Endurance training increases FFA oxidation and reduces triacylglycerol utilization in contracting rat soleus

We examined the effects of 8 wk of intense endurance training on free fatty acid (FFA) transporters and metabolism in resting and contracting soleus muscle using pulse-chase procedures. Endurance training increased maximal citrate synthase activity in red muscles (+54 to +91%; P less than or equal to 0.05) but failed to increase cytosolic fatty acid binding protein content, mRNA for fatty acyl-CoA synthase, and the putative FFA transporters or transport of palmitic acid into giant sarcolemmal vesicles. At rest, only triacylglycerol (TG) synthesis was significantly increased by training (+ 100.9 +/- 8.7 vs. +66.6 +/- 6.7 nmol/g wet wt; P less than or equal to 0.05). Muscle contraction increased TG synthesis (+46%; P less than or equal to 0.05) and palmitate oxidation(+115%; P less than or equal to 0.05) in untrained rats. Endurance training further enhanced synthesis of monoacylglycerol (MG), diacylglycerol (DG) and TG during contraction (+36, +69 and +71%, respectively; P less than or equal to 0.05), as well as exogenous palmitate oxidation (+41%; P less than or equal to 0.05) relative to untrained rats. Compared with those in untrained rats, TG breakdown and oxidation during contraction were reduced after training by 49 and 30%, respectively (P less than or equal to 0.05). In conclusion, endurance training 1) increases FFA oxidation and incorporation into endogenous lipid pools during contraction and 2) reduces the rate of intramuscular TG utilization during contraction when exogenous FFA availability is adequate. The enhanced FFA uptake subsequent to training appears to be independent of altered maximal transport rates of FFA into the muscle cell.