134 DIABETES, VOL. 48, JANUARY 1999 Differential Effects of Safflower Oil Versus Fish Oil Feeding on Insulin-Stimulated Glycogen Synthesis, Glycolysis, and Pyruvate Dehydrogenase Flux in Skeletal Muscle A 1 3 C Nuclear Magnetic Resonance Study Beat M. Jucker, Gary W. Cline, Nicole Barucci, and Gerald I. Shulman To examine the effects of safflower oil versus fish oil feeding on in vivo intramuscular glucose metabolism and relative pyruvate dehydrogenase ( PDH) versus tri- carboxylic acid ( TCA) cycle flux, rats were pair-fed on diets consisting of 1) 59% safflower oil, 2) 59% men- haden fish oil, or 3) 59% carbohydrate ( control) in calo- ries. Rates of glycolysis and glycogen synthesis were assessed by monitoring [1- 1 3 C]glucose label incorpora- tion into [1- 1 3 C]glycogen, [3- 1 3 C]lactate, and [3- 1 3 C]ala- nine in the hindlimb of awake rats via 1 3 C nuclear mag- netic resonance ( NMR) spectroscopy during a eugly- cemic ( ~6 mmol/l) hyperinsulinemic (~180 μU/ml) clamp. A steady-state isotopic analysis of lactate, ala- nine, and glutamate was used to determine the relative PDH versus TCA cycle flux present in muscle under these conditions. The safflower oil–fed rats were insulin resistant compared with control and fish oil–fed rats, as reflected by a markedly reduced glucose infusion rate ( G inf ) during the clamp (21.4 ± 2.3 vs. 31.6 ± 2.8 and 31.7 ± 1.9 mg · kg – 1 · min – 1 in safflower oil versus control and fish oil groups, respectively, P < 0.006). This decrease in insulin-stimulated glucose disposal in the safflower oil group was associated with a lower rate of glycolysis (21.7 ± 2.2 nmol · g – 1 · min – 1 ) versus control (62.1 ± 10.3 nmol · g – 1 · min – 1 , P < 0.001) and versus fish oil (45.7 ± 6.7 nmol · g – 1 · min – 1 , P < 0.04) , as no change in glycogen synthesis (103 ± 15, 133 ± 19, and 125 ± 14 nmol · g – 1 · min –1 in safflower oil, fish oil, and control, respectively) was detected. The intramuscular triglyceride ( TG) con- tent was increased in the safflower oil group ( 7.3 ± 0.8 μmol/g) compared with the control group (5.2 ± 0.8 μmol/g, P < 0.05) and the fish oil group ( 3.6 ± 1.1 μmol/g, P < 0.01). Conversely, the percent PDH versus TCA cycle flux was decreased in the safflower oil (43 ± 8%) versus the control (73 ± 8%, P < 0.01) and fish oil (64 ± 6% , P < 0.05) groups. These data suggest that the reduced insulin-stimulated glucose disposal attributed to safflower oil feeding was a consequence of reduced glycolytic flux associated with an increase in relative free fatty acid/ketone oxidation versus TCA cycle flux , whereas fish oil feeding did not alter glucose metabolism and may in part be protective of insulin-stimulated glu- cose disposal by limiting intramuscular TG deposition. Diabetes 48:134–140, 1999 I t is well established that chronic high-fat feeding can induce insulin resistance in rats (1–9). Furthermo re, it has been demonstrated that the dietary fat composition is a critical factor in this process because a diet high in long-chain -3 fatty acids (fish oil) has been found to pre- vent the development of insulin resistance, although the mechanism behind this protective effect remains unclear (10,11). Although many studies have demonstrated that the insulin resistance associated with high-fat feeding is sec- ondary to decreased muscle glycolysis (1,3,6,9,11,12), no studies to date have assessed its impact on pyruvate dehy- drogenase flux because of the difficulty in measuring this parameter. This is a critical factor because the classic mech- anism (glucose–free fatty acid [FFA] cycle) proposed by Ran- dle and colleagues (13,14) demonstrates that an increase in acetyl-CoA resulting from increased -oxidation of fat inhibits pyruvate dehydrogenase (PDH) flux by inactivating the PDH complex. Furthermore, the effect of -3 fatty ac ids on this flux is unknown. To address these questions, we used in vivo 13 C nuclear magnetic resonance (NMR) spectroscopy in awake rats to examine the impact of contrasting high-fat feeding protocols that either produce insulin resistance (safflower oil) or protect against insulin resistanc e (fish oil) o n the follo wing facto rs in insulin-stimulated muscle: 1) glycolysis, 2) glycogen synthesis, and 3) relative PDH versus tricarboxylic acid (TCA) cycle flux. RESEARCH DESIGN AND METHODS A nimals. Sprague-Dawley rats (Charles River, Raleigh, NC) were housed in an environmentally controlled room with a 12-h light/dark cycle. Rats (35–42 days old) were begun on a chronic high-fat feeding protocol that consisted of one of three different powdered diets containing, by calorie, 59% safflower oil, 23% car- bohydrate, and 18% protein (safflower oil diet, n = 10); 59% menhaden fish oil, From the Department of Internal Medicine (B.M.J., G.W.C., N.B., G.I.S) and the Ho ward Hughes Medical Institute (G.I.S.), Yale University School of Medicine, New Haven, Connecticut. Address correspondence and reprint requests to Dr. Beat M. Jucker, Department of Internal Medicine, Yale University School of Medicine, Fitkin 1, 333 Cedar St., P.O. Bo x 208020, New Haven, CT 06520-8020. E-mail: juc ke r@ m rc b s .m e d .yale .e d u. Received for publication 22 May 1998 and accepted in revised form 24 September 1998. ANOVA, analysis of variance; APE, atom percent excess; FFA, free fatty acid; G inf , glucose infusion rate; NMR, nuclear magnetic resonance; PDH, pyruvate dehydrogenase; TCA, tricarboxylic acid; TG, triglyceride; V g ly , rate of glycolysis; V g ly c , rate of glycogen synthesis. Downloaded from http://diabetesjournals.org/diabetes/article-pdf/48/1/134/364065/9892234.pdf by guest on 04 November 2022