Increased Fatigue Resistance of Skeletal Muscle with Elevated 2-Deoxy-ATP following Ribonucleotide Reductase Overexpression

Skeletal muscle myosin can use a variety of nucleotides to varying effectiveness as substrates for contraction. We previously demonstrated that complete replacement of ATP with 2 deoxy-ATP (dATP) in activation solutions increases contraction of demembranated rabbit skeletal muscle and that enhanced performance results from increased myosin binding and cycling kinetics, especially during sub-maximal calcium activation. Here we report on a transgenic mouse (Tg-RR) that overexpresses the enzyme, ribonucleotide reductase, which converts ADP to dADP (that is rapidly phosphorylated to dATP). This results in a ∼10x increase in dATP content of skeletal muscle, which still constitutes ≤ 1% of the adenosine triphosphate nucleotide pool. We are examining the contractile and metabolic properties of skeletal muscle in this transgenic model. Preliminary data indicates the phosphocreatine to ATP (PCr:ATP) ratio of Tg-RR mice is significantly elevated relative to wild type (Tg-WT) mice, suggesting the Tg-RR mice may have an energetic advantage due to increased high energy phosphate reserves. Furthermore, the Tg-RR mice ran for a longer period of time at increased speed and for longer distances in a graded exercise test. Direct in situ stimulation of the gastrocnemius muscle indicates improved resistance to fatigue, consistent with both the exercise and the metabolic (PCr:ATP) findings. Interestingly, our preliminary experiments suggest the Tg-RR mice produce significantly less peak force and show signs of atrophy. Ongoing experiments are examining changes in mitochondrial content and function as well as contractile properties of isolated muscles to determine the mechanisms underlying the increased fatigue resistance of the Tg-RR mice. These results suggest a new direction for developing interventions to improve exercise tolerance in human patients. Supported by HL11197 (MR).