Reactivation of peroxisome proliferator-activated receptor alpha is associated with contractile dysfunction in hypertrophied rat heart

In pressure overload-induced hypertrophy, the heart increases its reliance on glucose as a fuel while decreasing fatty acid oxidation. A key regulator of this substrate switching in the hypertrophied heart is peroxisome proliferator-activated receptor alpha (PPARalpha). We tested the hypothesis that down-regulation of PPARalpha is an essential component of cardiac hypertrophy at the levels of increased mass, gene expression, and metabolism by pharmacologically reactivating PPARalpha. Pressure overload (induced by constriction of the ascending aorta for 7 days in rats) resulted in cardiac hypertrophy, increased expression of fetal genes (atrial natriuretic factor and skeletal alpha-actin), decreased expression of PPARalpha and PPARalpha-regulated genes (medium chain acyl-CoA dehydrogenase and pyruvate dehydrogenase kinase 4), and caused substrate switching (measured ex vivo in the isolated working heart preparation). Treatment of rats with the specific PPARalpha agonist WY-14,643 (8 days) did not affect the trophic response or atrial natriuretic factor induction to pressure overload. However, PPARalpha activation blocked skeletal alpha-actin induction, reversed the down-regulation of measured PPARalpha-regulated genes in the hypertrophied heart, and prevented substrate switching. This PPARalpha reactivation concomitantly resulted in severe depression of cardiac power and efficiency in the hypertrophied heart (measured ex vivo). Thus, PPARalpha down-regulation is essential for the maintenance of contractile function of the hypertrophied heart.

Load-induced changes in vivo alter substrate fluxes and insulin responsiveness of rat heart in vitro

It has been observed that opposite changes in cardiac workload result in similar changes in cardiac gene expression. In the current study, the hypothesis that altered gene expression in vivo results in altered substrate fluxes in vitro was tested. Hearts were perfused for 60 minutes with Krebs-Henseleit buffer containing glucose (5 mmol/L) and oleate (0.4 mmol/L). At 30 minutes, either insulin (1 mU/mL) or epinephrine (1 micromol/L) was added. Hearts weighed 35% less after unloading and 25% more after aortic banding. Contractile function in vitro was decreased in transplanted and unchanged in banded hearts. Epinephrine, but not insulin, increased cardiac power. Basal glucose oxidation was initially decreased and then increased by aortic banding. The stimulatory effects of insulin or epinephrine on glucose oxidation were reduced or abolished by unloading, and transiently reduced by banding. Oleate oxidation correlated with cardiac power both before and after stimulation with epinephrine, whereas glucose oxidation correlated only after stimulation. Malonyl-coenzyme A levels did not correlate with rates of fatty acid oxidation. Pyruvate dehydrogenase was not affected by banding or unloading. It was concluded that atrophy and hypertrophy both decrease insulin responsiveness and shift myocardial substrate preference to glucose, consistent with a shift to a fetal pattern of energy consumption; and that the isoform-specific changes that develop in vivo do not change the regulation of key metabolic enzymes when assayed in vitro.

[5-3H]glucose overestimates glycolytic flux in isolated working rat heart: role of the pentose phosphate pathway

We set out to study the pentose phosphate pathway (PPP) in isolated rat hearts perfused with [5-3H]glucose and [1-14C]glucose or [6-14C]glucose (crossover study with 1- then 6- or 6- then 1-14C-labeled glucose). To model a physiological state, hearts were perfused under working conditions with Krebs-Henseleit buffer containing 5 mM glucose, 40 microU/ml insulin, 0.5 mM lactate, 0.05 mM pyruvate, and 0.4 mM oleate/3% albumin. The steady-state C1/C6 ratio (i.e., the ratio from [1-14C]glucose to [6-14C]glucose) of metabolites released by the heart, an index of oxidative PPP, was not different from 1 (1.06 +/- 0.19 for 14CO2, and 1.00 +/- 0.01 for [14C]lactate + [14C]pyruvate, mean +/- SE, n = 8). Hearts exhibited contractile, metabolic, and 14C-isotopic steady state for glucose oxidation (14CO2 production). Net glycolytic flux (net release of lactate + pyruvate) and efflux of [14C]lactate + [14C]pyruvate were the same and also exhibited steady state. In contrast, flux based on 3H2O production from [5-3H]glucose increased progressively, reaching 260% of the other measures of glycolysis after 30 min. The 3H/14C ratio of glycogen (relative to extracellular glucose) and sugar phosphates (representing the glycogen precursor pool of hexose phosphates) was not different from each other and was <1 (0.36 +/- 0.01 and 0.43 +/- 0.05 respectively, n = 8, P < 0.05 vs. 1). We conclude that both transaldolase and the L-type PPP permit hexose detritiation in the absence of net glycolytic flux by allowing interconversion of glycolytic hexose and triose phosphates. Thus apparent glycolytic flux obtained by 3H2O production from [5-3H]glucose overestimates the true glycolytic flux in rat heart.

Regulation of cardiac and skeletal muscle malonyl-CoA decarboxylase by fatty acids

Malonyl-CoA decarboxylase (MCD) catalyzes the degradation of malonyl-CoA, an important modulator of fatty acid oxidation. We hypothesized that increased fatty acid availability would increase the expression and activity of heart and skeletal muscle MCD, thereby promoting fatty acid utilization. The results show that high-fat feeding, fasting, and streptozotocin-induced diabetes all significantly increased the plasma concentration of nonesterified fatty acids, with a concomitant increase in both rat heart and skeletal muscle MCD mRNA. Upon refeeding of fasted animals, MCD expression returned to basal levels. Fatty acids are known to activate peroxisome proliferator-activated receptor-alpha (PPARalpha). Specific PPARalpha stimulation, through Wy-14643 treatment, significantly increased the expression of MCD in heart and skeletal muscle. Troglitazone, a specific PPARgamma agonist, decreased MCD expression. The sensitivity of MCD induction by fatty acids and Wy-14643 was soleus > extensor digitorum longus > heart. High plasma fatty acids consistently increased MCD activity only in solei, whereas MCD activity in the heart actually decreased with high-fat feeding. Pressure overload-induced cardiac hypertrophy, in which PPARalpha expression is decreased (and fatty acid oxidation is decreased), resulted in decreased MCD mRNA and activity, an effect that was dependent on fatty acids. The results suggest that fatty acids induce the expression of MCD in rat heart and skeletal muscle. Additional posttranscriptional mechanisms regulating MCD activity appear to exist.

Improved energy homeostasis of the heart in the metabolic state of exercise

We postulate that metabolic conditions that develop systemically during exercise (high blood lactate and high nonesterified fatty acids) are favorable for energy homeostasis of the heart during contractile stimulation. We used working rat hearts perfused at physiological workload and levels of the major energy substrates and compared the metabolic and contractile responses to an acute low-to-high work transition under resting versus exercising systemic metabolic conditions (low vs. high lactate and nonesterified fatty acids in the perfusate). Glycogen preservation, resulting from better maintenance of high-energy phosphates, was a consequence of improved energy homeostasis with high fat and lactate. We explained the result by tighter coupling between workload and total beta-oxidation. Total fatty acid oxidation with high fat and lactate reflected increased availability of exogenous and endogenous fats for respiration, as evidenced by increased long-chain fatty acyl-CoA esters (LCFA-CoAs) and by an increased contribution of triglycerides to total beta-oxidation. Triglyceride turnover (synthesis and degradation) also appeared to increase. Elevated LCFA-CoAs caused high total beta-oxidation despite increased malonyl-CoA. The resulting bottleneck at mitochondrial uptake of LCFA-CoAs stimulated triglyceride synthesis. Our results suggest the following. First, both malonyl-CoA and LCFA-CoAs determine total fatty acid oxidation in heart. Second, concomitant stimulation of peripheral glycolysis and lipolysis should improve cardiac energy homeostasis during exercise. We speculate that high lactate contributes to the salutary effect by bypassing the glycolytic block imposed by fatty acids, acting as an anaplerotic substrate necessary for high tricarbocylic acid cycle flux from fatty acid-derived acetyl-CoA.

Regulation of fatty acid oxidation of the heart by MCD and ACC during contractile stimulation

We tested the hypothesis that the level of malonyl-CoA, as well as the corresponding rate of total fatty acid oxidation of the heart, is regulated by the opposing actions of acetyl-CoA carboxylase (ACC) and malonyl-CoA decarboxylase (MCD). We used isolated working rat hearts perfused under physiological conditions. MCD in heart homogenates was measured specifically by (14)CO(2) production from [3-(14)C]malonyl-CoA, and ACC was measured specifically based on the portion of total carboxylase that is citrate sensitive. Increased heart work (1 microM epinephrine + 40% increase in afterload) elicited a 40% increase in total beta-oxidation of exogenous plus endogenous lipids, accompanied by a 33% decrease in malonyl-CoA. The basal activity and citrate sensitivity of ACC (reflecting its phosphorylation state) and citrate content were unchanged. AMP levels were also unchanged. MCD activity, when measured at a subsaturating concentration of malonyl-CoA (50 microM), was increased by 55%. We conclude that physiological increments in AMP during the work transition are insufficient to promote ACC phosphorylation by AMP-stimulated protein kinase. Rather, increased fatty acid oxidation results from increased malonyl-CoA degradation by MCD.

Myocardial glucose uptake measured with fluorodeoxyglucose: a proposed method to account for variable lumped constants

Quantitative assessment of myocardial glucose uptake by the glucose tracer analog 2-deoxy-2-[18F]fluoro-D-glucose (FDG) depends on a correction factor (lumped constant [LC]), which may vary. We propose that this variability is caused by different affinities of FDG and glucose for membrane transport and phosphorylation and can be predicted from the time course of FDG retention. We therefore measured the LC under steady-state metabolic conditions and compared the results with values predicted from the tracer retention alone.

METHODS

We measured rates of myocardial glucose uptake by tracer ([2-3H]glucose) and tracer analog methods (FDG) in isolated working Sprague-Dawley rat hearts perfused with Krebs buffer and glucose, or glucose plus insulin or beta-hydroxybutyrate. In separate experiments, we established the theoretical upper and lower limits for the LC (Rt and Rp), which are determined by the relative rates of FDG and glucose membrane transport (Rt, 1.73 +/- 0.22) and the relative rates of FDG and glucose phosphorylation (Rp, 0.15 +/- 0.04).

RESULTS

The LC was decreased in the presence of insulin or beta-hydroxybutyrate or both (from 1.14 +/- 0.3 to 0.58 +/- 0.16 [insulin], to 0.75 +/- 0.17 [beta-hydroxybutyrate] or to 0.53 +/- 0.17 [both], P < 0.05). The time-activity curves of FDG retention reflected these changes. Combining the upper and lower limits for the LC with the ratio between unidirectional and steady-state FDG uptake rates allowed the prediction of individual LCs, which agreed well with the actually measured values (r = 0.96, P < 0.001).

CONCLUSION

The LC is not a constant but is a predictable quotient. As a result of the fixed relation between tracer and tracee for both membrane transport and phosphorylation, the quotient can be determined from the FDG time-activity curve and true rates of myocardial glucose uptake can be measured.

Insulin improves functional and metabolic recovery of reperfused working rat heart

BACKGROUND

Glucose, insulin, and potassium solution improves left ventricular function in refractory pump failure. Direct effects of insulin on the heart cannot be determined in vivo. We hypothesized that insulin has a direct positive inotropic effect on the reperfused heart.

METHODS

Isolated working rat hearts were perfused with buffer containing glucose (5 mmol/L) plus oleate (1.2 mmol/L). Hearts were subjected to 15 minutes of ischemia and reperfused with or without insulin (100 microU/mL) for 40 minutes. Epinephrine (1 micromol/L) was added for the last 20 minutes.

RESULTS

Hearts recovered 51.1% of preischemic cardiac power in the absence and 76.4% in the presence of insulin (p < 0.05). Whereas oleate oxidation remained unchanged, glucose uptake and oxidation increased during reperfusion with epinephrine (p < 0.01). This increase was significantly greater when hearts were reperfused in the presence of insulin (p < 0.01). Insulin also prevented an epinephrine-induced glycogen breakdown during reperfusion (p < 0.05).

CONCLUSIONS

Insulin has a direct positive inotropic effect on postischemic rat heart. This effect is additive to epinephrine and occurs without delay. Increased rates of glucose oxidation and net glycogen synthesis are more protracted.

Regulation of energy metabolism of the heart during acute increase in heart work

We determined the contribution of all major energy substrates (glucose, glycogen, lactate, oleate, and triglycerides) during an acute increase in heart work (1 microM epinephrine, afterload increased by 40%) and the involvement of key regulatory enzymes, using isolated working rat hearts exhibiting physiologic values for contractile performance and oxygen consumption. We accounted for oxygen consumption quantitatively from the rates of substrate oxidation, measured on a minute-to-minute basis. Total beta-oxidation (but not exogenous oleate oxidation) was increased by the work jump, consistent with a decrease in the level of malonyl-CoA. Glycogen and lactate were important buffers for carbon substrate when heart work was acutely increased. Three mechanisms contributed to high respiration from glycogen: 1) carbohydrate oxidation was increased selectively; 2) stimulation of glucose oxidation was delayed at glucose uptake; and 3) glycogen-derived pyruvate behaved differently from pyruvate derived from extracellular glucose. Despite delayed activation of pyruvate dehydrogenase relative to phosphorylase, glycogen-derived pyruvate was more tightly coupled to oxidation. Also, glycogen-derived lactate plus pyruvate contributed to an increase in the relative efflux of lactate versus pyruvate, thereby regulating the redox. Glycogen synthesis resulted from activation of glycogen synthase late in the protocol but was timed to minimize futile cycling, since phosphorylase a became inhibited by high intracellular glucose.

Insulin does not change the intracellular distribution of hexokinase in rat heart

Preliminary evidence has suggested that hexokinase in rat heart changes its kinetic properties in response to insulin through translocation to the outer mitochondrial membrane. We reexamined this hypothesis in light of tracer kinetic evidence to the contrary. Our methods were as follows. Working rat hearts were perfused with Krebs-Henseleit buffer containing glucose (5 mmol/l) and sodium oleate (0.4 mmol/l). Dynamic glucose uptake was measured with [2-3H]glucose and with 2-deoxy-2-[18F]fluoroglucose (2-[18F]DG). Hexokinase activity was determined in the cytosolic and mitochondrial fractions. Our results are as follows. Uptake of glucose and uptake of 2-[18F]DG were parallel. Insulin (1 mU/ml) increased glucose uptake threefold but had no effect on 2-[18F]DG uptake. The tracer-to-tracee ratio decreased significantly. The Michaelis-Menten constant of hexokinase for 2-deoxyglucose was up to 10 times higher than for glucose. There was no difference in maximal reaction velocity. Two-thirds of hexokinase was bound to mitochondria. Insulin neither caused translocation nor changed Michaelis-Menten constant or maximal reaction velocity. In conclusion, the insulin-induced changes in the tracer-to-tracee ratio are due to a shift of the rate-limiting step for glucose uptake from transport to phosphorylation by hexokinase. Insulin does not affect the intracellular distribution or the kinetics of hexokinase.

Energy provision from glycogen, glucose, and fatty acids on adrenergic stimulation of isolated working rat hearts

We postulated that glycogen is a significant energy substrate compared with fatty acids and glucose in response to adrenergic stimulation of working rat hearts. Oxidation rates were determined at 1-min intervals by release of 3H2O from [9,10-(3)H]oleate (0.4 mM, 1% albumin) and 14CO2 from exogenous [U-14C]glucose (5 mM) or, by a pulse-chase method, from [14C]glycogen. We estimated the 14C enrichment of glycogen metabolized at each time point to determine true rates of glycogen use. Based on the pattern of glycogen enrichment over time, glycogenolysis did not exhibit a high degree of preference for newly synthesized glycogen. Epinephrine (1 microM) increased contractile performance 86% but did not stimulate oleate oxidation. The increased energy demand was supplied by carbohydrates, initially by a burst of glycogenolysis (contributing 35% to total ATP synthesis for 5 min) and followed by delayed increase in the use of exogenous glucose (eventually contributing 29% to ATP synthesis). On the basis of the release of 14CO2 and [14C]lactate specifically from glucose or glycogen, we found that a larger portion of glycogen was oxidized compared with exogenous glucose, augmenting the yield of ATP from glycogen. Thus the heart responds to an acute increase in energy demand by selective oxidation of glycogen.

Regulation of exogenous and endogenous glucose metabolism by insulin and acetoacetate in the isolated working rat heart. A three tracer study of glycolysis, glycogen metabolism, and glucose oxidation

Myocardial glucose use is regulated by competing substrates and hormonal influences. However, the interactions of these effectors on the metabolism of exogenous glucose and glucose derived from endogenous glycogen are not completely understood. In order to determine changes in exogenous glucose uptake, glucose oxidation, and glycogen enrichment, hearts were perfused with glucose (5 mM) either alone, or glucose plus insulin (40 microU/ml), glucose plus acetoacetate (5 mM), or glucose plus insulin and acetoacetate, using a three tracer (3H, 14C, and 13C) technique. Insulin-stimulated glucose uptake and lactate production in the absence of acetoacetate, while acetoacetate inhibited the uptake of glucose and the oxidation of both exogenous glucose and endogenous carbohydrate. Depending on the metabolic conditions, the contribution of glycogen to carbohydrate metabolism varied from 20-60%. The addition of acetoacetate or insulin increased the incorporation of exogenous glucose into glycogen twofold, and the combination of the two had additive effects on the incorporation of glucose into glycogen. In contrast, the glycogen content was similar for the three groups. The increased incorporation of glucose in glycogen without a significant change in the glycogen content in hearts perfused with glucose, acetoacetate, and insulin suggests increased glycogen turnover. We conclude that insulin and acetoacetate regulate the incorporation of glucose into glycogen as well as the relative contributions of exogenous glucose and endogenous carbohydrate to myocardial energy metabolism by different mechanisms.

Effects of insulin on glucose uptake by rat hearts during and after coronary flow reduction

We tested the hypothesis that low-flow ischemia increases glucose uptake and reduces insulin responsiveness. Working hearts from fasted rats were perfused with buffer containing glucose alone or glucose plus a second substrate (lactate, octanoate, or beta-hydroxybutyrate). Rates of glucose uptake were measured by 3H2O production from [2-3H]glucose. After 15 min of perfusion at a physiological workload, hearts were subjected to low-flow ischemia for 45 min, after which they were returned to control conditions for another 30 min. Insulin (1 mU/ml) was added before, during, or after the ischemic period. Cardiac power decreased by 70% with ischemia and returned to preischemic values on reperfusion in all groups. Low-flow ischemia increased lactate production, but the rate of glucose uptake during ischemia increased only when a second substrate was present. Hearts remained insulin responsive under all conditions. Insulin doubled glucose uptake when added under control conditions, during low-flow ischemia, and at the onset of the postischemic period. Insulin also increased net glycogen synthesis in postischemic hearts perfused with glucose and a second substrate. Thus insulin stimulates glucose uptake in normal and ischemic hearts of fasted rats, whereas ischemia stimulates glucose uptake only in the presence of a cosubstrate. The results are consistent with two separate intracellular signaling pathways for hexose transport, one that is sensitive to the metabolic requirements of the heart and another that is sensitive to insulin.

Substrate metabolism as a determinant for postischemic functional recovery of the heart

The mammalian myocardium meets its high energy needs through the oxidation of a variety of substrates, chiefly fatty acids. This review examines the hypothesis that efficient energy transfer in the heart occurs through a series of moiety-conserved cycles, which makes the heart an obligatory "omnivore." Ischemia results in a transformation of efficient metabolic cycles to less-efficient linear pathways. Substrate metabolism during reperfusion requires the replenishment of depleted cycles and is a major determinant for the return of contractile function. Although there is growing recognition of the concept that regulation of substrate flux through metabolic pathways is shared by several of the pathway enzymes it is apparent that glucose oxidation and glycogen resynthesis promote the return of normal contractile function in the postischemic heart. This concept is supported by clinical observations on the beneficial effects of a solution containing glucose, insulin, and potassium (GIK) for treatment of refractory left ventricular contractile failure after hypothermic ischemic arrest during cardiac surgery.

Preferential oxidation of glycogen in isolated working rat heart

We tested the hypothesis that glycogen is preferentially oxidized in isolated working rat heart. This was accomplished by measuring the proportion of glycolytic flux (oxidation plus lactate production) specifically from glycogen which is metabolized to lactate, and comparing it to the same proportion determined concurrently from exogenous glucose during stimulation with epinephrine. After prelabeling of glycogen with either 14C or 3H, a dual isotope technique was used to simultaneously trace the disposition of glycogen and exogenous glucose between oxidative and non-oxidative pathways. Immediately after the addition of epinephrine (1 microM), 40-50% of flux from glucose was directed towards lactate. Glycogen, however, did not contribute to lactate, being almost entirely oxidized. Further, glycogen utilization responded promptly to the abrupt increase in contractile performance with epinephrine, during the lag in stimulation of utilization of exogenous glucose, suggesting that glycogen serves as substrate reservoir to buffer rapid increases in demand. Preferential oxidation of glycogen may serve to ensure efficient generation of ATP from a limited supply of endogenous substrate, or as a mechanism to limit lactate accumulation during rapid glycogenolysis.

Metabolic fate of glucose in reversible low-flow ischemia of the isolated working rat heart

The acute adaptation of myocardial glucose metabolism in response to low-flow ischemia and reperfusion was investigated in isolated working rat hearts perfused with bicarbonate saline containing glucose (10 mM) and insulin (40 microU/ml). Reversible low-flow ischemia was induced by reducing coronary perfusion pressure from 100 to 35 cmH2O. Tritiated glucose was used to assess rates of glucose transport and phosphorylation, flux from glucose to pyruvate, and oxidation of exogenous glucose. Rates of glycogen synthesis and glycolysis were also assessed. With ischemia, cardiac power decreased by more than two-thirds. Rates of glucose uptake and flux from glucose to pyruvate remained unchanged, while glucose oxidation declined by 61%. Rates of lactate release more than doubled, and fractional enrichment of glycogen remained the same. During reperfusion, glucose oxidation returned to the preischemic values. When isoproterenol was added during ischemia, glucose uptake increased, glycogen decreased, and lactate release increased. No effect was seen with pacing. We conclude that during low-flow ischemia and with glucose as the only exogenous substrate, net glucose uptake remains unchanged. There is a reversible redirection between glycolysis and glucose oxidation, while glycogen synthesis continues during ischemia and is enhanced with reperfusion.

Fundamental limitations of [18F]2-deoxy-2-fluoro-D-glucose for assessing myocardial glucose uptake

BACKGROUND

The glucose tracer analog [18F]2-deoxy-2-fluoro-D-glucose (FDG) is widely used for assessing regional myocardial glucose metabolism in vivo. The reproducibility of this method has recently been questioned because of a discordant affinity of hexokinase for its substrates glucose and 2-deoxyglucose. We therefore compared rates of glucose utilization simultaneously with tissue time-activity curves of FDG uptake before and after changes in the physiological environment of the heart.

METHODS AND RESULTS

Isolated working rat hearts were perfused for 60 minutes with recirculating Krebs buffer containing glucose (10 mmol/L), FDG (1 microCi/mL), [2-3H]glucose (0.05 microCi/mL), and [U-14C]2-deoxyglucose (2-DG; 0.025 microCi/mL). Myocardial glucose uptake was measured by tracer ([2-3H]glucose) and tracer analog methods (FDG and 2-DG) before and after the addition of either insulin (1 mU/mL), epinephrine (1 mumol/L), lactate (40 mmol/L), or D,L-beta-hydroxybutyrate (40 mmol/L) at 30 minutes of perfusion and after acute changes in cardiac workload. Under steady-state conditions, myocardial rates of glucose utilization as measured by tritiated water (3H2O) production from metabolism of [2-3H]glucose, FDG uptake, and 2-DG retention were linearly related. The addition of competing substrates decreased glucose utilization immediately. The addition of insulin increased the rate of glucose utilization as measured by the glucose tracer but not as measured by the tracer analogs. The ratio of 3H2O release/myocardial FDG uptake increased by 111% after the addition of insulin, by 428% after the addition of lactate, and by 232% after the addition of beta-hydroxybutyrate. Epinephrine increased rates of glucose utilization and contractile performance, whereas there was no increase in glucose uptake with a comparable increase in workload alone. There was no change in the relation between the glucose tracer and the tracer analog either with epinephrine or with acute changes in workload.

CONCLUSIONS

The uptake and retention of FDG in heart muscle is linearly related to glucose utilization only under steady-state conditions. Addition of insulin or of competing substrates changes the relation between uptake of the glucose tracer and FDG. These observations preclude the determination of absolute rates of myocardial glucose uptake by the tracer analog method under non-steady-state conditions.

Glycogen turnover in the isolated working rat heart

The isolated working rat heart was adapted for simultaneous determination of glycogen synthesis and degradation using a dual isotope technique. After prelabeling of glycogen with [U-14C]glucose, glycogenolysis was determined continuously from the washout of 14CO2 plus [14C]lactate. Glycogen synthesis was determined during the same period from incorporation of [5-3H]glucose. In the absence of added hormones, hearts were predominantly glycogenolytic (1.5 mumol/min/g, dry weight), and there was simultaneous synthesis (11% of the rate of glycogenolysis). The percentage of glucose taken up by the heart that could traverse the glycogen pool as a consequence of glycogen turnover was minor (5%). Insulin (10 milliunits/ml) predictably stimulated glycogen synthesis (3.6-fold) and nearly abolished glycogenolysis. Addition of glucagon (1 microgram/ml) increased contractile performance and initially stimulated glycogenolysis (3.8-fold) until glycogen was largely depleted. Net tritium incorporation was unaffected by glucagon. Both hormones stimulated glycolytic flux from exogenous glucose (3H2O from [5-3H]glucose) as well as total glycolytic flux (3H2O plus glycogenolysis). The initial stimulation in total glycolytic flux with glucagon was largely from glycogen, explaining the lag in stimulation from exogenous glucose. The relationship between the specific radioactivity and amount of glycogen remaining after different degrees of glycogenolysis suggests that the preference of glycogenolysis for newly synthesized glycogen is only partial.

Dietary control and tissue specific expression of branched-chain alpha-ketoacid dehydrogenase kinase

The branched-chain alpha-ketoacid dehydrogenase complex, catalyst for the rate-limiting step of branched-chain amino acid catabolism, is controlled by a highly specific protein kinase (branched-chain alpha-ketoacid dehydrogenase kinase) that associates tightly with the complex. The activity state (proportion of the enzyme in its active, dephosphorylated state) of the complex varies dramatically in different rat tissues. The activity state of the complex in the liver is greater than that in any other tissue, and liver contains the lowest amount of kinase protein and kinase mRNA. However, protein malnutrition, a condition under which the complex is largely phosphorylated and inactive, resulted in a three- to fourfold increase in hepatic kinase activity with an accompanying increase in amounts of kinase protein and mRNA. Refeeding a 50% protein diet restored the normal activity state and the original levels of kinase protein and mRNA. The amount of kinase protein associated with the complex rather than changes in specific activity of the kinase appears responsible for observed differences in activity states of the complex in several rat tissues tested. Accordingly, the levels of kinase protein and mRNA measured are highest in tissues with greatest kinase activity (heart > kidney > liver), correlating reasonably well inversely with activity state of the branched-chain alpha-ketoacid dehydrogenase complex in the respective tissues. These observations suggest that the amount of kinase protein expressed in various tissues and in response to dietary protein deficiency is an important factor determining the activity state of the complex.

Metabolic recovery of isolated working rat heart after brief global ischemia

Functional recovery following ischemia and reperfusion in the isolated working rat heart perfused with glucose (11 mM) was examined in relation to pre- and postischemic levels of ATP, glycogen, glucose 6-phosphate, and the lactate-to-pyruvate ratio. The following variables were studied: feeding and fasting in vivo, addition of L-lactate (10 mM), dl-beta-hydroxybutyrate (10 mM), glucagon (0.01 and 1 micrograms/ml), and a 15-min anoxic perfusion before ischemia in vitro. Recovery was assessed as the percentage of preischemic power. Good correlation was found between functional recovery and the postischemic content of glycogen. Glycogen depletion by anoxia or glucagon before ischemia impaired recovery. There was no relationship among lactate produced, or the lactate-to-pyruvate ratio, and recovery. The addition of lactate or beta-hydroxybutyrate to hearts from fed rats increased the content of glycogen and glucose 6-phosphate, whereas addition of lactate, but not beta-hydroxybutyrate, improved recovery. There was a linear relationship between glycogen content and glucose 6-phosphate levels. In conclusion, the degree of return of oxidative metabolism and of net glycogen resynthesis reflects postischemic recovery of function. The results also suggest a role for anaplerosis of the citric acid cycle as an additional determinant of postischemic recovery.

A protein geranylgeranyltransferase from bovine brain: implications for protein prenylation specificity

A protein geranylgeranyltransferase (PGT) that catalyzes the transfer of a 20-carbon prenyl group from geranylgeranyl pyrophosphate to a cysteine residue in protein and peptide acceptors was detected in bovine brain cytosol and partially purified. The enzyme was shown to be distinct from a previously characterized protein farnesyltransferase (PFT). The PGT selectively geranylgeranylated a synthetic peptide corresponding to the C terminus of the gamma 6 subunit of bovine brain G proteins, which have previously been shown to contain a 20-carbon prenyl modification. Likewise, a peptide corresponding to the C terminus of human lamin B, a known farnesylated protein, selectively served as a substrate for farnesylation by the PFT. However, with high concentrations of peptide acceptors, both prenyl transferases were able to use either peptide as substrates and the PGT was able to catalyze farnesyl transfer. Among the prenyl acceptors tested, peptides and proteins with leucine or phenylalanine at their C termini served as geranylgeranyl acceptors, whereas those with C-terminal serine were preferentially farnesylated. These results suggest that the C-terminal amino acid is an important structural determinant in controlling the specificity of protein prenylation.

Spectrophotometric enzymatic assay for S-3-hydroxyisobutyrate

An enzymatic spectrophotometric end-point assay has been developed for determination of S-3-hydroxyisobutyrate in biological fluids. The assay measures NADH production at 340 nm after initiation of the reaction with rabbit liver 3-hydroxyisobutyrate dehydrogenase (EC 1.1.1.31). The assay is not affected by R-3-hydroxyisobutyrate, lactate, malate, 3-hydroxybutyrate, 2-methyl-3-hydroxybutyrate, 3-hydroxyisovalerate, 3-hydroxy-n-valerate, 2-methyl-3-hydroxy-valerate, and 3-hydroxypropionate. The assay does measure 2-ethyl-3-hydroxypropionate, a minor metabolite produced by catabolism of alloisoleucine. Application of the method to measure S-3-hydroxyisobutyrate in plasma obtained from normal, 48-h starved, and mildly and severely diabetic rats gave levels of 28, 42, 112, and 155 microM, respectively.

Regulation of the branched-chain alpha-ketoacid dehydrogenase and elucidation of a molecular basis for maple syrup urine disease

The hepatic branched-chain alpha-ketoacid dehydrogenase complex plays an important role in regulating branched-chain amino acid levels. These compounds are essential for protein synthesis but toxic if present in excess. When dietary protein is deficient, the hepatic enzyme is converted to the inactive, phosphorylated state to conserve branched-chain amino acids for protein synthesis. When dietary protein is excessive, the enzyme is in the active, dephosphorylated state to commit the excess branched-chain amino acids to degradation. Inhibition of protein synthesis by cycloheximide, even when the animal is starving for dietary protein, results in activation of the hepatic branched-chain alpha-ketoacid dehydrogenase complex to prevent accumulation of branched-chain amino acids. Likewise, the increase in branched-chain amino acids caused by body wasting during starvation and uncontrolled diabetes is blunted by activation of the hepatic branched-chain alpha-ketoacid dehydrogenase complex. The activity state of the complex is regulated in the short term by the concentration of branched-chain alpha-ketoacids (inhibitors of branched-chain alpha-ketoacid dehydrogenase kinase) and in the long term by alteration in total branched-chain alpha-ketoacid dehydrogenase kinase activity. cDNAs have been cloned and the primary structure of the mature proteins deduced for the E1 alpha subunit of the human and rat liver branched-chain alpha-ketoacid dehydrogenase complex. The cDNA and protein sequences are highly conserved for the two species. Considerable sequence similarity is also apparent between the E1 alpha subunits of the human branched-chain alpha-ketoacid dehydrogenase complex and the pyruvate dehydrogenase complex. Maple syrup urine disease is caused by an inherited deficiency in the branched-chain alpha-ketoacid dehydrogenase complex. The molecular basis of one maple syrup urine disease family has been determined for the first time. The patient was found to be a compound heterozygote, inheriting an allele encoding an abnormal E1 alpha from the father, and an allele which is not expressed from the mother. The only known animal model for the disease (Polled Hereford cattle) has also been characterized. The mutation in these animals introduces a stop codon in the leader peptide of the E1 alpha subunit, resulting in premature termination of translation. Two thiamine responsive patients have been studied. The deduced amino acid sequences of the mature E1 alpha subunit and its leader sequence were normal, suggesting that the defect in these patients must exist in some other subunit of the complex. 3-Hydroxyisobutyrate dehydrogenase and methylmalonate-semialdehyde dehydrogenase, two enzymes of the valine catabolic pathway, were purified from liver tissue and characterized.(ABSTRACT TRUNCATED AT 400 WORDS)

cDNA cloning of the E1 alpha subunit of the branched-chain alpha-keto acid dehydrogenase and elucidation of a molecular basis for maple syrup urine disease

We have cloned cDNAs encoding human and rat liver BCKDH E1 alpha subunits and deduced the primary structure of the mature protein. The sequences of the cDNA and protein are highly conserved between the two species. Significant sequence similarity has also been found between human BCKDH and PDH E1 alpha subunits. We have studied the molecular basis of MSUD by determining the enzyme activity and levels of BCKDH protein and mRNA, and by enzymatic amplification and sequencing of BCKDH E1 alpha-specific mRNA, from an MSUD patient and his parents. Different mutant alleles were identified in the two parents. The patient was a compound heterozygote, inheriting an allele encoding an abnormal E1 alpha from the father and an allele containing a defect in regulation from the mother. Our results demonstrate that a case of MSUD was caused by structural and regulatory mutations involving the E1 alpha subunit.

Nutritional and hormonal regulation of the activity state of hepatic branched-chain alpha-keto acid dehydrogenase complex

The hepatic branched-chain alpha-keto acid dehydrogenase complex plays an important role in regulating branched-chain amino acid levels. These compounds are essential for protein synthesis but are toxic if present in excess. When dietary protein is deficient, the hepatic enzyme is present in the inactive, phosphorylated state to allow conservation of branched-chain amino acids for protein synthesis. When dietary protein is excessive, the enzyme is in the active, dephosphorylated state to commit the excess branched-chain amino acids to degradation. Inhibition of protein synthesis by cycloheximide, even when the animal is starving for protein, results in activation of the hepatic branched-chain alpha-keto acid dehydrogenase complex to prevent accumulation of branched-chain amino acids. Likewise, the increase in branched-chain amino acids caused by body wasting during starvation and uncontrolled diabetes is blunted by activation of the hepatic branched-chain alpha-keto acid dehydrogenase complex. The activity state of the hepatic branched-chain alpha-keto acid dehydrogenase complex is regulated in the short term by the concentration of branched-chain alpha-keto acids (inhibitors of branched-chain alpha-keto acid dehydrogenase kinase) and in the long term by alteration in the total branched chain alpha-keto acid dehydrogenase kinase activity.

Purification and characterization of methylmalonate-semialdehyde dehydrogenase from rat liver. Identity to malonate-semialdehyde dehydrogenase

Methylmalonate semialdehyde dehydrogenase was purified from rat liver in order to define the distal portion of valine catabolism and related pathways in mammals. The purified enzyme is active with malonate semialdehyde and consumes both stereoisomers of methylmalonate semialdehyde, implicating a single semialdehyde dehydrogenase in the catabolism of valine, thymine, and compounds catabolized by way of beta-alanine. The oxidation of malonate and methylmalonate semialdehydes by this enzyme is CoA-dependent, the products being acetyl-CoA and propionyl-CoA, respectively. Expected activity with ethylmalonate semialdehyde as substrate was not found. Methylmalonate semialdehyde dehydrogenase was separated on DEAE-Sephacel into two isoforms which differ in mobility during nondenaturing polyacrylamide gel electrophoresis. The two forms are immunologically cross-reactive and exhibit the same N-terminal sequence, suggesting that one form is the product of the other. The monomer molecular mass, determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, was 58 kDa. The native molecular mass, estimated by gel filtration, was 250 kDa, suggesting a tetrameric structure.

Molecular cloning of a cDNA for the E1 alpha subunit of rat liver branched chain alpha-ketoacid dehydrogenase

We have isolated a cDNA encoding the branched chain alpha-ketoacid dehydrogenase E1 alpha subunit. A rat liver lambda gt11 expression library was screened with antibody reactive with the 2-oxoisovalerate dehydrogenase (lipoamide) component. A positive clone, lambda BZ304, contains a 1.7-kilobase pair cDNA insert with a 1323-base pair open reading frame. Translation of the open reading frame predicts the 24 residues of the previously reported phosphorylation sites 1 and 2 for the bovine kidney and rabbit heart enzymes. The N-terminal sequence of purified E1 alpha was determined, and this sequence was found 40 residues from the beginning of the deduced peptide sequence. Northern blots of rat liver and muscle RNA demonstrate a single mRNA species of approximately 1.8 kilobase pairs in each tissue, suggesting that this cDNA is nearly full length.

Activation of branched-chain alpha-ketoacid dehydrogenase in isolated hepatocytes by branched-chain alpha-ketoacids

The effects of branched-chain alpha-ketoacids on flux through and activity state of the branched-chain alpha-ketoacid dehydrogenase complex were studied in hepatocytes prepared from chow-fed, starved, and low-protein-diet-fed rats. Very low concentrations of alpha-ketoisocaproate caused a dramatic stimulation (50% activation at 20 microM) of alpha-ketoisovalerate decarboxylation in hepatocytes from low-protein-fed rats. alpha-Keto-beta-methylvalerate was also effective, but less so than alpha-ketoisocaproate. alpha-Ketoisocaproate did not stimulate alpha-ketoisovalerate decarboxylation by hepatocytes from chow-fed or starved rats. To a smaller degree, alpha-keto-beta-methylvalerate and alpha-ketoisovalerate stimulated alpha-ketoisocaproate decarboxylation by hepatocytes from low-protein-fed rats. The implied order of potency of stimulation of flux through branched-chain alpha-ketoacid dehydrogenase was alpha-ketoisocaproate greater than alpha-keto-beta-methylvalerate greater than alpha-ketoisovalerate, i.e., the same order of potency of these compounds as branched-chain alpha-ketoacid dehydrogenase kinase inhibitors. Fluoride, known to inhibit branched-chain alpha-ketoacid dehydrogenase phosphatase, largely prevented alpha-ketoisocaproate and alpha-chloroisocaproate activation of flux through the branched-chain alpha-ketoacid dehydrogenase. Assay of the branched-chain alpha-ketoacid complex in cell-free extracts of hepatocytes isolated from low-protein-diet-fed rats confirmed that alpha-ketoacids affected the activity state of the complex. Branched-chain alpha-ketoacids failed to activate flux in hepatocytes prepared from chow-fed and starved rats because essentially all of the complex was already in the dephosphorylated, active state. These findings indicate that inhibition of branched-chain alpha-ketoacid dehydrogenase kinase activity by branched-chain alpha-ketoacids is important for regulation of the activity state of hepatic branched-chain alpha-ketoacid dehydrogenase.

Preservation of the activity state of hepatic branched-chain 2-oxo acid dehydrogenase during the isolation of mitochondria

A comparison was conducted of current methods for estimation of the activity states (proportion of enzyme in active, dephosphorylated, form) of hepatic branched-chain 2-oxo acid dehydrogenase. Practically all of the enzyme was active in freeze-clamped liver obtained from chow-fed and 48 h-starved rats, regardless of the presence of fluoride in the extraction and assay media to inhibit phosphatase activity. Likewise, the enzyme was almost completely active in mitochondria isolated by a conventional method from livers of chow-fed and starved rats. However, when fluoride and 4-methyl-2-oxopentanoate were included in the mitochondrial isolation medium the activity state was decreased to 73% and 47% in mitochondria isolated from chow-fed and starved rats respectively. Furthermore, branched-chain 2-oxo acid dehydrogenase became partially inactivated upon incubation of isolated mitochondria on ice in fluoride- and/or 4-methyl-2-oxopentanoate-supplemented media. The rate of inactivation was greater in mitochondria prepared from starved than from chow-fed rats, which correlated with the lower activity state found in mitochondria of starved rats isolated in the fluoride- and 4-methyl-2-oxopentanoate-supplemented media. Thus the activity state of branched-chain 2-oxo acid dehydrogenase is underestimated in mitochondria isolated in media supplemented with fluoride plus 4-methyl-2-oxopentanoate.

Enzymatic determination of the branched-chain alpha-keto acids

A spectrophotometric endpoint assay for determination of branched-chain alpha-keto acids is described. The assay depends on measurement of the NADH produced after addition of branched-chain alpha-keto acid dehydrogenase. Interference by pyruvate and alpha-ketobutyrate was eliminated by pretreating the sample with pyruvate dehydrogenase. The method yielded a peripheral venous plasma value of 59 +/- 5 microM (mean +/- SE) for the branched-chain alpha-keto acids of five overnight fasted healthy humans.

Activities of branched-chain amino acid aminotransferase and branched-chain 2-oxo acid dehydrogenase complex in tissues of maternal and fetal sheep

Branched-chain amino acid aminotransferase (BCAAT) and branched-chain 2-oxo acid dehydrogenase (BCODH) activities were determined in sheep maternal and fetal liver, kidney, skeletal muscle, adipose and placenta in the fed and 5-day-starved states at 125 days gestation (term is 147 days). BCAAT activities were quite low in maternal skeletal muscle, low in fetal and maternal liver, high in fetal muscle, and very high in placenta. No significant changes occurred with maternal starvation. The high BCAAT activity of the placenta may provide the essential branched-chain 2-oxo acids for the fetus. BCODH activity was highest in liver and kidney of both the mother and fetus; it was largely in the active (dephosphorylated) state in maternal liver and kidney, and in fetal kidney, and about 50% active in fetal liver. The enzyme was largely inactive in fetal muscle, and about 50% active in maternal muscle. Although starvation had little effect on the activity of BCODH in fetal tissues, a significant decrease in activity was observed in maternal tissues, thereby potentially sparing branched-chain amino acids or the corresponding 2-oxo acids for maintenance of the fetus during compromised maternal nutrition.

Branched-chain 2-oxo acid dehydrogenase interferes with the measurement of the activity and activity state of hepatic pyruvate dehydrogenase

Oxidative decarboxylation of pyruvate by branched-chain 2-oxo acid dehydrogenase can result in overestimation of the expressed and total activity of hepatic pyruvate dehydrogenase. Pyruvate is a poor substrate for branched-chain 2-oxo acid dehydrogenase relative to the branched-chain oxo acids; however, the comparable total activities of the two complexes in liver, the much greater activity state of branched-chain 2-oxo acid dehydrogenase compared with pyruvate dehydrogenase in most physiological states, and the use of high pyruvate concentrations, explain the interference that can occur in conventional radiochemical or indicator-enzyme linked assays of pyruvate dehydrogenase. Goat antibody that specifically inhibited branched-chain 2-oxo acid dehydrogenase was used in this study to provide a more specific assay for pyruvate dehydrogenase.

Regulation of the branched-chain 2-oxo acid dehydrogenase complex in hepatocytes isolated from rats fed on a low-protein diet

Hepatocytes isolated from rats fed on a chow diet or a low-protein (8%) diet were used to study the effects of various factors on flux through the branched-chain 2-oxo acid dehydrogenase complex. The activity of this complex was also determined in cell-free extracts of the hepatocytes. Hepatocytes isolated from chow-fed rats had greater flux rates (decarboxylation rates of 3-methyl-2-oxobutanoate and 4-methyl-2-oxopentanoate) than did hepatocytes isolated from rats fed on the low-protein diet. Oxidizable substrates tended to inhibit flux through the branched-chain 2-oxo acid dehydrogenase, but inhibition was greater with hepatocytes isolated from rats fed on the low-protein diet. 2-Chloro-4-methylpentanoate (inhibitor of branched-chain 2-oxo acid dehydrogenase kinase), dichloroacetate (inhibitor of both pyruvate dehydrogenase kinase and branched-chain 2-oxo acid dehydrogenase kinase) and dibutyryl cyclic AMP (inhibitor of glycolysis) were effective stimulators of branched-chain oxo acid decarboxylation with hepatocytes from rats fed on a low-protein diet, but had little effect with hepatocytes from rats fed on chow diet. Activity measurements indicated that the branched-chain 2-oxo acid dehydrogenase complex was mainly (96%) in the active (dephosphorylated) state in hepatocytes from chow-fed rats, but only partially (50%) in the active state in hepatocytes from rats fed on a low-protein diet. Oxidizable substrates markedly decreased the activity state of the enzyme in hepatocytes from rats fed on a low-protein diet, but had much less effect in hepatocytes from chow-fed rats. 2-Chloro-4-methylpentanoate and dichloroacetate increased the activity state of the enzyme in hepatocytes from rats fed on a low-protein diet, but had no effect on the activity state of the enzyme in hepatocytes from chow-fed rats. The results indicate that protein starvation greatly increases the sensitivity of the hepatic branched-chain 2-oxo acid dehydrogenase complex to regulation by covalent modification.

Regulation of branched-chain alpha-ketoacid dehydrogenase complex by covalent modification

The branched-chain alpha-ketoacid dehydrogenase complex, like the pyruvate dehydrogenase complex, is an intramitochondrial enzyme subject to regulation by covalent modification. Phosphorylation causes inactivation and dephosphorylation causes activation of both complexes. The branched-chain alpha-ketoacid dehydrogenase kinase, believed distinct from pyruvate dehydrogenase kinase, is an integral component of the branched-chain alpha-ketoacid dehydrogenase complex and is sensitive to inhibition by branched-chain alpha-ketoacids, alpha-chloroisocaproate, phenylpyruvate, clofibric acid, octanoate and dichloroacetate. Phosphorylation of branched-chain alpha-ketoacid dehydrogenase occurs at two closely-linked serine residues (sites 1 and 2) of the alpha-subunit of the decarboxylase. HPLC and sequence data suggest homology of the amino acid sequence adjacent to phosphorylation sites 1 and 2 of complexes isolated from several different tissues. Stoichiometry for phosphorylation of all of the complexes studies was about 1 mol P/mol alpha-subunit for 95% inactivation and 1.5 mol P/mol alpha-subunit for maximally phosphorylated complex. Site 1 and site 2 were phosphorylated at similar rates until total phosphorylation exceeded 1 mol P/mol alpha-subunit. The complexes from rabbit kidney, rabbit heart, and rat heart showed 30-40% additional phosphorylation of the alpha-subunit beyond 95% inactivation. Site specificity studies carried out with the kinase partially inhibited with alpha-chloroisocaproate suggest that phosphorylation of site 1 is primarily responsible for regulation of the complex. The capacity of the branched-chain alpha-ketoacid dehydrogenase to oxidize pyruvate (Km = 0.8 mM, Vmax = 20% of that of alpha-ketoisovalerate) interferes with the estimation of activity state of the hepatic pyruvate dehydrogenase complex. The disparity between the activity states of the two complexes in most physiologic states contributes to this interference. An inhibitory antibody for branched-chain alpha-ketoacid dehydrogenase can be used to prevent interference with the pyruvate dehydrogenase assay. Almost all of the hepatic branched-chain alpha-ketoacid dehydrogenase in chow-fed rats is active (greater than 90% dephosphorylated). In contrast, almost all of the hepatic enzyme of rats fed a low-protein (8%) diet is inactive (greater than 85% phosphorylated). Fasting of chow-fed rats has no effect on the activity state of hepatic branched-chain alpha-ketoacid dehydrogenase, i.e. greater than 90% of the enzyme remains in the active state. However, fasting of rats maintained on low-protein diets greatly activates the hepatic enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)