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Objective  To study the effects of L-arginine (L-Arg) on total body aerobic capacity and muscle metabolism as assessed by  31 Phosphorus Magnetic Resonance Spectroscopy ( 31 P-MRS) in patients with MELAS (Mitochondrial Encephalomyopathy with Lactic Acidosis and Stroke-like episodes) syndrome.    Methods  We performed a case control study in 3 MELAS siblings (m.3243A>G tRNA leu(UUR)  in MTTL1 gene) with different % blood mutant mtDNA to evaluate total body maximal aerobic capacity (VO 2peak ) using graded cycle ergometry and muscle metabolism using  31 P-MRS. We then ran a clinical trial pilot study in MELAS sibs to assess response of these parameters to single dose and a 6-week steady-state trial of oral L-Arginine.    Results  At baseline (no L-Arg), MELAS had lower serum Arg (p = 0.001). On  31 P-MRS muscle at rest, MELAS subjects had increased phosphocreatine (PCr) (p = 0.05), decreased ATP (p = 0.018), and decreased intracellular Mg 2+  (p = 0.0002) when compared to matched controls. With L-arginine therapy, the following trends were noted in MELAS siblings on cycle ergometry: (1) increase in mean % maximum work at anaerobic threshold (AT) (2) increase in % maximum heart rate at AT (3) small increase in VO 2peak . On   31 P-MRS  the following mean trends were noted: (1) A blunted decrease in pH after exercise (less acidosis) (2) increase in Pi/PCr ratio (ADP) suggesting increased work capacity (3) a faster half time of PCr recovery (marker of mitochondrial activity) following 5 minutes of moderate intensity exercise (4) increase in torque.    Significance  These results suggest an improvement in aerobic capacity and muscle metabolism in MELAS subjects in response to supplementation with L-Arg. Intramyocellular hypomagnesemia is a novel finding that warrants further study.    Classification of Evidence  Class III evidence that L-arginine improves aerobic capacity and muscle metabolism in MELAS subjects.    Trial Registration  ClinicalTrials.gov  NCT01603446 .

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L-Arginine Affects Aerobic Capacity and Muscle Metabolism in MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-Like Episodes) Syndrome