ATP Consumption by Sarcoplasmic Reticulum Ca2+ Pumps Accounts for 40-50% of Resting Metabolic Rate in Mouse Fast and Slow Twitch Skeletal Muscle

The main purpose of this study was to directly quantify the relative contribution of Ca2+ cycling to resting metabolic rate in mouse fast (extensor digitorum longus, EDL) and slow (soleus) twitch skeletal muscle. Resting oxygen consumption of isolated muscles (VO2, mu L/g wet weight/s) measured polarographically at 30 degrees C was similar to 20% higher (P<0.05) in soleus (0.326 +/- 0.022) than in EDL (0.261 +/- 0.020). In order to quantify the specific contribution of Ca2+ cycling to resting metabolic rate, the concentration of MgCl2 in the bath was increased to 10 mM to block Ca2+ release through the ryanodine receptor, thus eliminating a major source of Ca2+ leak from the sarcoplasmic reticulum (SR), and thereby indirectly inhibiting the activity of the sarco(endo) plasmic reticulum Ca2+-ATPases (SERCAs). The relative (%) reduction in muscle VO2 in response to 10 mM MgCl2 was similar between soleus (48.0 +/- 3.7) and EDL (42.4 +/- 3.2). Using a different approach, we attempted to directly inhibit SERCA ATPase activity in stretched EDL and soleus muscles (1.42x optimum length) using the specific SERCA inhibitor cyclopiazonic acid (CPA, up to 160 mu M), but were unsuccessful in removing the energetic cost of Ca2+ cycling in resting isolated muscles. The results of the MgCl2 experiments indicate that ATP consumption by SERCAs is responsible for 40-50% of resting metabolic rate in both mouse fast-and slow-twitch muscles at 30 degrees C, or 12-15% of whole body resting VO2. Thus, SERCA pumps in skeletal muscle could represent an important control point for energy balance regulation and a potential target for metabolic alterations to oppose obesity.