Muscle energy metabolism: structural and functional features in different types of porcine striated muscles

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Abstract
Striated muscles exhibit a wide range of metabolic activity levels. Heart and diaphragm are muscles with continuous contractile performance, which requires life-long function. In contrast, skeletal muscles like longissimus muscle can adapt metabolism from resting to different stages of exercise. The aim of this study was to compare the morphological features of these three muscles and the expression of genes that are important for energy metabolism. Therefore, histochemical studies were performed for determination of muscle fibre type composition. Oxidative and glycolytic capacity was assessed by measuring isocitrate dehydrogenase (ICDH) and lactate dehydrogenase (LDH) activities. The mRNA expression of glucose transporter 4 (GLUT 4), growth hormone receptor (GHR) and AMP-activated kinase (AMPK) α1 and α2 subunits was studied by semiquantitative Northern blotting. Heart, and to a slightly lesser extent diaphragm were highly oxidative muscles characterised by high expression of oxidative muscle fibres and ICDH activity. Longissimus muscle exhibited the highest percentage of glycolytic fibres and LDH activity. GLUT 4 mRNA was lowest in heart reflecting the dependency of heart muscle on fatty acids as major energy source. Higher expression of GLUT 4 in diaphragm indicated that glucose is an important energy substrate in this oxidative muscle. Highest GLUT 4 expression in longissimus should be essential for the refilling of glycogen stores after exercise. AMPK subunits, which are important stimulators of GLUT 4 protein insertion into the sarcolemma, are also highest expressed in longissimus muscle indicating the strong capacity to adapt energy metabolism to large changes in energy demand. Interestingly, AMPK α1 subunit expression on protein level is strongly restricted to muscle fibres containing type I myosin in this muscle. GHR mRNA expression was also highest in longissimus muscle indicating that an enhanced effect of growth hormone, which is described to be diabetogenic, could be involved in the lower insulin sensitivity of glycolytic muscles.

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