The interaction of diet and exercise on skeletal muscle adaptations in rats

이종삼 School of Medical Sciences RMIT University 2002 해외박사

Physical exercise and dietary manipulations are major factors that induce significant metabolic, biochemical and cellular changes in skeletal muscle. In this thesis the interaction of exercise and diet on a variety of metabolic, enzymatic, mitogenic and genetic adaptations in rat skeletal muscle and liver was investigated. In order to determine the interaction of diet and training on skeletal muscle and liver adaptations and their metabolic consequences for endurance (Chapter three), eighty rats performed a baseline treadmill run to exhaustion at 16 m·min^(-1) (RUN1). Animals were then divided into one of two dietary conditions: high-carbohydrate (CHO) or high-fat (FAT). Each dietary group was then divided into one of four subgroups: sedentary control that performed no training (NT); low-intensity running (8 m·min^(-1); LOW) and two groups who trained at their maximal voluntary running speed (28 m·min^(-1); VMAX). Training volume was identical for LOW and VMAX (1,000 m·session^(-1)) and animals ran 4 times·wk^(-1) for 8 wk. To assess the interaction of the higher intensity exercise with diet, a second endurance test (RUN2) was undertaken after 6 wk at both 16 m·min^(-1) and 28 m·min^(-1). It was hypothesised that a high-fat diet in combination with low-intensity training would evoke the greatest metabolic adaptations for fat metabolism in skeletal muscle and improve endurance running capacity to a greater extent than when either low or more intense training was undertaken on a high carbohydrate diet. Compared to CHO, FAT increased the activities of citrate synthase, β-hydroxyacyl-CoA dehydrogenase and carnitine palmitoyl-transferase (P<0.01). NT animals ran 77% longer at 16 m·min^(-1) after FAT than CHO (239±28 vs. 135±30 min, P<0.05). There was no effect of diet on run time for LOW when rats were tested at 16 m·min^(-1) (454±86 vs. 427±75 min for CHO and FAT). However, VMAX rats fed FAT ran longer than CHO at 28 m·min^(-1) (100±28 vs. 58±11 min, P<0.05). In contrast to the original hypothesis, there was no additive effect of a high-fat diet on endurance performance when rats performed low-intensity training. Indeed, running performance was only enhanced by a high-fat diet after the more intense training programme. Although exercise capacity can be greatly influenced by dietary manipulation (Chapter three) a chronic high-fat diet is associated with poor health prognosis. Thus, whereas exercise improves carbohydrate metabolism by increasing insulin sensitivity and glucose transporter (GLUT)-4 protein in skeletal muscle, a high-fat diet reduces glucose tolerance and insulin sensitivity. Of interest is how glucose metabolism might be altered when undertaking regular exercise (which promotes glucose transport) in the face of a high-fat diet (which reduces glucose transport). It was hypothesised that exercise would partially (but not completely) preserve the increases in glucose metabolism (i.e., GLUT-4 content) despite a diet-induced impairment as a result of fat feeding. Therefore in the second experiment (Chapter four), the interaction of exercise and diet on GLUT-4 protein and messenger ribonucleic acid (mRNA) expression in both type I (soleus) and type II (extensor digitorum longus, [EDL]) skeletal muscle was determined. Forty eight rats were randomly assigned into one of two dietary conditions: a high-fat or a high-carbohydrate diet. Animals in each dietary condition were then randomly allocated to one of two subgroups: a sedentary control group that performed NT, and a group that undertook 8 wk of treadmill running training (28 m·min^(-1) for 1 km·session^(-1), 4 times·wk^(-1)). GLUT-4 protein expression in NT rats was similar in both muscles after 8 wk of either CHO or FAT. However, there was a training-induced increase in GLUT-4 protein in the soleus in rats fed either CHO or FAT (P<0.05) and in the EDL in animals fed CHO (P<0.05). Exercise training only increased GLUT-4 gene expression in animals fed CHO (soleus: 100% ↑ P=0.009, EDL: 142% ↑ P=0.002). FAT trained rats had a significant decrease in mRNA in the EDL (↓ 45%, P<0.05) but not the soleus (↓ 14%, NS), but the exercise-induced increases in GLUT-4 protein were largely preserved. Exercise is a complex physiological stimulus that activates multiple biochemical and biophysical aspects of cellular function. The effects of exercise on metabolic/morphological responses depend on the intensity, duration and frequency of this stimulus. Exercise/contraction activates specific molecular signalling pathways that transduce extracellular impulses into intracellular responses. One specific pathway is the mitogen-activated protein (MAP) kinase pathway. Activation of the MAP kinase signalling cascade has been proposed as a possible pathway whereby extracellular signals are transmitted to their intracellular targets. In the final experiment (Chapter five), the effect of a chronic programme of either low- or moderate- to high-intensity running (as described in the first experiment) on the activation of the p38 MAP kinase and the extracellular-signal regulated protein kinase (ERK) 1 / 2 pathways was investigated. It was hypothesised that a chronic programme of either low- or moderate- to high-intensity treadmill running would result in differential activation of the ERK 1 / 2 and p38 MAP kinase pathways in rat skeletal muscle. A novel feature of this study was that the intensity of training sessions was calculated so that animals covered the same distance and had similar glycogen utilisation, despite the fact that rats undertaking the less intense protocol spent almost four times as long running. A single bout of either low- or moderate- to high-intensity exercise following 8 wk of training led to a 2-fold increase in the phosphorylation of ERK 1 / 2 (P<0.05) and a 2-3 fold increase in p38 MAP kinase (P<0.01) compared to sedentary values. The ERK 1 / 2 phosphorylation in muscle sampled 48 h after the last exercise bout was similar to sedentary values, while p38 MAP kinase phosphorylation was 70-80% lower than sedentary. One bout of exercise increased total ERK 1 / 2 and p38 MAP kinase expression with the magnitude of this increase being independent of prior exercise intensity or duration. The ERK 1 / 2 expression was increased 3-4 fold in muscle sampled 48 h after the last exercise bout irrespective of the prior training programme (P<0.05), but p38 MAP kinase expression was significantly lower than sedentary values (P<0.05). It was concluded that either the activation threshold for exercise-stimulation of the MAP kinase pathway in rat soleus muscle is independent of the intensity and duration of either the last exercise bout or the prior training regimen, or that the total energy turnover (i.e., muscle glycogen utilisation during the exercise bout) determines the phosphorylation of ERK 1 / 2 and p38 MAP kinases. Taken collectively, these studies show that exercise and diet elicit a multitude of metabolic effects in skeletal muscle including changes in substrate metabolism, gene expression and protein synthesis. These adaptations are the result of both the cummulative (but independent) effects of chronic diet and/or exercise exposure (i.e., increases in enzyme activity), and the acute response to a single bout of exercise (i.e., activation of intracellular signalling cascades). The results of the studies in this thesis have provided new insight into the complex interaction of diet and exercise on a variety of physiological and cellular functions in rat skeletal muscle.