Dissociation of Muscle Insulin Resistance from Alterations in Mitochondrial Substrate Preference.

Alterations in muscle mitochondrial substrate preference have been postulated to play a major role in the pathogenesis of muscle insulin resistance. In order to examine this hypothesis, we assessed the ratio of mitochondrial pyruvate oxidation (V) to rates of mitochondrial citrate synthase flux (V) in muscle. Contrary to this hypothesis, we found that high-fat-diet (HFD)-fed insulin-resistant rats did not manifest altered muscle substrate preference (V/V) in soleus or quadriceps muscles in the fasting state. Furthermore, hyperinsulinemic-euglycemic (HE) clamps increased V/V in both muscles in normal and insulin-resistant rats. We then examined the muscle V/V flux in insulin-sensitive and insulin-resistant humans and found similar relative rates of V/V, following an overnight fast (∼20%), and similar increases in V/V fluxes during a HE clamp. Altogether, these findings demonstrate that alterations in mitochondrial substrate preference are not an essential step in the pathogenesis of muscle insulin resistance.