Pyruvate dehydrogenase activities and rates of lipogenesis during the fed-to-starved transition in liver and brown adipose tissue of the rat.

The percentages of pyruvate dehydrogenase complex (PDH) in the active form (PDHa) in two lipogenic tissues (liver and brown adipose tissue) in the fed state were 12.0% and 13.4% respectively. After acute (0.5 h) insulin treatment, PDHa activities had increased by 77% in liver and by 234% in brown fat. Significant decreases in PDHa activities were observed in both tissues by 5 h after the removal of food. The patterns of decline in PDHa activities in the two lipogenic tissues were similar in that the major decreases in activities were observed within the first 7 h of starvation. The significant decreases in PDHa activities observed after starvation for 6 h were accompanied by decreased rates of lipogenesis. Hepatic and brown-fat PDHa activities after acute (30 min) exposure to exogenous insulin were less in 6 h-starved than in fed rats, but the absolute increases in PDHa activities over the 30 min exposure period were similar in fed and 6 h-starved rats. Increases in PDHa activities were paralleled by increases in lipid synthesis in both tissues. Re-activation of PDH in response to insulin treatment or chow re-feeding after 48 h starvation occurred more rapidly in brown adipose tissue than in liver. The results are discussed in relation to the importance of the activity of the PDH complex as a determinant of the total rate of lipogenesis during the fed-to-starved transition and after insulin challenge or re-feeding.     

Comparison of tissue pyruvate dehydrogenase activities on re-feeding rats fed ad libitum or meal-fed rats with a chow-diet meal.

Meal-fed rats and rats fed ad libitum had similar rates of hepatic glycogenesis at 60 min after the initiation of re-feeding a chow meal after 22 h starvation, but hepatic PDHa (active form of pyruvate dehydrogenase) activities were 4-fold higher in the meal-fed group. In heart, PDHa activities were 3-fold higher before re-feeding and 2-fold higher after re-feeding in the meal-fed group compared with the group fed ad lib. The blood metabolite profile suggested diminished fat oxidation in starved meal-fed rats and accelerated flux through PDH in meal-fed re-fed rats compared with the group fed ad lib.     

Pyruvate overrides inhibition of PDH during exercise after a low-carbohydrate diet

The effects of carbohydrate deprivation on the regulation of pyruvate dehydrogenase (PDH) were studied at rest and during moderate-intensity exercise. An inhibitory effect of a chronic low-carbohydrate diet (LCD) on the active form of PDH (PDHa) mediated by a stable increase in PDH kinase (PDHK) activity has recently been reported (Peters SJ, Howlett RA, St. Amand TA, Heigenhauser GJF, and Spriet LL. Am J Physiol Endocrinol Metab 275: E980-E986, 1998.). In the present study, seven males cycled at 65% maximal O(2) uptake for 30 min after a 6-day LCD. Exercise was repeated 1 wk later after a mixed diet (MD). Muscle biopsies were sampled from the vastus lateralis at rest and at 2 and 30 min of exercise. At rest, PDHa activity (0.18 +/- 0.04 vs. 0.63 +/- 0.18 mmol x min(-1) x kg wet wt(-1)), muscle glycogen content (310.2 +/- 36.9 vs. 563.9 +/- 32.6 mmol/kg dry wt), and muscle lactate content (2.6 +/- 0.3 vs. 4.2 +/- 0.6 mmol/kg dry wt) were significantly lower after the LCD. Resting muscle acetyl-CoA (10.8 +/- 1.9 vs. 7.4 +/- 0.8 micromol/kg dry wt) and acetylcarnitine (5.3 +/- 1.4 vs. 1.6 +/- 0.3 mmol/kg dry wt) contents were significantly elevated after the LCD. During exercise, PDHa, glycogenolytic rate (LCD 5.8 +/- 0.4 vs. MD 6.9 +/- 0.2 mmol x min(-1) x kg dry wt(-1)), and muscle concentrations of acetylcarnitine, pyruvate, and lactate increased to the same extent in both conditions. The results of the present study suggest that inhibition of resting PDH by elevated PDHK activity after a LCD may be overridden by the availability of muscle pyruvate during exercise.     

Pyruvate dehydrogenase activity in several tissues of genetically obese hyperglycemic mice

The active form (PDHa) and total activity of pyruvate dehydrogenase (PDH) were measured in homogenates from heart muscle, epididymal fat pads and liver of genetically obese hyperglycemic mice and compared with similar data derived from lean controls or Swiss albino mice. Both PDHa and total PDH activities were similar in heart muscle from all mice with a precipitous decrease in the PDHa upon fasting. Adipose tissue and liver of obese mice had a PDHa level that was almost two-fold higher than either lean control or Swiss albino mice. Fasting for 24 hours decreased the elevated activity of PDHa in adipose tissue and liver in obese mice to a value that was comparable to lean control or Swiss albino mice, fasted similarly. The elevation in both the active form and total activity of pyruvate dehydrogenase in livers from obese mice could explain the increased provision of acetyl-CoA units necessary for the accelerated hepatic lipogenesis observed with this mouse, a model for human obesity and insulin resistance.     

Regulation of PDH in human arm and leg muscles at rest and during intense exercise

To test the hypothesis that pyruvate dehydrogenase (PDH) is differentially regulated in specific human muscles, regulation of PDH was examined in triceps, deltoid, and vastus lateralis at rest and during intense exercise. To elicit considerable glycogen use, subjects performed 30 min of exhaustive arm cycling on two occasions and leg cycling exercise on a third day. Muscle biopsies were obtained from deltoid or triceps on the arm exercise days and from vastus lateralis on the leg cycling day. Resting PDH protein content and phosphorylation on PDH-E1 alpha sites 1 and 2 were higher (P < or = 0.05) in vastus lateralis than in triceps and deltoid as was the activity of oxidative enzymes. Net muscle glycogen utilization was similar in vastus lateralis and triceps ( approximately 50%) but less in deltoid (likely reflecting less recruitment of deltoid), while muscle lactate accumulation was approximately 55% higher (P < or = 0.05) in triceps than vastus lateralis. Exercise induced (P < or = 0.05) dephosphorylation of both PDH-E1 alpha site 1 and site 2 in all three muscles, but it was more pronounced at PDH-E1 alpha site 1 in triceps than in vastus lateralis (P < or = 0.05). The increase in activity of the active form of PDH (PDHa) after 10 min of exercise was more marked in vastus lateralis ( approximately 246%) than in triceps ( approximately 160%), but when it was related to total PDH-E1 alpha protein content, no difference was evident. In conclusion, PDH protein content seems to be related to metabolic enzyme profile, rather than myosin heavy chain composition, and less PDH capacity in triceps is a likely contributing factor to higher lactate accumulation in triceps than in vastus lateralis.     

The acute effects of differential dietary fatty acids on human skeletal muscle pyruvate dehydrogenase activity.

Pyruvate dehydrogenase (PDH) is an important regulator of carbohydrate oxidation during exercise, and its activity can be downregulated by an increase in dietary fat. The purpose of this study was to determine the acute metabolic effects of differential dietary fatty acids on the activation of the PDH complex (PDHa activity) at rest and at the onset of moderate-intensity exercise. University-aged male subjects (n = 7) underwent two fat-loading trials spaced at least 2 wk apart. Subjects consumed approximately 300 g saturated (SFA) or n-6 polyunsaturated fatty acid (PUFA) fat over the course of 5 h. Following this, participants cycled at 65% of their maximum oxygen uptake for 15 min. Muscle biopsies were taken before and following fat loading and at 1 min exercise. Plasma free fatty acids increased from 0.15 +/- 0.07 to 0.54 +/- 0.19 mM over 5 h with SFA and from 0.11 +/- 0.04 to 0.35 +/- 0.13 mM with n-6 PUFA and were significantly lower throughout the n-6 PUFA trial. PDHa activity was unchanged following fat loading but increased at the onset of exercise in the SFA trial, from 1.18 +/- 0.27 to 2.16 +/- 0.37 mmol x min(-1) x kg wet wt(-1). This effect was negated in the n-6 PUFA trial (1.04 +/- 0.20 to 1.28 +/- 0.36 mmol x min(-1) x kg wet wt(-1)). PDH kinase was unchanged in both trials, suggesting that the attenuation of PDHa activity with n-6 PUFA was a result of changes in the concentrations of intramitochondrial effectors, potentially intramitochondrial NADH or Ca(2+). Our findings suggest that attenuated PDHa activity contributes to the preferential oxidation of n-6 PUFA during moderate-intensity exercise.     

Effects of insulin on pyruvate dehydrogenase in circulating lymphocytes from normal and diabetic rats

1. The in vivo and in vitro conditions which allow a response of rat circulating lymphocyte PDH to insulin are investigated. 2. In vivo tests show that inactive PDH (PDHi) prevails in diabetic rats and active PDH (PDHa) in hyperinsulinemic rats; in treated with insulin diabetic rats the PDHa/PDHi ratio (1.7) is similar to that of normal rats (PDHa/PDHi ratio = 2). 3. In vitro tests show a responsiveness of PDH to insulin only when 50 microM Ca2+ -Mg2+ and intact lymphocytes are used in the incubation medium. Insulin concentrations and contact time are important variables.     

Effect of insulin on pyruvate dehydrogenase in a mixture of plasma membranes and mitochondria from normal and alloxan treated rat brains

In a mixture of plasma membranes/mitochondria from normal rat brain, pyruvate dehydrogenase (PDH) is present in the active (PDHa) and the inactive (PDHi) form; the latter is converted into the former by preincubation with Ca2+ and Mg2+ and represents about 40% of total PDH (PDHt = PDHa + PDHi). Incubation with increasing insulin concentrations activates PDHa and PDHt, the maximum being reached at 25 microU/ml insulin; inhibition appears with further insulin increase. In a mixture of plasma membranes and mitochondria from alloxan rat brain PDHa activity markedly decreases; no activation is achieved by preincubation with Ca2+ and Mg2+. However an activating effect of Ca2+ and Mg2+ appears when the mixture is added and incubated with increasing insulin concentrations. PDHa and PDHt activity reaches a maximum of stimulation at 25 microU/ml insulin; the activation is reduced at higher concentrations of insulin though no inhibition appears. ATP partially inhibits PDHa in normal and alloxan rat brain plasma membrane/mitochondria mixtures; this effect is completely cancelled by 25 microU/ml insulin.     

Effects of reduced free fatty acid availability on skeletal muscle PDH activation during aerobic exercise. Pyruvate dehydrogenase

This study investigated the effect of reduced free fatty acid (FFA) availability on pyruvate dehydrogenase activation (PDHa) and carbohydrate metabolism during moderate aerobic exercise. Eight active male subjects cycled for 40 min at 55% Vo(2 peak) on two occasions. During one trial, subjects ingested 20 mg/kg body mass of the antilipolytic drug nicotinic acid (NA) during the hour before exercise to reduce FFA. Nothing was ingested in the control trial (CON). Blood and expired gas measurements were obtained throughout the trials, and muscle biopsy samples were obtained immediately before exercise and at 5, 20, and 40 min of exercise. Plasma FFA were lower in the NA trial (0.13 +/- 0.01 vs. 0.48 +/- 0.03 mM, P < 0.05), and the respiratory exchange ratio (RER) was increased with NA (0.93 +/- 0.01 vs. 0.89 +/- 0.01, P < 0.05), resulting in a 14.5 +/- 1.8% increase in carbohydrate oxidation compared with CON. PDHa increased rapidly in both trials at exercise onset but was approximately 15% higher (P < 0.05) throughout exercise in the NA trial (2.44 +/- 0.19 and 2.07 +/- 0.12 mmol x kg wet muscle(-1) x min(-1) for NA and CON at 40 min). Muscle glycogenolysis was 15.3 +/- 9.6% greater in the NA trial vs. the CON trial but did not reach statistical significance. Glucose 6-phosphate contents were elevated (P < 0.05) in the NA trial at 30 and 40 min of exercise, but pyruvate and lactate contents were unaffected. These data demonstrate that the reduction of exogenous FFA availability increased the activation of PDH and carbohydrate oxidation during moderate aerobic exercise in men. The increased activation of PDH was not explained by changes in muscle pyruvate or the ATP/ADP ratio but may be related to a decrease in the NADH/NAD(+) ratio or an epinephrine-induced increase in calcium concentration.     

"In vitro" effects of insulin on the PDH complex of the isolated perfused heart of rats fed a sucrose-rich diet.

We have recently reported that the "in situ" myocardial concentrations of the active form of the Pyruvate Dehydrogenase Complex (PDHa) were significantly decreased in hearts obtained from normal rats fed for 3 weeks on an isocaloric sucrose rich (63%) diet (SRD) when compared to age matched controls fed on the standard laboratory chow (STD). Since, on the one hand SRD rats present glucose intolerance and impaired "in vivo" insulin action and, on the other hand the effects of insulin on the interconversion of heart PDH remains a controversial matter, we found it relevant to study the effects of insulin on the PDH complex in the "in vitro" perfused (Langendorff technique) heart preparations obtained from SRD rats. After a 35 minute perfusion period with 5.5 mM glucose as the only nutrient in the perfusate, PDHa as a percentage of total PDH was found to remain significantly lower in SRD hearts (M +/- SEM 32.6 +/- 2.3) when compared to STD hearts (68.3 +/- 4.6, P less than 0.05) in spite of comparable total PDH activities in both groups of animals. Although the addition of insulin to the perfusate (20 mu/ml) resulted in a significant increase in the percentage of PDHa (45.8 +/- 3.4) of SRD heart, values attained still remained significantly lower than those obtained in STD controls (67.5 +/- 3.6; P less than 0.05). Simultaneously, the addition of insulin to the perfusate, significantly reduced the Acetyl-CoA/CoASH ratio in SRD hearts although this ratio remained still much higher than those observed in STD controls under the same experimental conditions.     

Exercise-induced pyruvate dehydrogenase activation is not affected by 7 days of bed rest

To test the hypothesis that physical inactivity impairs the exercise-induced modulation of pyruvate dehydrogenase (PDH), six healthy normally physically active male subjects completed 7 days of bed rest. Before and immediately after the bed rest, the subjects completed an oral glucose tolerance test (OGTT) and a one-legged knee extensor exercise bout [45 min at 60% maximal load (W(max))] with muscle biopsies obtained from vastus lateralis before, immediately after exercise, and at 3 h of recovery. Blood samples were taken from the femoral vein and artery before and after 40 min of exercise. Glucose intake elicited a larger (P ≤ 0.05) insulin response after bed rest than before, indicating glucose intolerance. There were no differences in lactate release/uptake across the exercising muscle before and after bed rest, but glucose uptake after 40 min of exercise was larger (P ≤ 0.05) before bed rest than after. Muscle glycogen content tended to be higher (0.05< P ≤ 0.10) after bed rest than before, but muscle glycogen breakdown in response to exercise was similar before and after bed rest. PDH-E1α protein content did not change in response to bed rest or in response to the exercise intervention. Exercise increased (P ≤ 0.05) the activity of PDH in the active form (PDHa) and induced (P ≤ 0.05) dephosphorylation of PDH-E1α on Ser2?3, Ser2?? and Ser3??, with no difference before and after bed rest. In conclusion, although 7 days of bed rest induced whole body glucose intolerance, exercise-induced PDH regulation in skeletal muscle was not changed. This suggests that exercise-induced PDH regulation in skeletal muscle is maintained in glucose-intolerant (e.g., insulin resistant) individuals.     

Time courses of the responses of pyruvate dehydrogenase activities to short-term starvation in diaphragm and selected skeletal muscles of the rat.

In the fed state, the percentages of the pyruvate dehydrogenase complex (PDH) in the active form (PDHa) in diaphragm and a selection of skeletal muscles (adductor longus, soleus, extensor digitorum longus, tibialis anterior, gastrocnemius) ranged from 8% (soleus) to 38% (gastrocnemius). Major decreases in PDHa activities in all of these muscles were observed after 15 h of starvation, by which time activities were less than 40% of the fed values. In general, the response to starvation was observed more rapidly in muscles of high oxidative capacity. The patterns of changes in skeletal-muscle PDH activities during the fed-to-starved transition are discussed in relation to changes in lipid-fuel supply and oxidation.     

Effect of maternal exercise on fetal liver glycogen late in gestation in the rat

To determine the effect of maternal exercise on fetal liver glycogen content, fed and fasted rats that were pregnant for 20.5 or 21.5 days were run on a rodent treadmill for 60 min at 12 m/min with a 0% grade or 16 m/min up a 10% grade. The rats were anesthetized by intravenous injection of pentobarbital sodium, and fetal and maternal liver and plasma samples were collected and frozen. Fetal liver glycogenolysis did not occur as a result of maternal exercise. Fetal blood levels of lactate increased 22-60%, but glucose, plasma glucagon, and insulin were unchanged during maternal exercise. Maternal liver glycogen decreased as a result of exercise in all groups of rats except the fasted 20.5-day-pregnant group. Plasma free fatty acids increased in all groups and blood lactate increased in fed (20.5 days) and fasted (21.5 days) pregnant rats. Maternal glucose, glucagon, and insulin values remained constant during exercise. The fetus appears to be well-protected from metabolic stress during moderate-intensity maternal exercise.     

Effect of phenylephrine on pyruvate dehydrogenase activity in rat hepatocytes and its interaction with insulin and glucagon

In isolated rat hepatocytes phenylephrine promotes a rapid increase in the amount of pyruvate dehydrogenase present in its active form (PDHa). This action is mediated by alpha 1-adrenergic receptors and is not observed in Ca2+-depleted hepatocytes. It is mimicked by the Ca2+ ionophore A23187. No changes in metabolites known to affect PDH activity are measured 3 min after addition of phenylephrine. Glucagon also increases PDHa, its action is additive to that of phenylephrine. The action of phenylephrine on PDHa could be explained by an increase in mitochondrial free Ca2+.     

Insulin-induced hypoglycemia in fed and fasted exercising rats.

To determine running performance and hormonal and metabolic responses during insulin-induced hypoglycemia, fed and fasted male rats (315 +/- 3 g) were infused with insulin (100 mU/ml, 1.5 ml/h) or saline (1.5 ml/h) for 60 min and then killed at rest or after running on the treadmill (21 m/min, 15% grade). Insulin-infused fed rats ran poorly during the second 10 min of a 20-min exercise test. They were capable of running a total of 43 +/- 5 min, compared with 138 +/- 6 min for saline-infused fed rats. Fasted insulin-infused rats were able to run only 12.8 +/- 0.8 min, compared with 122 +/- 15 min for fasted saline-infused rats. In fasted rats, blood glucose was 1.6 +/- 0.1 mM after 60 min of insulin infusion and 1.2 +/- 0.1 mM after running to exhaustion. Artificial increase of plasma free fatty acids had no effect on performance. Intravenous infusion of glucose at the time of fatigue produced an immediate recovery, allowing the formerly fatigued rats to run 20 min without development of fatigue. These results provide evidence that severe hypoglycemia can be a significant cause of fatigue, even if it occurs early in the course of an exercise bout.     

Down‐regulation of Pyruvate Dehydrogenase Phosphatase in Obese Subjects is a Defect that Signals Insulin Resistance

Objective: The objective of this study was to determine whether down‐regulation of pyruvate dehydrogenase phosphatase (PDP) is responsible for poorly active pyruvate dehydrogenase (PDH) in circulating lymphocytes (CLs) of obese subjects (ObS), and if so, whether it improves when their plasma insulin rises. Research Methods and Procedures: PDH activity was compared in lysed CLs of 10 euglycemic ObS and 10 sex‐ and age‐matched controls before and during plasma insulin enhancement in an oral glucose tolerance test. It was evaluated without (PDHa) or with Mg/Ca or Mg at various concentrations to assess PDP1 or PDP2 activities or with Mg/Ca and exogenous PDP to determine total PDH activity (PDHt), which is an indirect measure of the amount of PDH. The insulin sensitivity index was calculated, and PDP1 and PDP2 mRNA was sought in the CLs. Results: At T₀ in ObS, PDHt was normal, whereas PDHa and PDP1 activity was below normal at all Mg/Ca concentrations. PDP2 activity was undetectable in both groups. PDP1 and PDP2 mRNA was identified, and insulin sensitivity index and PDHa were directly correlated. During the oral glucose tolerance test, plasma insulin rose considerably more in ObS than in controls; PDHa and PDP1 activity also increased but remained significantly below normal, and PDHt was unvaried in both groups. Discussion: PDP1 is down‐regulated in CLs of ObS because it is poorly sensitive to Mg/Ca; this defect is attenuated when plasma insulin is greatly enhanced.     

Activation of glycogen synthase in myocardium induced by intermittent hypoxia is much lower in fasted than in fed rats

Obstructive sleep apnea is characterized by intermittent obstruction of the upper airway, which leads to intermittent hypoxia. Myocardial glycogen is a major energy resource for heart during hypoxia. Previous studies have demonstrated that intermittent hypoxia rapidly degrades myocardial glycogen and activates glycogen synthase (GS). However, the underlying mechanisms remain undefined. Because sleep apnea/intermittent hypoxia usually happens at night, whether intermittent hypoxia leads to GS activation in the postabsorptive state is not known. In the present study, male adult rats were studied after either an overnight fast or ad libitum feeding with or without intermittent ventilatory arrest (3 90-s periods at 10-min intervals). Hearts were quickly excised and freeze-clamped. Intermittent hypoxia induced a significant decrease in myocardial glycogen content in fed rats and stimulated GS in both fasted and fed rats. However, the portion of GS in the active form increased by approximately 38% in fasted rats compared with a larger, approximately 130% increase in fed rats. The basal G-6-P content was comparable in fasted and fed animals and increased approximately threefold after hypoxia. The basal phosphorylation states of Akt and GSK-3beta and the activity of protein phosphatase 1 (PP1) were comparable between fasted and fed control rats. Hypoxia significantly increased Akt phosphorylation and PP1 activity only in fed rats. In contrast, hypoxia did not induce significant change in GSK-3beta phosphorylation in either fasted or fed rats. We conclude that hypoxia activates GS in fed rat myocardium through a combination of rapid glycogenolysis, elevated local G-6-P content, and increased PP1 activity, and fasting attenuates this action independent of local G-6-P content.     

PDH activation during in vitro muscle contractions in PDH kinase 2 knockout mice: effect of PDH kinase 1 compensation

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate oxidation in skeletal muscle. PDH is deactivated by a set of PDH kinases (PDK1, PDK2, PDK3, PDK4), with PDK2 and PDK4 being the most predominant isoforms in skeletal muscle. Although PDK2 is the most abundant isoform, few studies have examined its physiological role. The role of PDK2 on PDH activation (PDHa) at rest and during muscle stimulation at 10 and 40 Hz (eliciting low- and moderate-intensity muscle contractions, respectively) in isolated extensor digitorum longus muscles was studied in PDK2 knockout (PDK2KO) and wild-type (WT) mice (n = 5 per group). PDHa activity was unexpectedly 35 and 77% lower in PDK2KO than WT muscle (P = 0.043), while total PDK activity was nearly fourfold lower in PDK2KO muscle (P = 0.006). During 40-Hz contractions, initial force was lower in PDK2KO than WT muscle (P < 0.001) but fatigued similarly to ～75% of initial force by 3 min. There were no differences in initial force or rate of fatigue during 10-Hz contractions. PDK1 compensated for the lack of PDK2 and was 1.8-fold higher in PDK2KO than WT muscle (P = 0.019). This likely contributed to ensuring that resting PDHa activity was similar between the groups and accounts for the lower PDH activation during muscle contraction, as PDK1 is a very potent inhibitor of the PDH complex. Increased PDK1 expression appears to be regulated by hypoxia inducible factor-1α, which was 3.5-fold higher in PDK2KO muscle. It is clear that PDK2 activity is essential, even at rest, in regulation of carbohydrate oxidation and production of reducing equivalents for the electron transport chain. In addition, these results underscore the importance of the overall kinetics of the PDK isoform population, rather than total PDK activity, in determining transformation of the PDH complex and PDHa activity during muscle contraction.     

Activation of pyruvate dehydrogenase by insulin in isolated brown fat cells.

Isolated fat cells from rat brown adipose tissue in vitro respond to insulin with an increase of pyruvate dehydrogenase (EC 1.2.4.1) activity due to conversion of the inactive form of the enzyme (PDHb) to the active form (PDHa). Like in white adipocytes this effect depends on the presence of glucose or 2-deoxyglucose in the medium. The interrelationship between the steady state of the PDH-system and the phosphorylation state of the adenine nucleotides was studied in white adipose tissue. While insulin in the presence of 2-deoxyglucose caused a large fall of the tissue ATP/ADP ratio which could explain the increase of PDHa activity, the ATP/ADP ratio remained unchanged during incubations with insulin and glucose. Thus it appears that other factors than the ATP/ADP ratio are involved in the regulation of PDH activity by insulin the nature of which remains to be elucidated.     

The effect of insulin on the uptake and metabolic fate of glucose in isolated perfused hearts of dyslipemic rats

Male Wistar rats chronically (15 weeks) fed a sucrose-rich diet (SRD; 63% w/w) developed hypertriglyceridemia and impaired glucose homeostasis. Hearts from these animals were isolated and perfused using the Langendorff recirculating method. Glucose at levels similar to those found in the animal in vivo was used as the only exogenous substrate. The hearts were perfused for 30 minutes in the presence or absence of insulin (30 mU/mL) in the perfusion medium. In the absence of the hormone, glucose uptake was impaired and the glucose utilization was reduced, with a significant increase of lactate release. Glucose oxidation, which was estimated from the activation state of the enzyme pyruvate dehydrogenase complex (PDHc), was depressed mainly due to both an increase of PDH kinase and a decrease of PDHa (active form of PDHc) activities. Although the addition of insulin in the perfusion medium improved the above parameters, it was unable to normalize them. The present results suggest that at least two different mechanisms might contribute to insulin resistance and to the impaired glucose metabolism in the perfused hearts of the dyslipemic SRD-fed animals: (1) reduced basal and insulin-stimulated glucose uptake and its utilization or (2) increased availability and oxidation of lipids (low PDHa and high PDH kinase activities), which in turn decrease glucose uptake and utilization. Thus, this nutritional experimental model may be useful to study how impaired glucose homeostasis, increases plasma free fatty acid levels and hypertriglyceridemia could contribute to heart tissue malfunction.