In pursuit of the glycogen–[Ca2+] connection

Glycogen is the principal storage form of glucose (energy) in animal cells and is present as insoluble granules of different sizes in various locations within the muscle cells. While it has been known for over 40 years that depletion of intracellular glycogen is associated with reduced muscle performance (Bergstrom et al. 1967), there is an increasing body of evidence suggesting that the glycogen-dependent impairment of muscle function is not simply related to the role of glycogen as the main source of energy storage in muscle. It is difficult to extract unambiguous evidence for a more specific role of glycogen on muscle function from observations on intact muscle preparations, because changes in total muscle glycogen content entail a suite of metabolic changes, which in turn, are known to affect the cascade of processes involved in excitation–contraction coupling, and therefore muscle performance (Stephenson et al. 1999). A study on muscle samples from elite cross-country skiers, published in this issue of The Journal of Physiology by Ortenblad and colleagues (2011), provides compelling evidence of a causal relationship between the level of muscle glycogen and the relative rate of Ca2+ release from the sarcoplasmic reticulum (SR) induced in vitro, under controlled conditions. In this study the authors used homogenates of arm and leg muscle samples obtained by needle biopsy from 10 elite cross-country skiers, in whom the pools of muscle glycogen were manipulated by intense exercise followed by either carbohydrate, or water ingestion, and the SR Ca2+ release was induced by 4-chloro-m-cresol (CmC), a potent activator of the SR Ca2+ release channels. In arm muscle, the relative rate of SR Ca2+ release was significantly reduced as the glycogen content was decreased after exercise. Importantly, it did not recover after exercise as long as the glycogen content was maintained low in subjects ingesting only water, but fully recovered when the glycogen content returned to pre-exercise levels following ingestion of carbohydrates. In contrast to the results obtained on arm muscle samples, the glycogen content in the leg muscle samples decreased much less after exercise and the decrease in the relative rate of SR Ca2+ release was not statistically significant. The difference between the arm and leg muscle results appears to be due to differences in fibre type composition of the arm muscle samples (containing predominantly fast-twitch fibres) and the leg muscle samples (containing predominantly slow-twitch fibres), and to the different level of utilization of the arm and leg muscles in the cross-country skiing event. These results unequivocally show that in fast-twitch human muscle expressing predominantly myosin heavy chain IIa, there is a connection between the relative rates of SR Ca2+ release in vitro induced by CmC, and muscle glycogen content. Based on observations made in their previous work with regard to the role of glycogen in modulating muscle contraction, the authors have further investigated whether there were any correlations between the observed changes in the rate of SR Ca2+ release and the glycogen present in three intracellular pools: intramyofibrillar, intermyofibrillar and subsarcolemmal. A statistically significant correlation between SR Ca2+ release and glycogen was only found with respect to the intramyofibrillar pool of glycogen. What is the basis of the mechanism responsible for glycogen modulation of Ca2+ release from the SR? The authors’ preferred answer to this question is that the intramyofibrillar glycogen together with the glycogenolytic enzymes known to be associated with muscle glycogen provide glycolytic intermediates in a microenvironment that locally supplies metabolic energy for processes occurring at the level of the triad, where the transverse tubules are in close contact with the SR cisternae containing the releasable Ca2+ and the Ca2+ release channels. However, not all triads in muscle have intramyofibrillar glycogen in their proximity and in many cases glycogen from the intermyofibrillar pool is closer to the Ca2+ release channels in the SR. Therefore, in order to further test this hypothesis, the main criterion for defining intracellular glycogen pools should be the proximity of glycogen granules to the triads rather the position of glycogen granules with respect to the myofibrils. A tighter correlation between the SR Ca2+ release rate and the glycogen pool closest to triads than that reported in this paper between the SR Ca2+ releases rate and the intramyofibrillar glycogen pool (r2= 0.23) could be viewed as supporting the authors’ preferred hypothesis. However, even if this were the case, one cannot discard the possibility that the connection between glycogen and SR Ca2+ release reported by Ortenblad et al. (2011) is not related to glycogen as an energy source, but to the action of enzymes associated with the glycogen granules, which themselves can modulate the function of the SR Ca2+ release channels in their proximity. For example, the SR Ca2+ release channels can be phosphorylated/dephosphorylated by kinases/phosphatases involved in glycogen metabolism that are associated with glycogen and SR membranes. In this context it is worth noting that glycogen phosphorylase a, the more active form of glycogen phosphorylase, by itself has been found to be a negative regulator of SR Ca2+- release (Hirata et al. 2003). The reduced rate of CmC-induced Ca2+ release from the SR when the glycogen was depleted or stayed depleted, reported in the study by Ortenblad et al. (2011), parallels observations made on isolated intact muscle preparations subjected to glycogen manipulation with respect to the myoplasmic [Ca2+] reached during tetanic contraction (Chin & Allen, 1997). This strongly supports the view that the glycogen–[Ca2+] connection demonstrated by Ortenblad et al. (2011) in fast-twitch fibres would also operate in the intact muscle in vivo.