The effect of therapeutic exercise and metabolic acidosis on skeletal muscle wasting in chronic kidney disease

Muscle wasting and increased proteolysis is a major problem in chronic kidney disease (CKD). Exercise is potentially beneficial, but has been under-investigated in pre-dialysis CKD and could theoretically worsen acidosis through exercise-induced lactic acid generation. We therefore investigated effects of 6 months walking exercise with and without additional alkali therapy. 40 patients were recruited (23 male and 17 female, median age 58, range 20-83, mean eGFR±SEM 25.7±1.2ml/min/1.73m2). 20 undertook walking exercise at a Borg Rating of Perceived Exertion Rate (RPE) of 12-14 for at least 30 minutes, 5 times a week. The other 20 continued with normal physical activity (non-exercising controls). In addition to standard oral bicarbonate therapy (STD), 10 patients in each group were randomised to receive additional bicarbonate (XS). Blood and vastus lateralis muscle biopsies were drawn at baseline, one and six months. 18 exercisers (including 8 in XS group) and 14 controls (6 in XS group) completed the 6 month study. Exercise tolerance increased after 1 and 6 months in the exercisers, but not the controls, accompanied by a reduced acute lactate response in the XS, but not the STD exercising group. After 6 months of exercise, 9 intramuscular free amino acids showed striking depletion in the STD, but not XS bicarbonate group. This suggests an inhibition of active amino acid transporters, possibly the SNAT2 transporters that are inhibited by acidosis. Studies with cultured myotubes identified glucocorticoid as a possible mediator of acid s inhibitory effect on SNAT2. The preservation of amino acid concentrations in the XS exercising group was accompanied by strong suppression of ubiquitin E3-ligases MuRF-1 and MAFbx which activate proteolysis through the ubiquitin-proteasome pathway. However, other anabolic indicators (Protein Kinase B activation and suppression of the 14kDa actin fragment) were unaffected in the exercising XS group. Possibly because of this, overall suppression of myofibrillar proteolysis (3-methyl histidine excretion) and increased lean body mass (DEXA) were not observed in the exercising patients. As XS alkali had no effect in non-exercisers, it is concluded that alkali effects in the exercisers arose by countering exercise-induced acidosis. Sulphuric acid produced from the catabolism of sulphur-containing amino acids ingested in the diet is the main contributor to the daily titratable acid load and hence acidosis in CKD. In these patients the amount of sulphate excreted in urine over 24h varied widely between individuals. This directly correlated with 3-methyl histidine excretion suggesting that sulphate excretion may be a better clinical indicator of acidotic patients at long-term risk of cachexia than conventional measures such as venous bicarbonate. Studies with cultured myotubes confirmed that skeletal muscle is a source of sulphuric acid and showed that production of this acid is partly suppressed by L-Glutamine a potential novel way to control acidosis in CKD.