2008/2009Background. Muscle atrophy is determined by specific molecular pathways controlling protein synthesis and degradation. Physical inactivity and muscle unloading are normally related to decreased muscle mass, but pathways controlling muscle atrophy during inactivity in humans are not presently clarified. Previous publications showed in animals a link between muscle atrophy and oxidative stress or inflammation. Inactivity was shown to be associated to inflammation and oxidative stress upregulation. Inflammation is controlled by several factors and polyunsaturated fatty acids (PUFA) of n-3 and n-6 series are known to play an anti-inflammatory and proinflammatory role, respectively: their relative content in cell membranes reflects the inflammatory condition at whole body level. Oxidative stress derives from reactive oxygen species (ROS) production in excess to the scavenging activity of antioxidant systems. Homocysteine is a non-proteinogenic amino acid: increased homocysteine concentration in plasma is directly linked to oxidative tissue damage. Glutathione (GSH) is an antioxidant tripeptide: upregulated GSH availability is considered a response to occurred ROS production. Glutamine, is synthesized by muscles and it is utilized by immune cells: its depletion occurs in critically ill patients. Nutrition can play a pivotal role during inactivity: dietary protein supplementation can ameliorate protein turnover while energy intake restriction was linked to enhanced muscle atrophy. Aims and experimental design. The aim of the present thesis is to investigate in human healthy volunteers: i) the impact of physical inactivity on inflammation and oxidative stress as potential mechanisms underlying muscle atrophy; ii) the impact of energy balance, and of protein supplementation during inactivity on muscle atrophy, oxidative stress and glutamine availability. Different metabolic tests were performed in five separate experimental bed rest campaigns: STBR-IP, WISE, Valdoltra Bed Rest 2006 – 2007 – 2008. Results. Eucaloric experimental bed rest at normal protein intake: decreased n-3 and increased n-6 PUFA in red blood cell membranes; increased arachidonic to eicosapentaenoic acid ratio (Valdoltra Bed Rest 2006 – 2007 – 2008); increased homocysteine plasma concentration lowering clearance by remethylation (WISE); reduced plasma glutamine concentrations lowering its de novo synthesis (STBR-IP); decreased muscle thickness and myofibres pennation angle; increased muscle protein carbonylation and GSH availability in direct correlation with atrophy progression (Valdoltra Bed Rest 2006 – 2007). High protein intake during bed rest significantly lowered homocysteine concentrations by increased clearance by transsulfuration. Calorie restriction (20% reduction) during muscle unloading failed to affect changes of glutamine kinetics and availability mediated by inactivity (STBR-IP). During bed rest, excessive fat mass gain, when matched to stable fat mass maintenance, significantly: reduced vastus lateralis muscle thickness; increased plasma leptin, myeloperoxidase and C-reactive protein levels; enhanced, in red blood cells, activity and availability of GSH (Valdoltra Bed Rest 2006 – 2007). Discussion. The experimental bed rest mediated increase in n-6 relatively to n-3 polyunsaturated fatty acids of erythrocytes membranes confirm bed rest can play a pro-inflammatory role at whole body level. Upregulated levels of homocysteine plasma concentrations after bed rest underline that immobility can induce whole body oxidative stress and cardiovascular risk. Significant correlation between changes in protein carbonylation, as marker of oxidative damage, or antioxidant GSH availability, with muscle atrophy induced by bed rest, strongly suggest that physical inactivity can induce muscle atrophy by pathways involving oxidative stress. Carbonylated proteins are, in fact, directly and rapidly degraded by the proteasome system. Moreover, dietary protein supplementation during physical inactivity potentially reduces oxidative damage lowering plasma homocysteine concentration. This evidence is confirmed by publications showing, in animals, that high protein diet can increase homocysteine transsulfuration rate. Interestingly, during inactivity, positive energy balance, when matched to fat mass maintenance, was here shown to induce greater muscle atrophy together with greater upregulation of whole body inflammation and oxidative stress. Thus, such observations further suggest that inactivity mediated atrophy can be triggered and regulated by oxidative stress and inflammation. Calorie restriction probably failed to affect glutamine kinetics due to the low extent of energy intake reduction, but glutamine metabolism can be hypothesized to be independent from energy balance during inactivity.XXII Cicl
