The role of sarcoplasmic calcium in skeletal muscle training adaptation

Current research shows a clear correlation between strong mitochondrial capacity, healthy muscle and general public health. A sedentary lifestyle increases the risk of a whole host of so called ‘western diseases’, while an active lifestyle reduce the risk of said diseases. Thus, well-functioning muscles are a necessity for general health. So far endurance exercise is the most effective method to improve muscle function. This thesis will focus on the cellular mechanisms that regulate muscle performance and how these can be improved.

In the first study, we show that supplemented dietary nitrate enhances Ca2+ handling and submaximal force in mouse fast twitch muscle. Continuing this, in study two, we show that the increased submaximal force enhances voluntary activity in mice, presumably due to a shifted perceived effort of running. In study three we show that mild stress from cold exposure can enhance mitochondrial biogenesis resulting in improved fatigue resistance without exercise. The cold environment seems to induce a sarcoplasmic reticulum (SR) Ca2+ leak in the skeletal muscle. In study four we investigated why short (180s) high intensity interval training works better for enhancing endurance than regular low-intensity exercise. We show that oxidants formed during exercise causes ryanodine receptor modifications, which result in a SR Ca2+ leak and this in turn likely triggers transcription to improved mitochondrial capacity. In study five we show that inducing a mild SR Ca2+ leak, either with exercise or pharmacological tools, drive mitochondrial biogenesis. In study six we show that a in a model of ageing, a degenerative mitochondrial problem causes myopathy via reduced SR Ca2+ release.

Ca2+ is a central player in muscle function. This thesis shows that diet, exercise and age have the ability to affect skeletal muscle Ca2+ handling. Most importantly, Ca2+ signals can improve mitochondrial function, resulting in improved muscle function. However degenerative mitochondria causes reduced Ca2+handling that leads to muscle weakness. This is one of the reasons an active lifestyle is so important for the elderly, because it improves the mitochondrial function rather than being degraded. Perhaps in the future, inducing a small SR Ca2+ leak could minimize some of the risks associated with sedentary lifestyle.

List of scientific papers

I. Hernández A, Schiffer TA, Ivarsson N, Cheng AJ, Bruton JD, Lundberg JO, Weitzberg E and Westerblad H (2012). Dietary nitrate increases tetanic [Ca2+]i in mouse fast-twitch muscle. J Physiol 590, 3575-3583.
https://doi.org/10.1113/jphysiol.2012.232777

II. Ivarsson N, Schiffer TA, Hernández A, Lanner JT, Lundberg JO, Weitzberg E and Westerblad H. Dietary nitrate markedly improves voluntary running in mice. [Manuscript]

III. Bruton JD, Aydin J, Yamada T, Shabalina IG, Ivarsson N, Zhang SJ, Wada M, Tavi P, Nedergaard J, Katz A and Westerblad H (2010). Increased fatigue resistance linked to Ca2+-stimulated mitochondrial biogenesis in muscle fibers of cold-acclimated mice. J Physiol 588, 4275-4288.
https://doi.org/10.1113/jphysiol.2010.198598

IV. Place N, Ivarsson N, Venckunas T, Neyroud D, Brazaitis M, Cheng AJ, Ochala J, Kamandulis S, Girard S, Volungevičius G, Paužas H, Mekideche A, Kayser B, Martinez-Redondo V, Ruas JL, Bruton JD, Truffert A, Lanner JT, Skurvydas A and Westerblad H. (2015). Ryanodine receptor fragmentation and sarcoplasmic reticulum Ca2+ leak after one session of high-intensity interval exercise. [Accepted]
https://doi.org/10.1073/pnas.1507176112

V. Ivarsson N, Cheng AJ, Bruton JD, Westerblad H and Lanner JT. (2015). Sarcoplasmic reticulum Ca2+ leak and increased endurance in mouse muscle. [Manuscript]

VI. Ivarsson N, Yamada T, Hernández A, Fahlström, A, Cheng, AJ, Zhang SJ, Bruton JD, Ulfhake B, Westerblad H (2012). Impaired mitochondrial respiration and decreased fatigue resistance followed by severe muscle weakness in skeletal muscle of mitochondrial DNA mutator mice. J Physiol, 590, 6187-6197.
https://doi.org/10.1113/jphysiol.2012.240077