Blood Glycerol Is an Important Precursor for Intramuscular Triacylglycerol Synthesis

The utilization of blood glycerol and glucose as precursors for intramuscular triglyceride synthesis was examined in rats using an intravenous infusion of [2-14C]glycerol and [6-3H]glucose or [6-14C]glucose. In 24-h fasted rats, more glycerol than glucose was incorporated into intramuscular triglyceride glycerol in soleus (69 ± 23 versus 4 ± 1 nmol/μmol triglyceride/h, respectively, p = 0.02 glycerol versus glucose) and in gastrocnemius (25 ± 5 versus 9 ± 2 nmol/μmol triglyceride/h, respectively, p = 0.02). Blood glucose was utilized more than blood glycerol for triglyceride glycerol synthesis in quadriceps. In fed rats, the blood glycerol incorporation rates (4 ± 2, 8 ± 3, and 9 ± 3 nmol/μmol triglyceride/h) were similar (p > 0.3) to those of glucose (5 ± 2, 8 ± 2, and 5 ± 2 nmol/μmol triglyceride/h for quadriceps, gastrocnemius, and soleus muscle, respectively). Glucose incorporation into intramuscular triglycerides was less with [6-3H]glucose than with [6-14C]glucose, suggesting an indirect pathway for glucose carbon entry into muscle triglyceride. The isotopic equilibrium between plasma and intramuscular free glycerol ([U-13C]glycerol) was complete in quadriceps and gastrocnemius, but not soleus, within 2 h after beginning the tracer infusion. We conclude that blood glycerol is a direct and important precursor for muscle triglyceride synthesis in rats, confirming the presence of functionally important amounts of glycerol kinase in skeletal muscle.