Abstract AKT signaling plays a crucial role in muscle physiology, and is activated by stimuli, including insulin, growth factors, and exercise. Three AKT isoforms have been identified in mammals, and they possess both distinct and redundant functions. However, it is currently unknown what the predominant AKT isoform is in primary human skeletal myotubes, and very little is known regarding the effects of insulin and insulin‐like growth factor‐I (IGF‐I) on AKT isoforms activation in human myotubes. Thus, we sought to determine the abundances of each AKT isoform in primary human skeletal myotubes and their responses to insulin or IGF‐I. Analysis of protein lysates by liquid chromatography‐parallel reaction monitoring/mass spectrometry revealed that AKT1 was the most abundant AKT isoform and AKT3 was the least‐abundant isoform. Next, myotubes were treated with either 100 nM insulin or 10 nM IGF‐I for 5, 20, 45, or 60 min. In response to insulin, there was a significant treatment effect on phosphorylation of AKT1 and AKT2, but not AKT3 (p
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