Glucose-fatty acid cycle to inhibit glucose utilization and oxidation is not operative in fatty acid-cultured islets.

Abstract

The glucose-fatty acid cycle of Randle entails two elements: decreased pyruvate dehydrogenase (PDH) activity, which inhibits glucose oxidation, and inhibition of phosphofructokinase (PFK) by a rise in citrate so that glucose-6-phosphate (G-6-P) levels increase, thereby inhibiting hexokinase activity and hence glucose utilization. Chronic exposure of islets to long-chain fatty acids (FA) is reported to lower PDH activity, but the effect on glucose oxidation and glucose-induced insulin secretion is uncertain. We investigated rat islets that were cultured for 4 days with 0.25 mmol/l oleate/5.5 mmol/l glucose. Glucose oxidation was doubled at 2.8 mmol/l glucose and unchanged at 27.7 mmol/l glucose in the FA-cultured islets despite a 35% decrease in assayed PDH activity. Pyruvate content was increased 60%, which may well compensate for the decreased PDH activity and maintain flux through the citric acid cycle. However, a greater diversion of pyruvate metabolism through the pyruvate-malate shuttle is suggested by unchanged pyruvate carboxylase Vmax and a fourfold higher release of malate from isolated mitochondria. The FA-cultured islets also showed increased basal glucose usage and insulin secretion together with a lowered level of G-6-P and 50% reductions in citrate synthase Vmax and the citrate content. Thus, the effects of chronic FA exposure on islet glucose metabolism differ from the glucose-fatty acid interactions reported in some other tissues.

Chronic High Glucose Lowers Pyruvate Dehydrogenase Activity in Islets through Enhanced Production of Long Chain Acyl-CoA

“…We studied ␤-cells exposed to states of excess fatty acids in which glucose-induced insulin secretion is preserved, and we have identified another potential regulatory role for PC and its downstream pathways. We cultured islets for 4 days with 0.25 mmol/liter oleate and 5.5 mmol/liter glucose, and we observed that glucose oxidation was increased despite pyruvate dehydrogenase (PDH) activity being lowered by 30% (17). We made a similar finding in isolated islets from insulin-resistant, hyperlipidemic, nondiabetic Zucker fatty rats (18).…”

“…Islets are more complex because of the presence and high activity of PC (22). In our studies of fatty acid-associated lowering of PDH activity (17,18), PC activity was unaffected or slightly increased, and flux through the malate-pyruvate shuttle was more than doubled, as was the concentration of pyruvate. We proposed from these findings that islets are protected against the mitochondrial dysmetabolism that occurs with excess fatty acids in other tissues because of having an alternate pyruvate metabolism pathway through PC and that diversion through this pathway results in augmented flux through the pyruvate shuttles, which raises the level of cytoplasmic pyruvate to an extent that overcomes the lowered PDH activity through mass action.…”

“…Malic enzyme was measured as described previously (17). Islets (100) were sonicated in 100 l of 50 mmol/liter triethanolamine, pH 7.4, 3 mmol/liter MnCl 2 , 0.02% BSA.…”

“…CO 2 was trapped in the filter paper by injecting 100 l of 1 N KOH into the center well. For PDH activity, we set 1 unit equal to 1 mol of CO 2 Malate dehydrogenase was measured as described previously (17). Islets (100) were homogenized in 100 l of 10 mmol/liter HEPES, pH 7.4, 250 mmol/liter sucrose, 2.5 mmol/liter EDTA, 2 mmol/liter cysteine, 0.02% BSA.…”

“…Active PDH was measured as described previously (17). Islets (300) were homogenized on ice in 0.3 ml of 50 mmol/liter HEPES, pH 7.5, 0.2 mmol/liter KCl, 3 mmol/liter EDTA, 5 mmol/liter dithiothreitol, 0.1 mmol/liter N ␣ -p-tosyl-L-lysine chloromethyl ketone, 0.1 mmol/liter trypsin inhibitor, 0.02 trypsin inhibitory units/ml aprotinin, 2% rat serum, 0.25% (v/v) Triton X-100; then freeze-thawed three times; and passed through a 0.5-ml insulin syringe 10 times.…”