No abstract

The protein serine/threonine kinase Akt/protein kinase B has been recognized as a critical signaling mediator for multiple cell systems. The function of Akt in skeletal muscle is not well understood, and whether contractile activity stimulates Akt activity has been controversial. In the current study, contraction in situ, induced via sciatic nerve stimulation, significantly increased Akt Ser 473 phosphorylation in multiple muscle types including the extensor digitorum longus (13-fold over basal), plantaris (5.8-fold), red gastrocnemius (4.7-fold), white gastrocnemius (3.3-fold), and soleus (1.6-fold). In addition to increasing phosphorylation, contraction in situ significantly increased the activity of all three Akt isoforms (Akt1 > Akt2 > Akt3) with maximal activation occurring at 2.5 min and returning to base line with 15 min of contraction. Akt phosphorylation and activity were also increased when isolated muscles were contracted in vitro in the absence of systemic factors, although to a much lesser extent. The phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 fully inhibited contraction-stimulated Akt phosphorylation and activity but did not diminish contraction-stimulated glycogen synthase kinase-3 phosphorylation and glycogen synthase activity. These results demonstrate that contraction increases Akt phosphorylation and activity in skeletal muscle and that this stimulation is rapid, transient, muscle fiber typespecific, and wortmannin-and LY294002-inhibitable. Akt signaling is not necessary for the regulation of glycogen synthase activity in contracting skeletal muscle.Insulin and contractile activity are the major regulators of glucose uptake, glycogen synthesis, and protein synthesis in adult skeletal muscle. The signal transduction mechanism by which insulin induces these metabolic responses has been the focus of intense research, and it is now well established that the activation of phosphatidylinositol 3-kinase (PI3K) 1 is critical for insulin actions, as nearly all physiological responses of mammalian cells to insulin are prevented by pharmacologic inhibition or by overexpression of dominant negative mutants of PI3K (1). In contrast, the molecular signaling mechanisms by which contractile activity leads to changes in muscle metabolic processes remain largely undefined. What is known is that insulin and contraction utilize different signaling pathways leading to glucose uptake and glycogen synthesis in skeletal muscle because contraction-stimulated glucose uptake occurs through a PI3K-independent mechanism (2-4), and contraction-stimulated glycogen synthase activation can occur in the absence of PI3K activation (5). Downstream mediators beyond PI3K in insulin signaling have been characterized (6, 7); however, whether insulin and contraction signaling converge at a point that is distal to PI3K is unknown.Akt is a serine/threonine kinase whose catalytic domain is closely related to those of protein kinase A and protein kinase C (thus also termed protein kinase B). Three isoforms of Akt (A...

Akt/protein kinase B is a serine/threonine kinase that has emerged as a critical signaling component for mediating numerous cellular responses. Contractile activity has recently been demonstrated to stimulate Akt signaling in skeletal muscle. Whether physiological exercise in vivo activates Akt is controversial, and the initiating factors that result in the stimulation of Akt during contractile activity are unknown. In the current study, we demonstrate that treadmill running exercise of rats using two different protocols (intermediate high or high-intensity exhaustive exercise) significantly increases Akt activity and phosphorylation in skeletal muscle composed of various fiber types. To determine if Akt activation during contractile activity is triggered by mechanical forces applied to the skeletal muscle, isolated skeletal muscles were incubated and passively stretched. Passive stretch for 10 min significantly increased Akt activity (2-fold) in the fast-twitch extensor digitorum longus (EDL) muscle. However, stretch had no effect on Akt in the slow-twitch soleus muscle, although there was a robust phosphorylation of the stress-activated protein kinase p38. Similar to contraction, stretch-induced Akt activation in the EDL was fully inhibited in the presence of the phosphatidylinositol 3-kinase inhibitor wortmannin, whereas glycogen synthase kinase-3 (GSK3) phosphorylation was only partially inhibited. Stretch did not cause dephosphorylation of glycogen synthase on GSK3-targeted sites in the absence or presence of wortmannin. We conclude that physiological exercise in vivo activates Akt in multiple skeletal muscle fiber types and that mechanical tension may be a part of the mechanism by which contraction activates Akt in fast-twitch muscles.