IN VIVO MUSCLE GLYCOGEN AND PHOSPHATE METABOLISM IN RELATION TO BODY COMPOSITION IN YOUNG PIGS OF DIFFERENT GENOTYPES STUDIED BY NUCLEAR MAGNETIC RESONANCE TECHNIQUES A.M. Scholz1, A.D. Mitchell2, H. Song3 and P.C. Wang3 'Humboldt University, Institute of Animal Sciences, Berlin, Germany, 2USDA, ARS, Growth Biology Lab, Beltsville, MD, 3Howard University, Department of Radiology, Washington, DC SUMMARY Three ryanodine receptor 1 gene variants (AW: homozygous stress stable, Nn: heterozygous and nn: homozygous stress susceptible) served as a model for stress susceptibility with 96 pigs in each study originating from 6 breeding lines. Their age varied between 4 and 12 weeks and the body weight between 6 and 20 kg. Pig breeding programs which use only the normal genotype (NN) will need to apply supplementary selection criteria to reach further improvements in the constitution of the animals and finally in the carcass and meat quality. 13C and 3,P nuclear magnetic resonance spectroscopy were applied noninvasively in vivo and in a few pigs also post mortem to study the metabolic processes in the biceps femoris muscle after halothane exposure. In contrast to no visible effects of the halothane challenge test, the heterozygous defective allele carriers showed a reduction in the levels of glycogen (57 />), phosphocreatine (44 %), ATP (10.5 %) and muscle tissue pH (0.28) coupled with an increase in inorganic phosphate (355 %) and body temperature (1.71 °C). Overall, these changes were intermediate compared to the dramatic response in the homozygous nn genotype and to the very slow processes in NN. Results of 'H magnetic resonance imaging provided the evidence that the defective allele carriers are leaner than normal pigs already at approximately 11 kg live weight. However, fatter pigs are not necessarily more stress stable. Keywords: Swine, muscle metabolism, body composition, nuclear magnetic resonance, ryanodine receptor. INTRODUCTION Since Fujii et al. (1991) found an association between susceptibility to Malignant Hyperthermia and the alteration of C1843 to T1843 in the DNA encoding the Ca” release channel of skeletal muscle sarcoplasmic reticulum (ryanodine receptor 1 - RYR1), the RYR1 gene test has been used intensively in numerous pig breeding programs. As dam lines (and to some extent sire lines) are being ultimately cleared from all defective allele carriers, it will be necessary to use supplementary selection criteria based on the thorough knowledge of metabolic processes to achieve further genetic improvements. Nuclear Magnetic Resonance (NMR) techniques like spectroscopy and imaging are very powerful tools to manifest such selection criteria noninvasively and continuously both in vivo and post mortem. Previous in vivo 31P NMR studies established that especially homozygous stress susceptible pigs respond with faster phosphocreatine (PCr) decay, faster declining pH level (higher glycogen depletion), and a simultaneous increase of inorganic phosphate (Pi) combined with an increased adenosine triphosphate (ATP) depletion to muscle stressors. Controversial 125