In vitro studies revealed that insulin resistance might be associated with the intracellular formation of ceramide, the second messenger in the sphingomyelin signaling pathway. The aim of the present study was to examine the content and composition of fatty acids in ceramide and sphingomyelin in human muscle and to evaluate their relationships with insulin sensitivity. The study was conducted on 27 male subjects with normal glucose tolerance. Euglycemic-hyperinsulinemic clamps and biopsies of vastus lateralis muscle were performed. In 10 subjects, additional biopsies were taken after a 4-h clamp and after a clamp with concurrent Intralipid/heparin infusion. We identified 13 ceramides and sphingomyelins according to fatty acid residues. Insulin sensitivity was related to total ceramide content (r = −0.49, P = 0.01) and to ceramide consisting of palmitic (r = −0.48, P = 0.011), palmitoleic (r = −0.45, P = 0.019), mirystic (r = −0.42, P = 0.028), and nervonic acid (r = −0.39, P = 0.047). Hyperinsulinemia did not affect estimated muscle parameters. Intralipid/heparin infusion resulted in a 24.73% decrease in insulin sensitivity (P = 0.007) and a 47.81% increase in ceramide content (P = 0.005). These changes were significantly related to each other (r = −0.64, P = 0.046). A relationship with the decrease in insulin sensitivity was also observed for ceramides consisting of palmitic (r = −0.68, P = 0.03) and linoleic (r = −0.66, P = 0.038) acid. Our data indicate that the sphingomyelin signaling pathway in muscle might be an important factor determining the development of insulin resistance in humans.

Intramuscular lipid pool turnover [triacylglycerols (TG), phospholipids (PL), mono- and diacylglycerols (MG, DG)] and the oxidation of endogenous and exogenous lipids were determined with pulse-chase studies in incubated muscles of varied oxidative potential [soleus strips (SOL)--> epitrochlearis --> flexor digitorum brevis]. Incorporation of palmitate into TG and PL pools and its oxidation were linearly related to time and exogenous palmitate concentration in all muscles. Total palmitate incorporation (deposition and oxidation) was greatest in SOL. However, palmitate incorporation into TG was similar in all muscles when expressed as a percentage of the total incorporation. In contrast, palmitate incorporation into PL was greatest in the least oxidative muscle. Palmitate oxidation, incorporation into TG, and citrate synthase activity were all strongly correlated with muscle cytosolic fatty acid-binding protein content (r = 0.96, 1.0, and 0.98, respectively). During the chase, reducing exogenous palmitate from 1.0 mM to 0.5 or 0 mM resulted in a significant (approximately 30%) loss of [(14)C]palmitate from the TG pool in SOL and a significant increase in (14)CO(2) production from endogenous stores. No significant loss of (14)C label from lipid pools occurred in the less oxidative muscles, suggesting a closely regulated interaction between energy provision from exogenous and endogenous lipid pools in oxidative muscle. Glucose oxidation increased significantly in all muscles in the absence of palmitate. The loss of (14)C label from TG in SOL during the chase without palmitate was not accompanied by a significant change in TG content. This suggests that, during rest, there is a small subpool of TG with a relatively rapid turnover.