Regulation of Hepatic Fat and Glucose Oxidation in Rats with Lipid-Induced Hepatic Insulin Resistance

Pyruvate dehydrogenase plays a critical role in the regulation of hepatic glucose and fatty acid oxidation; however, surprisingly little is known about its regulation in vivo. In this study we examined the individual effects of insulin and substrate availability on the regulation of pyruvate dehydrogenase flux (V-PDH) to tricarboxylic acid flux (V-TCA) in livers of awake rats with lipid-induced hepatic insulin resistance. V-PDH/V-TCA flux was estimated from the [4-C-13] glutamate/[3-C-13] alanine enrichments in liver extracts and assessed under conditions of fasting and during a hyperinsulinemic-euglycemic clamp, whereas the effects of increased plasma glucose concentration on V-PDH/V-TCA flux was assessed during a hyperglycemic clamp in conjunction with infusions of somatostatin and insulin to maintain basal concentrations of insulin. The effects of increases in both glucose and insulin on V-PDH/V-TCA were examined during a hyperinsulinemic-hyperglycemic clamp. The effects of chronic lipid-induced hepatic insulin resistance on this flux were also examined by performing these measurements in rats fed a high-fat diet for 3 weeks. Using this approach we found that fasting V-PDH/V-TCA was reduced by 95% in rats with hepatic insulin resistance (from 17.2 +/- 1.5% to 1.3 +/- 0.7%, P < 0.00001). Surprisingly, neither hyperinsulinemia per se or hyperglycemia per se were sufficient to increase V-PDH/V-TCA flux. Only under conditions of combined hyperglycemia and hyperinsulinemia did V-PDH/V-TCA flux increase (44.6 +/- 3.2%, P < 0.0001 versus basal) in low-fat fed animals but not in rats with chronic lipid-induced hepatic insulin resistance. Conclusion: These studies demonstrate that the combination of both hyperinsulinemia and hyperglycemia are required to increase V-PDH/V-TCA flux in vivo and that this flux is severely diminished in rats with chronic lipid-induced hepatic insulin resistance. (HEPATOLOGY 2011;53:1175-1181)