Intense Resistance Exercise Induces Early and Transient Increases in Ryanodine Receptor 1 Phosphorylation in Human Skeletal Muscle

Background: While ryanodine receptor 1 (RyR1) critically contributes to skeletal muscle contraction abilities by mediating Ca(2+)ion oscillation between sarcoplasmatic and myofibrillar compartments, AMP-activated protein kinase (AMPK) senses contraction-induced energetic stress by phosphorylation at Thr(172). Phosphorylation of RyR1 at serine(2843) (pRyR1Ser(2843)) results in leaky RyR1 channels and impaired Ca2+ homeostasis. Because acute resistance exercise exerts decreased contraction performance in skeletal muscle, preceded by high rates of Ca2+-oscillation and energetic stress, intense myofiber contractions may induce increased RyR1 and AMPK phosphorylation. However, no data are available regarding the time-course and magnitude of early RyR1 and AMPK phosphorylation in human myofibers in response to acute resistance exercise. Purpose: Determine the effects and early time-course of resistance exercise on pRyR1Ser(2843) and pAMPKThr(172) in type I and II myofibers. Methods: 7 male subjects (age 23 +/- 2 years, height: 185 +/- 7 cm, weight: 82 +/- 5 kg) performed 3 sets of 8 repetitions of maximum eccentric knee extensions. Muscle biopsies were taken at rest, 15, 30 and 60 min post exercise. pRyR1Ser(2843) and pAMPKThr(172) levels were determined by western blot and semi-quantitative immunohistochemistry techniques. Results: While total RyR1 and total AMPK levels remained unchanged, RyR1 was significantly more abundant in type II than type I myofibers. pRyR1Ser(2843) increased 15 min and peaked 30 min (p<0.01) post exercise in both myofiber types. Type I fibers showed relatively higher increases in pRyR1Ser(2843) levels than type II myofibers and remained elevated up to 60 min post resistance exercise (p<0.05). pAMPKThr(172) also increased 15 to 30 min post exercise (p<0.01) in type I and II myofibers and in whole skeletal muscle. Conclusion: Resistance exercise induces acutely increased pRyR1Ser(2843) and concomitantly pAMPKThr(172) levels for up to 30 min in resistance exercised myofibers. This provides a time-course by which pRyR1Ser(2843) can mechanistically impact Ca2+ handling properties and consequently induce reduced myofiber contractility beyond immediate fatiguing mechanisms.