Skeletal Muscle AMP-activated Protein Kinase Is Essential for the Metabolic Response to Exercise in Vivo

Abstract

AMP-activated protein kinase (AMPK) has been postulated as a super-metabolic regulator, thought to exert numerous effects on skeletal muscle function, metabolism, and enzymatic signaling. Despite these assertions, little is known regarding the direct role(s) of AMPK in vivo, and results obtained in vitro or in situ are conflicting. Using a chronically catheterized mouse model (carotid artery and jugular vein), we show that AMPK regulates skeletal muscle metabolism in vivo at several levels, with the result that a deficit in AMPK activity markedly impairs exercise tolerance. Compared with wild-type littermates at the same relative exercise capacity, vascular glucose delivery and skeletal muscle glucose uptake were impaired; skeletal muscle ATP degradation was accelerated, and arterial lactate concentrations were increased in mice expressing a kinase-dead AMPK␣2 subunit (␣2-KD) in skeletal muscle. Nitric-oxide synthase (NOS) activity was significantly impaired at rest and in response to exercise in ␣2-KD mice; expression of neuronal NOS (NOS) was also reduced. Moreover, complex I and IV activities of the electron transport chain were impaired 32 ؎ 8 and 50 ؎ 7%, respectively, in skeletal muscle of ␣2-KD mice (p < 0.05 versus wild type), indicative of impaired mitochondrial function. Thus, AMPK regulates neuronal NOS expression, NOS activity, and mitochondrial function in skeletal muscle. In addition, these results clarify the role of AMPK in the control of muscle glucose uptake during exercise. Collectively, these findings demonstrate that AMPK is central to substrate metabolism in vivo, which has important implications for exercise tolerance in health and certain disease states characterized by impaired AMPK activation in skeletal muscle.The ubiquitously expressed serine/threonine AMP-activated protein kinase (AMPK) 2 is an ␣␤␥ heterotrimer postulated to play a key role in the response to energetic stress (1, 2), because of its sensitivity to increased cellular AMP levels (3). Pharmacological activation of AMPK (primarily via the AMP analogue ZMP) increases catabolic processes such as GLUT4 translocation (4, 5), glucose uptake (6, 7), long chain fatty acid (LCFA) uptake (8), and substrate oxidation (6). Concomitantly, pharmacological activation of AMPK inhibits anabolic processes, and in skeletal muscle genetic reduction of the catalytic AMPK␣2 subunit eliminates these pharmacological effects (9 -12). Thus, AMPK has been proposed to act as a metabolic master switch (2, 13, 14). Physiologically, exercise at intensities sufficient to increase free cytosolic AMP (AMP free ) levels is a potent stimulus of AMPK, preferentially activating AMPK␣2 in skeletal muscle (15)(16)(17). The metabolic profile of skeletal muscle during moderate to high intensity exercise is remarkably similar to skeletal muscle in which AMPK has been pharmacologically activated (i.e. increases in catabolic processes). This is consistent with the hypothesis that AMPK activation is required for the metabolic response to increased cellular stress....

AMPKα2deficiency uncovers time dependency in the regulation of contraction-induced palmitate and glucose uptake in mouse muscle

“…These conclusions are consistent with previous studies in which AMPK␣ 2 was shown to be a necessary signal in the regulation of substrate metabolism during muscle contraction or treadmill exercise (24,25). However, our data are not consistent with some additional reports in which low AMPK␣ 2 activity did not affect muscle glucose clearance during treadmill exercise (26), glucose uptake in contracting extensor digitorum longus muscle, which is a mixed-fiber type muscle (10), or FA oxidation in isolated muscle during muscle contraction or following treadmill exercise (4,10,25,29,30).…”

“…Consistent with previous data in the same mouse model (31,32), DN mice had lower resting muscle glycogen content and did not utilize glycogen to the same extent as the WT mice during muscle contraction. However, this is not consistent with recent reports suggesting similar or higher rates of glycogen utilization during treadmill exercise in muscle of mice with low AMPK␣ 2 activity (24,26). Given that glycogen use and mean glucose uptake and FA oxidation were lower in the DN mice during muscle contraction, alternate fuel sources must have been used to maintain force production.…”

“…The perfusate consisted of Krebs-Henseleit solution, 5% BSA (Millipore, Billerica, MA), 550 M albumin-bound palmitate (4:1), 24 Ci of albumin-bound [1-14 C]palmitate, 6 mM glucose, and 3 mM caffeine (Sigma, St. Louis, MO) dissolved in Krebs-Henseleit buffer in the caffeine-treated groups. The perfusate was kept at 37°C and was continuously gassed with 95% O 2-5% CO2.…”

“…However, studies utilizing transgenic mouse models where AMPK is either inactive or knocked out reveal conflicting results (24 -26, 30). Some studies suggest that AMPK is necessary in the regulation of glucose and FA metabolism (24,25), whereas other studies show that AMPK is not necessary to measure increases in substrate metabolism during exercise or other metabolic challenges (26,30).…”

“…However, studies utilizing transgenic mouse models where AMPK is either inactive or knocked out reveal conflicting results (24 -26, 30). Some studies suggest that AMPK is necessary in the regulation of glucose and FA metabolism (24,25), whereas other studies show that AMPK is not necessary to measure increases in substrate metabolism during exercise or other metabolic challenges (26,30).Therefore, the purpose of this study was to determine 1) whether a caffeine-induced increase in intracellular [Ca 2ϩ ] regulates muscle glucose uptake, FA uptake, and FA oxidation solely via AMPK␣ 2 activation and 2) whether the ␣ 2 -subunit of AMPK is essential to increase glucose uptake, FA uptake, and FA oxidation in contracting skeletal muscle in a hindlimb perfusion system. Although the perfusion system is not as physiologically relevant as tracer studies in live animals, it allowed us to measure glucose uptake, FA uptake, and FA oxidation at various time points and provided an in situ model.…”