Glucose regulates leucine-induced insulin release and the expression of the branched chain ketoacid dehydrogenase E1 alpha subunit gene in pancreatic islets

Branched-chain Amino Acids: Catabolism in Skeletal Muscle and Implications for Muscle and Whole-body Metabolism

Branched-chain amino acids (BCAAs) are critical for skeletal muscle and whole-body anabolism and energy homeostasis. They also serve as signaling molecules, for example, being able to activate mammalian/mechanistic target of rapamycin complex 1 (mTORC1). This has implication for macronutrient metabolism. However, elevated circulating levels of BCAAs and of their ketoacids as well as impaired catabolism of these amino acids (AAs) are implicated in the development of insulin resistance and its sequelae, including type 2 diabetes, cardiovascular disease, and of some cancers, although other studies indicate supplements of these AAs may help in the management of some chronic diseases. Here, we first reviewed the catabolism of these AAs especially in skeletal muscle as this tissue contributes the most to whole body disposal of the BCAA. We then reviewed emerging mechanisms of control of enzymes involved in regulating BCAA catabolism. Such mechanisms include regulation of their abundance by microRNA and by post translational modifications such as phosphorylation, acetylation, and ubiquitination. We also reviewed implications of impaired metabolism of BCAA for muscle and whole-body metabolism. We comment on outstanding questions in the regulation of catabolism of these AAs, including regulation of the abundance and post-transcriptional/post-translational modification of enzymes that regulate BCAA catabolism, as well the impact of circadian rhythm, age and mTORC1 on these enzymes. Answers to such questions may facilitate emergence of treatment/management options that can help patients suffering from chronic diseases linked to impaired metabolism of the BCAAs.

The impact of IUGR on pancreatic islet development and β-cell function

Placental insufficiency is a primary cause of intrauterine growth restriction (IUGR). IUGR increases the risk of developing type 2 diabetes mellitus (T2DM) throughout life, which indicates that insults from placental insufficiency impair β-cell development during the perinatal period because β-cells have a central role in the regulation of glucose tolerance. The severely IUGR fetal pancreas is characterized by smaller islets, less β-cells, and lower insulin secretion. Because of the important associations among impaired islet growth, β-cell dysfunction, impaired fetal growth, and the propensity for T2DM, significant progress has been made in understanding the pathophysiology of IUGR and programing events in the fetal endocrine pancreas. Animal models of IUGR replicate many of the observations in severe cases of human IUGR and allow us to refine our understanding of the pathophysiology of developmental and functional defects in islet from IUGR fetuses. Almost all models demonstrate a phenotype of progressive loss of β-cell mass and impaired β-cell function. This review will first provide evidence of impaired human islet development and β-cell function associated with IUGR and the impact on glucose homeostasis including the development of glucose intolerance and diabetes in adulthood. We then discuss evidence for the mechanisms regulating β-cell mass and insulin secretion in the IUGR fetus, including the role of hypoxia, catecholamines, nutrients, growth factors, and pancreatic vascularity. We focus on recent evidence from experimental interventions in established models of IUGR to understand better the pathophysiological mechanisms linking placental insufficiency with impaired islet development and β-cell function.

Leucine stimulates insulin secretion via down-regulation of surface expression of adrenergic α2A receptor through the mTOR (mammalian target of rapamycin) pathway: implication in new-onset diabetes in renal transplantation

The amino acid leucine is a potent secretagogue, capable of inducing insulin secretion. It also plays an important role in the regulation of mTOR activity, therefore, providing impetus to investigate if a leucine-sensing mechanism in the mTOR pathway is involved in insulin secretion. We found that leucine-induced insulin secretion was inhibited by both the mTOR inhibitor rapamycin as well as the adrenergic α2 receptor agonist clonidine. We also demonstrated that leucine down-regulated the surface expression of adrenergic α2A receptor via activation of the mTOR pathway. The leucine stimulatory effect on insulin secretion was attenuated in diabetic Goto-Kakizaki rats that overexpress adrenergic α2A receptors, confirming the role of leucine in insulin secretion. Thus, our data demonstrate that leucine regulates insulin secretion by modulating adrenergic α2 receptors through the mTOR pathway. The role of the mTOR pathway in metabolic homeostasis led us to a second important finding in this study; retrospective analysis of clinical data showed that co-administration of rapamycin and clonidine was associated with an increased incidence of new-onset diabetes in renal transplantation patients over those receiving rapamycin alone. We believe that inhibition of mTOR by rapamycin along with activation of adrenergic α2 receptors by clonidine represents a double-hit to pancreatic islets that synergistically disturbs glucose homeostasis. This new insight may have important implications for the clinical management of renal transplant patients.

Metabonomics research of diabetic nephropathy and type 2 diabetes mellitus based on UPLC-oaTOF-MS system

Ultra performance liquid chromatography (UPLC) coupled with orthogonal acceleration time-of-flight (oaTOF) mass spectrometry has showed great potential in diabetes research. In this paper, a UPLC-oaTOF-MS system was employed to distinguish the global serum profiles of 8 diabetic nephropathy (DN) patients, 33 type 2 diabetes mellitus (T2DM) patients and 25 healthy volunteers, and tried to find potential biomarkers. The UPLC system produced information-rich chromatograms with typical measured peak widths of 4 s, generating peak capacities of 225 in 15 min. Furthermore, principal component analysis (PCA) was used for group differentiation and marker selection. As shown in the scores plot, the distinct clustering between the patients and controls was observed, and DN and T2DM patients were also separated into two individual groups. Several compounds were tentatively identified based on accurate mass, isotopic pattern and MS/MS information. In addition, significant changes in the serum level of leucine, dihydrosphingosine and phytosphingosine were noted, indicating the perturbations of amino acid metabolism and phospholipid metabolism in diabetic diseases, which having implications in clinical diagnosis and treatment.

Beta-cell alpha-ketoglutarate hydroxylases may acutely participate in insulin secretion

The presence of Fe(II) alpha-ketoglutarate hydroxylases in rat and human pancreatic islets and INS-1 832/13 cells was demonstrated with the reverse transcriptase polymerase chain reaction (PHD1, 2, and 3; lysyl hydroxylases 1, 2, and 3; and phytanoyl-coenzyme A hydroxylase were seen) and/or immunoblotting (high levels of proline hydroxylase P4Halpha1, PHD2, and PHD4 and low levels of PHD2 and PHD3 in human islets, and high levels of PHD2 in rat islets and INS-1 cells were seen). Prolyl hydroxylase enzyme activity in INS-1 832/13 cells was purified with polyproline affinity chromatography. Inhibitors of alpha-ketoglutarate hydroxylases lowered glucose-induced and leucine-plus-glutamine-induced insulin release in rat pancreatic islets, suggesting that there may be acute unknown effects of alpha-ketoglutarate hydroxylases in insulin secretion. It is possible that an increase in mitochondrially generated alpha-ketoglutarate derived from insulin secretagogue carbon and translocated to the cytosol may be part of the signal for insulin secretion.

Feasibility of pathways for transfer of acyl groups from mitochondria to the cytosol to form short chain acyl-CoAs in the pancreatic beta cell

The mitochondria of pancreatic beta cells are believed to convert insulin secretagogues into products that are translocated to the cytosol where they participate in insulin secretion. We studied the hypothesis that short chain acyl-CoA (SC-CoAs) might be some of these products by discerning the pathways of SC-CoA formation in beta cells. Insulin secretagogues acutely stimulated 1.5-5-fold increases in acetoacetyl-CoA, succinyl-CoA, malonyl-CoA, hydroxymethylglutaryl-CoA (HMG-CoA), and acetyl-CoA in INS-1 832/13 cells as judged from liquid chromatography-tandem mass spectrometry measurements. Studies of 12 relevant enzymes in rat and human pancreatic islets and INS-1 832/13 cells showed the feasibility of at least two redundant pathways, one involving acetoacetate and the other citrate, for the synthesis SC-CoAs from secretagogue carbon in mitochondria and the transfer of their acyl groups to the cytosol where the acyl groups are converted to SC-CoAs. Knockdown of two key cytosolic enzymes in INS-1 832/13 cells with short hairpin RNA supported the proposed scheme. Lowering ATP citrate lyase 88% did not inhibit glucose-induced insulin release indicating citrate is not the only carrier of acyl groups to the cytosol. However, lowering acetoacetyl-CoA synthetase 80% partially inhibited glucose-induced insulin release indicating formation of SC-CoAs from acetoacetate in the cytosol is important for insulin secretion. The results indicate beta cells possess enzyme pathways that can incorporate carbon from glucose into acetyl-CoA, acetoacetyl-CoA, and succinyl-CoA and carbon from leucine into these three SC-CoAs plus HMG-CoA in their mitochondria and enzymes that can form acetyl-CoA, acetoacetyl-CoA, malonyl-CoA, and HMG-CoA in their cytosol.

Synergistic potent insulin release by combinations of weak secretagogues in pancreatic islets and INS-1 cells

Insulin secretion by the beta cell depends on anaplerosis in which insulin secretagogues are metabolized by mitochondria into molecules that are most likely exported to the extramitochondrial space where they have signaling roles. However, very little is known about the products of anaplerosis. We discovered an experimental paradigm that has begun to provide new information about these products. When various intracellular metabolites were applied in combination to overnight-cultured rat or human pancreatic islets or to INS-1 832/13 cells, they interacted synergistically to strongly stimulate insulin release. When these same metabolites were applied individually to these cells, insulin stimulation was poor. Discerning the contributions of the individual compounds to metabolism has begun to allow us to dissect some of the pathways involved in insulin secretion, which was not possible from studying individual secretagogues. Monomethyl succinate (MMS) combined with a barely stimulatory concentration of alpha-ketoisocaproate (KIC) (2 mm) stimulated insulin release in cultured rat islets 18-fold (versus 21-fold for 16.7 mm glucose). MMS plus low glucose (2 mm) or pyruvate (5 mm) gave 11- and 9-fold stimulations. These agents also potentiated MMS-induced insulin release in fresh islets, and KIC plus MMS gave synergistic insulin release in cultured human islets. In INS-1 cells, neither MMS nor KIC (10 mm) was an insulin secretagogue, but when added together KIC (2 mm) and MMS stimulated insulin release 7-fold (versus 12-fold for glucose). In islets and INS-1 cells, conditions that stimulated insulin release caused large relative increases in acetoacetate, which is a precursor of pathways to short chain acyl-CoAs. Liquid chromatography-tandem mass spectrometry measurements of acetyl-CoA, acetoacetyl-CoA, succinyl-CoA, hydroxymethylglutaryl-CoA, and malonyl-CoA confirmed that they were increased by insulin secretagogues. The results suggest a new mechanism of insulin secretion in which anaplerosis increases short chain acyl-CoAs that have roles in insulin exocytosis.

Decreased nutrient-stimulated insulin secretion in chronically hypoglycemic late-gestation fetal sheep is due to an intrinsic islet defect

We measured in vivo and in vitro nutrient-stimulated insulin secretion in late gestation fetal sheep to determine whether an intrinsic islet defect is responsible for decreased glucose-stimulated insulin secretion (GSIS) in response to chronic hypoglycemia. Control fetuses responded to both leucine and lysine infusions with increased arterial plasma insulin concentrations (average increase: 0.13 +/- 0.05 ng/ml leucine; 0.99 +/- 0.26 ng/ml lysine). In vivo lysine-stimulated insulin secretion was decreased by chronic (0.37 +/- 0.18 ng/ml) and acute (0.27 +/- 0.19 ng/ml) hypoglycemia. Leucine did not stimulate insulin secretion following acute hypoglycemia but was preserved with chronic hypoglycemia (0.12 +/- 0.09 ng/ml). Isolated pancreatic islets from chronically hypoglycemic fetuses had normal insulin and DNA content but decreased fractional insulin release when stimulated with glucose, leucine, arginine, or lysine. Isolated islets from control fetuses responded to all nutrients. Therefore, chronic late gestation hypoglycemia causes defective in vitro nutrient-regulated insulin secretion that is at least partly responsible for diminished in vivo GSIS. Chronic hypoglycemia is a feature of human intrauterine growth restriction (IUGR) and might lead to an islet defect that is responsible for the decreased insulin secretion patterns seen in human IUGR fetuses and low-birth-weight human infants.

New insights into amino acid metabolism, beta-cell function and diabetes

Specific amino acids are now known to acutely and chronically regulate insulin secretion from pancreatic beta-cells in vivo and in vitro. Understanding the molecular mechanisms by which amino acids regulate insulin secretion may identify novel targets for future diabetes therapies. Mitochondrial metabolism is crucial for the coupling of amino acid and glucose recognition to the exocytosis of the insulin granules. This is illustrated by in vitro and in vivo observations discussed in the present review. Mitochondria generate ATP, which is the main coupling factor in insulin secretion; however, the subsequent Ca2+ signal in the cytosol is necessary, but not sufficient, for full development of sustained insulin secretion. Hence mitochondria generate ATP and other coupling factors serving as fuel sensors for the control of the exocytotic process. Numerous studies have sought to identify the factors that mediate the amplifying pathway over the Ca2+ signal in nutrient-stimulated insulin secretion. Predominantly, these factors are nucleotides (GTP, ATP, cAMP and NADPH), although metabolites have also been proposed, such as long-chain acyl-CoA derivatives and the key amino acid glutamate. This scenario highlights further the importance of the key enzymes or transporters, glutamate dehydrogenase, the aspartate and alanine aminotransferases and the malate/aspartate shuttle, in the control of insulin secretion. Therefore amino acids may play a direct or indirect (via generation of putative messengers of mitochondrial origin) role in insulin secretion.

Perspective: emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion

The importance of mitochondrial biosynthesis in stimulus secretion coupling in the insulin-producing beta-cell probably equals that of ATP production. In glucose-induced insulin secretion, the rate of pyruvate carboxylation is very high and correlates more strongly with the glucose concentration the beta-cell is exposed to (and thus with insulin release) than does pyruvate decarboxylation, which produces acetyl-CoA for metabolism in the citric acid cycle to produce ATP. The carboxylation pathway can increase the levels of citric acid cycle intermediates, and this indicates that anaplerosis, the net synthesis of cycle intermediates, is important for insulin secretion. Increased cycle intermediates will alter mitochondrial processes, and, therefore, the synthesized intermediates must be exported from mitochondria to the cytosol (cataplerosis). This further suggests that these intermediates have roles in signaling insulin secretion. Although evidence is quite good that all physiological fuel secretagogues stimulate insulin secretion via anaplerosis, evidence is just emerging about the possible extramitochondrial roles of exported citric acid cycle intermediates. This article speculates on their potential roles as signaling molecules themselves and as exporters of equivalents of NADPH, acetyl-CoA and malonyl-CoA, as well as alpha-ketoglutarate as a substrate for hydroxylases. We also discuss the "succinate mechanism," which hypothesizes that insulin secretagogues produce both NADPH and mevalonate. Finally, we discuss the role of mitochondria in causing oscillations in beta-cell citrate levels. These parallel oscillations in ATP and NAD(P)H. Oscillations in beta-cell plasma membrane electrical potential, ATP/ADP and NAD(P)/NAD(P)H ratios, and glycolytic flux are known to correlate with pulsatile insulin release. Citrate oscillations might synchronize oscillations of individual mitochondria with one another and mitochondrial oscillations with oscillations in glycolysis and, therefore, with flux of pyruvate into mitochondria. Thus citrate oscillations may synchronize mitochondrial ATP production and anaplerosis with other cellular oscillations.

Distinguishing features of leucine and alpha-ketoisocaproate sensing in pancreatic beta-cells

Culturing rat islets in high glucose (HG) increased 1-(14)C-alpha-ketoisocaproate (KIC) oxidation compared with culturing them in low glucose. Leucine caused insulin secretion (IS) in low glucose but not in HG rat islets, whereas KIC did so in both. Pretreatment with HG for 40 min abolished leucine stimulation of IS by mouse islets and prevented the cytosolic Ca(2+) rise without inhibiting IS and Ca(2+) increments caused by KIC. When islets were pretreated without glucose and glutamine, aminooxyacetic acid (AOA) markedly decreased KIC effects. When islets were pretreated without glucose and with glutamine, AOA potentiated leucine effects but attenuated KIC effects. AOA stimulated glutamine oxidation in the presence but not the absence of +/-2-amino-2-norbornane-carboxylic acid, a nonmetabolized leucine analog. Pretreatment with HG and glutamine partially reversed AOA inhibition of KIC effects. Glucose increased intracellular ATP and GTP, whereas it decreased ADP and GDP in beta HC9 cells. Glutamate dehydrogenase activity of beta HC9 cell extracts was increased by leucine and attenuated by GTP, but it was potentiated by ADP. In conclusion, leucine and KIC stimulated beta-cells via distinct mechanisms. Glutamate dehydrogenase is the sensor of leucine, whereas transamination plays an important role in KIC stimulation of pancreatic beta-cells.

The succinate mechanism of insulin release

Nutrient secretagogues can increase the production of succinyl-CoA in rat pancreatic islets. When succinate esters are the secretagogue, succinyl-CoA can be generated via the succinate thiokinase reaction. Other secretagogues can increase production of succinyl-CoA secondary to increasing alpha-ketoglutarate production by glutamate dehydrogenase or mitochondrial aspartate aminotransferase followed by the alpha-ketoglutarate dehydrogenase reaction. Although secretagogues can increase the production of succinyl-CoA, they do not increase the level of this metabolite until after they decrease the level of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). This suggests that the generated succinyl-CoA initially reacts with acetoacetate to yield acetoacetyl-CoA plus succinate in the succinyl-CoA-acetoacetate transferase reaction. This would be followed by acetoacetyl-CoA reacting with acetyl-CoA to generate HMG-CoA in the HMG-CoA synthetase reaction. HMG-CoA will then be reduced by NADPH to mevalonate in the HMG-CoA reductase reaction and/or cleaved to acetoacetate plus acetyl-CoA by HMG cleavage enzyme. Succinate derived from either exogenous succinate esters or generated by succinyl-CoA-acetoacetate transferase is metabolized to malate followed by the malic enzyme reaction. Increased production of NADPH by the latter reaction then increases reduction of HMG-CoA and accounts for the decrease in the level of HMG-CoA produced by secretagogues. Pyruvate carboxylation catalyzed by pyruvate carboxylase will supply oxaloacetate to mitochondrial aspartate aminotransferase. This would enable this aminotransferase to supply alpha-ketoglutarate to the alpha-ketoglutarate dehydrogenase complex and would, in part, account for secretagogues increasing the islet level of succinyl-CoA after they decrease the level of HMG-CoA. Mevalonate could be a trigger of insulin release as a result of its ability to alter membrane proteins and/or cytosolic Ca(2+). This is consistent with the fact that insulin secretagogues decrease the level of the mevalonate precursor HMG-CoA. In addition, inhibitors of HMG-CoA reductase interfere with insulin release and this inhibition can be reversed by mevalonate.

Leucine and its keto acid enhance the coordinated expression of genes for branched-chain amino acid catabolism in Arabidopsis under sugar starvation

Branched-chain alpha-keto acid dehydrogenase (BCKDH), a multienzyme complex, plays a key role in branched-chain amino acid catabolism. However, it remains unclear whether expression of each subunit is coordinately regulated in plants, which should be important for the efficient assembly of subunits into a functional multienzyme complex. We show that the transcripts from the Arabidopsis E1alpha subunit gene accumulated in dark-adapted leaves and in sugar-starved suspension cells. These results are complementary to our previous report that the transcripts for the E1beta and E2 subunit genes accumulated in sugar-starved cells. Expression of the E1alpha gene is likely to be regulated by hexokinase-mediated sugar signaling, indicating that sugar plays a regulatory role in the coordinated expression of BCKDH subunit genes. Furthermore, Leu and its metabolite alpha-ketoisocaproate have synergistic effects on the enhanced expression of BCKDH subunit genes under sugar starvation. We hence suggest that branched-chain amino acids activate their own degradation pathway in sugar-starved cells through co-induction of each subunit gene of BCKDH.

Purification and properties of a folate-catabolizing enzyme

We have identified and purified to homogeneity an enzyme from rat liver that catalyzes the oxidative catabolism of 5-formyltetrahydrofolate to p-aminobenzoylglutamate and a pterin derivative. Purification of the enzyme utilized six column matrices, including a pterin-6-carboxylic acid affinity column. Treatment of crude rat liver extracts with EDTA or heat decreased the specific activity of the enzyme by up to 85%. Peptides generated from the purified protein were sequenced and found to be identical to primary sequences present within rat light chain or heavy chain ferritin. Commercial rat ferritin did not display catabolic activity, but activity could be acquired with iron loading. The purified enzyme contained 2000 atoms of iron/ferritin 24-mer and displayed similar electrophoretic properties as commercial rat liver ferritin. The ferritin-catalyzed reaction displayed burst kinetics, and the enzyme catalyzed only a single turnover in vitro. Expression of rat heavy chain ferritin cDNA resulted in increased rates of folate turnover in cultured Chinese hamster ovary cells and human mammary carcinoma cells and reduced intracellular folate concentrations in Chinese hamster ovary cells. These results indicate that ferritin catalyzes folate turnover in vitro and in vivo and may be an important factor in regulating intracellular folate concentrations.

Glutamate is not a messenger in insulin secretion

Experiments do not support a recent claim that glutamate formed from the amination of citric acid cycle-derived alpha-ketoglutarate is a messenger in glucose-induced insulin secretion (Maechler, P., and Wollheim, C. (1999) Nature 402, 685-689). Glucose, leucine, succinic acid methyl ester, and alpha-ketoisocaproic acid all markedly stimulate insulin release but do not increase glutamate levels in pancreatic islets. Increasing the intracellular glutamate levels to 10-fold higher than basal levels by adding glutamine to islets does not stimulate insulin release. When leucine, in addition to glutamine, is applied to islets, insulin release is almost as high as with glucose alone. This is consistent with the known ability of leucine to allosterically activate glutamate deamination by glutamate dehydrogenase, which can supply alpha-ketoglutarate to the citric acid cycle. Experiments with mitochondria from pancreatic islets suggest that flux through the glutamate dehydrogenase reaction is quiescent during glucose-induced insulin secretion. These experiments support the traditional idea that when insulin release is associated with flux through glutamate dehydrogenase, the flux is in the direction of alpha-ketoglutarate.

Glucose-stimulated expressed sequence tags from rat pancreatic islets

Available evidence suggests that glucose, the most potent physiologic insulin secretagogue, capacitates glucose-induced insulin secretion by stimulating synthesis of various proteins in the beta cell. To obtain more clues about proteins that might be involved in insulin secretion, rat pancreatic cDNA libraries were screened by differential hybridization for non-preproinsulin transcripts that were increased when pancreatic islets were cultured for 1 day at a high (20 mM) versus a low concentration (1 mM) of glucose. More than 100,000 pfu were initially screened. After repeated rescreening, 33 transcripts were 1-3-fold higher in the presence of the high glucose. For comparison, preproinsulin transcripts were 4-8-fold higher at the high concentration of glucose. The sequences of 12 clones were > or = 85% similar to published sequences. These included annexin, calbindin, protein kinase C receptor, the G protein beta subunit, the guanyl cyclase A/atrial natriuretic peptide receptor and the serotonin 5HT-2 receptor. As previously reported, ferritin H chain transcripts were discovered to be 3-6-fold higher in the presence of the high glucose (MacDonald et al., FASEB J. 8, 777-781, 1994). Unidentified glucose responsive clones have been assigned GenBank accession numbers N55606-N55636, N65938 and N65939. The results implicate the proteins encoded by these mRNAs in insulin secretion.

Function of the human insulin promoter in primary cultured islet cells

Pancreatic islet beta cells regulate the rate of insulin gene transcription in response to a number of nutrients, the most potent of which is glucose. To test for its regulation by glucose, the promoter sequence was isolated from the human insulin gene. When linked to chloramphenicol acetyltransferase and transfected into primary islet cultures, the human insulin promoter is activated by glucose. In parallel islet transfections, glucose also activates the L-pyruvate kinase and islet amyloid chain ketoacid dehydrogenase E1a promoter, but it does not affect the beta cell glucose kinase promoter. Using deletion and substitution mutations of the proximal human insulin promoter, we mapped a metabolic response element to the E box, E1, at -100 base pairs relative to the transcription start site. Although the isolated E1 element responds to glucose, inclusion of either of two AT-rich sequences, A1 or A2/C1 on either side of E1, results in dramatic synergistic activation. Inclusion of A2/C1 also increases the response to glucose. The A2-E1-A1 region alone, however, does not explain all of the activity of the human insulin promoter in cultured islets, and other transcriptionally important elements likely to contribute to the glucose response as well.

Rat muscle branched-chain ketoacid dehydrogenase activity and mRNAs increase with extracellular acidemia

The rate-limiting enzyme in branched-chain amino acid catabolism is branched-chain ketoacid dehydrogenase (BCKAD). In rats fed NH4Cl to induce acidemia, we find increased basal BCKAD activity as well as maximal activity in skeletal muscle. Concurrently, there is a > 10-fold increase in mRNAs of BCKAD subunits in skeletal muscle plus an increase in cardiac muscle but not in liver or kidney. There was no increase in mRNA for malate dehydrogenase or for cytosolic glyceraldehyde-3-phosphate dehydrogenase. Evaluation of the translation capacity of BCKAD mRNAs in muscle of acidemic rats yielded more immunoreactive BCKAD whether the proteins were synthesized from muscle RNA using rabbit reticulocyte lysate or directly using postmitochondrial homogenates. Although the RNA from muscle of acidemic rats yielded twice as much BCKAD protein, we found no net increase in mitochondrial BCKAD protein in muscle by Western blotting. Because there is increased proteolysis in muscle of rats with acidemia, the increase in mRNA might be a mechanism to augment BCKAD synthesis and activity in muscle.