Contribution of sarcoplasmic reticulum calcium pumping to resting mouse muscle metabolism

Few studies have quantified resting mouse muscle metabolism and even fewer studies have separated the contribution of sarcoplasmic reticulum (SR) Ca2+ pumping to resting metabolic rate. Furthermore, the studies that have attempted to quantify the contribution of Ca2+ pumping have used indirect methods to inhibit SR Ca2+ ATPase activity. The purpose of this study is to directly quantify resting muscle oxygen consumption and the contribution of SR Ca2+ pumping to resting oxygen consumption in mouse hindlimb muscles by using CPA to specifically inhibit Ca2+ pump activity in intact muscles at rest. The TIOX system was used to measure resting muscle VO2 of extensor digitorum longus (EDL) and soleus (SOL) muscles at 30oC and 20oC. C57BL mice aged 8-12 weeks were used with an average whole body mass of 23.8 g and EDL and SOL dry weights averaging 1.88 mg and 1.8 mg, respectively. All muscle VO2 measurements are expressed per gram dry weight. There were no differences (P>0.1) in resting muscle VO2 between EDL and SOL muscles at either 30oC (EDL, 2.05 µL/g/s; SOL, 2.27 µL/g/s) or 20oC (EDL, 0.62 µL/g/s; SOL, 0.71 µL/g/s). The average Q10 (3.1) was determined from EDL and SOL VO2 measures at 20oC and 30oC. The contribution of Ca2+ pumping by the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) was measured at 30oC using a range of CPA concentrations (1-15 µM) . There was a concentration-dependent effect of CPA on oxygen consumption with increasing CPA concentrations up to 10 µM resulting in progressively greater reductions in muscle oxygen consumption. Specifically, 1, 5, 10, and 15 µM CPA caused an 11, 35.4, 49.5, and 50.3% reduction in VO2. There were no differences (P>0.1) between 10 and 15 µM CPA indicating that 10 µM CPA induces maximal inhibition of SERCA in isolated muscle preparations. The results indicate that the Ca2+ pumping by SERCA is responsible for ~50% of oxygen consumption in resting mouse EDL and SOL muscle. This is the first study to use a direct inhibitor of SERCA to quantify the contribution of Ca2+ cycling to resting oxygen consumption and therefore is a more accurate reflection of the actual contribution of SERCA to resting muscle oxygen consumption compared to previous findings. These results suggest that SERCA energy consumption accounts for a large portion of resting muscle metabolism and may represent a potential therapeutic target for metabolic alterations to oppose obesity.

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