Results and discussion
To show the dynamic nature of substrate oxidation, we measured substrate oxidation for human myotubes exposed to alternating stepwise increased concentrations of glucose and PA (Fig. 1A and B). The inhibition of glucose oxidation by PA could be partly circumvented by increasing the glucose concentration. Increasing the PA concentration further decreased glucose oxidation again. A reciprocal oxidative pattern was seen in the opposite experiment starting with PA (Fig. 1B). These studies show, that four regulatory principles operate simultaneously in human myotubes to determine substrate oxidation and these are integrated in a dynamic continuum at low-moderate substrate levels: (1) lipid reduced glucose oxidation; (2) glucose reduced lipid oxidation; high concentrations of glucose (3) and palmitate (4) increased their own oxidation, despite inhibited by the other. The Randle effect mediates through inhibition of the pyruvate dehydrogenase (PDH) complex by end products of fatty acid oxidation, such as acetyl-CoA and NADH. The depression of lipid oxidation by glucose is suggested to be mediated through an increase of malonyl-CoA (MCoA), which is a potent inhibitor of acyl-CoA uptake into mitochondria by inhibition of carnitine palmitoyltransferase-1 (CPT1) [15] and [16]. Accordingly, we showed previously that both PA oxidation and the suppression of glucose oxidation by PA could be partly inhibited when the entrance of palmitoyl-CoA into the mitochondria was inhibited by the CPT1-inhibitor etoximir [5].