrical stimulation, even though phospho-AMPK did increase. These results suggest that AMPKK is activated during electrical stimulation of both trained and nontrained muscle by mechanisms other than covalent modification.

The Molecular Bases of Training Adaptation

“…Activation of AMPK in skeletal muscle has been shown in a variety of systems including in vitro / in vivo contraction, pharmacological activators and transgenic animals (Nielsen et al 2003b;Sakamoto et al 2004b;Atherton et al 2005;Jorgensen et al 2005;Toyoda et al 2006). Current evidence suggests that isoform specific AMPK activation occurs in a time, intensity, fibre-type and mode-specific manner and exercise-induced contraction of skeletal muscle in vivo has been clearly shown to up-regulate AMPK activity (Durante et al 2002;Nielsen et al 2003b;Yu et al 2003;Frøsig et al 2004;Atherton et al 2005;Hurst et al 2005;McConell et al 2005;Sriwijitkamol et al 2005;Taylor et al 2005;Wojtaszewski et al 2005;Toyoda et al 2006).…”

“…Indeed, endurance exercise-induced activation of AMPK is predictable given its ability to increase ATP regeneration via fat oxidation to meet the demands of prolonged lowintensity contraction. Interestingly, this exercise response seems to be dependent on the adaptive state of the muscle as evidence exists to demonstrate that short-term and chronic aerobic training reduces the acute AMPK response to prolonged and intermittent sub-maximal exercise (Durante et al 2002;Yu et al 2003;Hurst et al 2005;McConell et al 2005). …”

LKB1 and AMPK and the regulation of skeletal muscle metabolism

“…Two studies [63,64] have shown that exercise training increases LKB1 protein content, concomitant with elevated expression of the transcriptional coactivator PGC1 in rat skeletal muscle [64]. This occurs despite reportedly decreased AMPK kinase activity [63,65]. In studies of muscle-specific LKB1 knockout mice [37,66 •• ], it was shown that LKB1 is a critical regulator of exercise capacity and basal mitochondrial function.…”

Characterization of the liver kinase B1-mouse protein-25 -Ste-20-related adaptor protein complex in adult mouse skeletal muscle

“…The work identifying this AMPKK was done on liver and cells in culture, and not skeletal muscle. LKB1, MO25 protein, and STRAD mRNA were subsequently shown to be expressed in skeletal muscle (13,24). Three groups reported marked reductions in resting and contracting muscle AMPK activity and phosphorylated AMPK in muscle-specific LKB1 knockout (MLKB1KO) mice, implying that LKB1 is a component of the AMPKK of skeletal muscle (9,21,34).…”