Muscle glycogenolysis is not activated by changes in cytosolic P‐metabolites: A ³¹P and ¹H MRS demonstration

Abstract

Skeletal muscle contraction and glycogenolysis are closely coupled. The standard explanation for this coupling, as taught in modern biochemistry textbooks, is that the metabolic products of contraction (ADP, AMP, P i ) feed back to activate glycogenolytic enzymes, thus providing for resynthesis of ATP. However, both in vivo 31 P MRS analyses and chemical analyses of muscle extracts have provided results that are contrary to this theory, at least in its simplest form. The MRS studies suffer from ambiguous assumptions. More importantly, in 31 P MRS studies the dependent and independent variables are often confounded because the glycogenolytic rate is calculated from the same data which are used to calculate the other metabolic variables. The analysis of biopsies has been necessarily quite limited, and suffers from a different set of experimental artifacts. Thus, the problem of contraction-glycogenolysis-coupling was reassessed using a quantitatively accurate 1 H MRS method. It is confirmed that glycogenolysis and contractions are closely coupled during repetitive exercise, while glycogenolysis and P-metabolite concentrations are not. A simple metabolic feedback system cannot explain contraction-glycogenolysis-coupling.Magn Despite its fundamental importance in all prokaryotic and eukaryotic life, the in vivo regulation of glycolysis is still not fully understood. In standard textbook descriptions, glycolysis is activated by a feedback mechanism at phosphofructokinase (PFK) by metabolites such as ADP and P i , direct products of the actomyosin-ATPase, and AMP, which is in equilibrium with ADP through adenylate kinase (1). By this simple feedback mechanism, glycolysis is activated whenever the direct and indirect products of ATP hydrolysis are increased. However, during the past 20 years it has been shown that, as calculated from changes in the 31 P MRS spectrum of intact ex vivo and in vivo animal and human muscle, glycolysis is closely coupled to contractions and not to metabolite levels (2-6).Similar results have been obtained using muscle biopsies and extracts (7,8).Close coupling of glycolysis or glycogenolysis to contraction is referred to as contraction-glycolysis-coupling or contraction-glycogenolysis-coupling (CGC). In this article, glycogenolysis refers to the degradation of glycogen to lactate. In the absence of oxygen, CGC is characterized by a partial metabolic recovery from contraction, followed by metabolic stasis during anaerobic rest, i.e., very low glycogenolytic rate with little to no net change in metabolite concentrations. During and closely following contraction the glycogenolytic rate is increased by orders of magnitude over the resting rate and then it falls to near zero without restoring P-metabolites to their precontractile values. In other words, glycogenolysis and contractions are coupled temporally. Glycogenolysis and contractions are also coupled quantitatively, since the amount of lactate (Lac -) produced is proportional to the magnitude of mechanical activity.The methods previously us...

Phosphocreatine recovery kinetics following low- and high-intensity exercise in human triceps surae and rat posterior hindlimb muscles

“…The flux through these reactions is controlled, in part, by substrate availability of P i and allosteric regulation by free cytosolic AMP and ADP, which would both be expected to be elevated following high-intensity exercise. However, some studies (10,14,28,36) have observed no evidence of PCr recovery following exercise in ischemic conditions, suggesting that glycolytic flux terminates abruptly at the end of exercise and that factors more temporally coupled to muscle contraction, such as free cytosolic Ca 2ϩ , are required for activation of glycogenolysis/glycolysis. On the other hand, Crowther et al (4) observed that glycolytic flux remained high at end exercise for 3 s following ischemic exercise and de- Data are means Ϯ SE; n ϭ 10 humans; n ϭ 15 rats (n ϭ 7 low, 8 high).…”

Tribute to R. G. Boutilier: The role for skeletal muscle in the hypoxia-induced hypometabolic responses of submerged frogs

“…This may partly reflect the fact that lactate efflux is favoured in isolated muscle preparations that are perifused with lactate-free solution (Boutilier et al, 1986). However, the chief reason seems to be that glycolytic activation is minimal in non-contracting anoxic muscle (Hsu and Dawson, 2003). Interestingly, Vezzoli et al (Vezzoli et al, 2004) observed that the time course of suppression of ATP synthesis rate was essentially paralleled by changes in the phosphocreatine (PCr) breakdown rate, perhaps indicating that anaerobic energy turnover in resting and/or hypometabolic muscle is controlled to a large extent by ATP supply from PCr hydrolysis.…”

“…The time courses of liver glycogen depletion and the onset of muscle glycogenolysis are essentially mirrored by the rise in plasma lactate levels in the first few weeks of hypoxic submergence (Fig.·4). In isolated frog muscle, energy turnover during acute anoxia at 4°C is apparently supported entirely by PCr hydrolysis; glycolytic activation seems tightly coupled to the onset of increased contractions (Hsu and Dawson, 2003). The clear activation of muscle glycogenolysis in vivo may mean that some low level of muscular activity occurs continuously during the initial stages of hypoxic coldsubmergence and/or that circulatory factors are important for mobilising muscle glycogen.…”

“…As discussed above, energy provision during acute anoxia/ischaemia in quiescent isolated skeletal muscle (over the temperature range 4-25°C) is highly dependent on PCr hydrolysis (Hsu and Dawson, 2003;Vezzoli et al, 2004). Complete and sustained muscle anoxia or ischemia is likely to be a rare occurrence in vivo because of the frogs' need to avoid severe environmental hypoxia.…”

Phosphate uptake in rat skeletal muscle is reduced during isometric contractions

“…In the absence of a steady state, at work rates above oxidative capacity, PCr may become nearly depleted (5), whereas P i concentration can continue to increase because of further ATP hydrolysis. Although the direct effect of elevated P i concentration on glycogenolysis and glycolysis can be debated (8,18,32,38), higher cellular P i may also foster a greater rate of P i efflux (35) and induce a net loss of phosphate from the cell (14).…”