Skeletal Muscle Lipids

The extent to which the intracellular lipids of monkey gastrocnemius and soleus muscles are used as a fuel for contractile activity in postabsorptive state was investigated. Under conditions in vivo, one set of the gastrocnemius and soleus muscles was caused to undergo vigorous contractile activity for 5 hours while the contralateral muscles served as quiescent controls. The concentration of the various classes of phospholipids and triglyceride in the skeletal muscle was not affected by contractile activity. The significance of this finding is discussed. The conclusion is drawn that intracellular muscle lipids are not used as a net source of fuel for the increased energy metabolism of contracting muscle even after prolonged periods of fasting. It is further concluded that the lipid fuel used by skeletal muscle during contractile activity is derived entirely from sources outside the muscle cell. The question of the effect of muscle activity on the turnover of muscle lipid esters was also studied by means of isotopic tracer techniques. The data provide no evidence that the rate of turnover is increased during muscular activity, and the conclusion is tentatively drawn that the rate of turnover of skeletal muscle lipids is not influenced by contractile activity.