Intracellular and extracellular skeletal muscle triglyceride metabolism during alternating intensity exercise in humans
Abstract
The main purpose of this study was to evaluate non-invasively with magnetic resonance spectroscopy (H-MRS) changes in the concentrations of intracellular (IT) and extracellular (between muscle fibres) triglycerides (ET) in skeletal muscles of trained males (age range: 24–38 years) during two standard exercise protocols of alternating velocities.
Protocol 1 consisted of locomotion in a shuttle manner between two lines 30 m apart at four different velocities (1, 2, 3 and 4 m s) which were alternated every minute in a standard routine for 90 min, whereas Protocol 2 included locomotion between two lines 20 m apart at only three velocities (2, 2.7 and 4 m s) until volitional exhaustion. The heart rate during both protocols fluctuated between 140 and 200 beats min.
Using pre-exercise muscle water to quantify individual total creatine (TCr) that was utilized as an internal standard and assuming that TCr does not change during exercise, subjects’ mean IT and ET concentrations in soleus (Sol) muscle before Protocol 1 (n = 8) were 45.8 ± 4.8 mmol (kg dry weight) (mean ± s.e.m.) and 93.1 ± 14.1 mmol (kg dry weight), respectively. After the exercise, the concentrations of IT and ET were not significantly different from the values at rest. Before Protocol 2 (n = 4), IT concentrations in Sol, gastrocnemius (Gast) and tibialis (Tib) muscles were 46.4 ± 13.6, 35.0 ± 12.1 and 23.1 ± 4.8 mmol (kg dry weight), respectively, and were not affected by the exhaustive exercise. The ET concentrations in Sol, Gast and Tib were 136.4 ± 38.1, 175.3 ± 86.5 and 79.3 ± 20.0 mmol (kg dry weight), respectively, and they did not change significantly after exhaustion.
The study showed that levels of IT and ET were not affected by alternating intensity exercise to fatigue. This suggests that IT and ET in human Sol, Gast and Tib muscles do not contribute significantly to the energy turnover during this type of exercise. Energy for this type of muscle contraction may arise primarily from muscle phosphocreatine (PCr) and glycogen breakdown, circulating glucose and fatty acids from triglycerides other than those encountered within and between muscle cells.
Direct examination of the utilization of energy substrates in exercising human muscle has been possible primarily through the use of biochemical analyses of muscle biopsies (Hultman, 1967; Gollnick, Piehl & Saltin, 1974). More recently, P-magnetic resonance spectroscopy (MRS) and C-MRS have been used in in vivo measurements of human muscle energy metabolism during exercise (Ross, Radda, Gadian, Rocker, Esiri & Falconer-Smith, 1981; Price, Rothman, Avison, Buonamico & Shulman, 1991). From these and other studies it has been established that, during short duration intense exercise, muscle phosphocreatine (PCr) and glycogen breakdown enable a high rate of energy turnover. On the other hand, during prolonged low intensity exercise, there are relatively smaller decrements in muscle PCr and glycogen. Primarily circulating glucose and fatty acids are believed to provide the energy for low intensity muscle contraction (Hultman, 1967; Carlson, Ekelund & Fröberg, 1971; Gollnick et al. 1974; Essen, Hagenfeldt & Kaijser, 1977; Kiens, Essen-Gustavson, Christensen & Saltin, 1993). The possible role of intramuscular triglycerides (IT) in skeletal muscle energy metabolism during exercise is less clear. Furthermore, to our knowledge there is no information regarding the physiological function of the extramuscular triglycerides (ET, triglycerides between muscle cells) during muscle contraction.
Several groups have attempted to assess the possible contribution of ITs to energy metabolism during exercise in humans with rather conflicting results. Some of these studies show that IT decrease after continuous exercise (Essen et al. 1977; Hurley, Nemeth, Martin, Hagberg, Dalsky & Holloszy, 1986; Cleroux, Van Nguyen, Taylor & Leenen, 1989; Romijn et al. 1993; Sidossis, Coggan, Gastaldelli & Wolfe, 1995), whereas others report no significant changes after 25 min to 2 h of bicycle exercise or after 2 h of one leg knee extension dynamic exercise (Standl, Lotz, Dexel, Janka & Kolb, 1980; Jansson & Kaijser, 1982; Kiens et al. 1993; Wendling, Peters, Heigenhauser & Spriet, 1996; Starling, Trappe, Parcell, Kerr, Fink & Costill, 1997). As the results have been obtained using protocols of similar exercise modes and durations, it is unclear if ITs are a significant energy source during prolonged muscle contraction. It has also been suggested that during intermittent exercise, the regulation of IT metabolism might differ from continuous exercise (Essen et al. 1977).
Until recently, studies investigating the role of IT in exercise in humans had been carried out by direct biochemical analysis of biopsy material and by tracer estimation of IT oxidation. However, the use of the muscle biopsy technique in humans presents ethical and practical considerations especially in longitudinal studies. Furthermore, IT quantification from biopsies is not without complication and can show great variability (Wendling et al. 1996). Recently, a non-invasive measurement of intra- and extramuscular lipids by means of H-MRS was proposed by Schick, Eismann, Jung, Bongers, Bunse & Lutz (1993) and later used by Boesch, Slotboom, Hoppeler & Kreis (1997). Intramyocellular fat utilization in one subject was measured with H-MRS after a 3 h mountain bike ride and a reduction of intramuscular lipids in the tibialis anterior muscle was observed (Boesch et al. 1997). This suggests that this technique could be utilized to assess the potential role of IT under different physiological conditions.
In light of the results of Romijn et al. (1993), who quantified muscle triglyceride utilization at three different exercise intensities using direct and indirect measurements, we hypothesized that IT would also be used during continuous alternating intensity exercise. This type of exercise reflects the muscular contraction found in some physically demanding industrial professions as well as in diverse exercise and sports situations. The purpose of this study was to evaluate non-invasively IT and also ET metabolism during continuous locomotion with alternating velocities.
Acknowledgments
The MRUI software package (VARPRO) was kindly provided by Dr A. van den Boogaart, Katholieke Universiteit Leuven, Belgium. Financial support provided by the Medical Research Council and Picker International are gratefully acknowledged.