Metabolic and hormonal changes after exercise in relation to muscle glycogen concentrations

To test whether, in horses, the concentration of muscle glycogen can be influenced by increasing the uptake of glucose into the muscle cells or by providing a gluconeogenic precursor, 9 trained half-bred riding horses performed on a treadmill a 1.5 h competition exercise test (CET). Each horse performed CET 3 times and 30 min after CET, each was given one of the following solutions: isotonic glucoseelectrolyte (GE) solution, GE supplemented with 50 g leucine (GEL) to increase insulin secretion, or GE supplemented with 200 ml propionic acid (GEP), a gluconeogenic precursor. Administration of GE solutions caused no increase in plasma glucose concentration. The highest concentration of insulin was measured after GEL, but also in the GE group the concentration of insulin increased. GEP completely inhibited the increase in insulin concentration. Concentration of glucagon was increased 6 and 22.5 h after CET. None of the post exercise treatments influenced significantly the glycogen content at 22.5 h after CET. This indicates that neither i) elevation of insulin concentration to increase muscle-uptake of glucose, nor ii) increase in the availability of a glucose precursor, propionic acid, was able to increase accumulation of glycogen in the middle gluteal muscle. The basal entry of glucose into the muscle cells occurs by GLUT1 transporter, but with insulin stimulation another transporter, GLUT-4, is translocated to the sarcolemma from the intracellular pool (Barnard and Youngren 1992). In human athletes it has been found that the amount of GLUT-4 correlates with insulin-stimulated glucose uptake (Hardin et al. 1995) and, in trained athletes, has been reported to correlate with glycogen accumulation during early recovery (McCoy et al. 1996; Nakatani et al. 1997). Although blood glucose concentration is the main regulator of insulin secretion, in addition, amino acids, such as leucine, arginine and glycine, may increase its secretion (Kuhara et al. 1991). In the present study, leucine was administered together with glucose. In man, high-carbohydrate diets when given shortly after exercise are known to enhance the repletion of muscle glycogen stores (Bergstrom and Hultman 1966), but in horses no such effect has been demonstrated (Snow et al. 1987; Davie et al. 1994). Furthermore, large amounts of soluble carbohydrates cannot be recommended for the horse, because they may cause gastrointestinal disorders or laminitis. Therefore, the horses in this study received propionic acid, an efficient precursor of glucose (Ford and Simmons 1985; Simmons and Ford 1991). ' To study the effects of insulin and a glucose precursor on muscle glycogen, the horses performed an endurance exercise test on a treadmill to deplete their glycogen stores, and received leucine or propionic acid together with glucose during early recovery. Introduction Materials and methods In the horse, compared to other species, the post exercise repletion of muscle glycogen stores is slow (Bergstrom and Hultman 1966; Snow et al. 1987; Davie et al. 1994; Casey et al. 1995; Hyyppa et al. 1997). We have speculated that this could be, in part, due to low concentrations of lipid metabolites during recovery, which may divert glucose to energy production instead of to glycogen synthesis (Hyyppa et al. 1997). Rapid repletion of muscle glycogen stores may, however, be beneficial, if horses are required to race on 2 consecutive days. In the present experiments we used 2 strategies to study the role of glucose availability in the resynthesis of glycogen. First we wanted to enhance the uptake of glucose by muscle by increasing the plasma concentration of insulin, and secondly to enhance the rate of glucose synthesis in the liver by providing a gluconeogenetic precursor.