Transcriptional control of cardiac fuel metabolism and mitochondrial function.

As a persistent pump, the mammalian heart demands a high-capacity mitochondrial system. Significant progress has been made in delineating the gene regulatory networks that control mitochondrial biogenesis and function in striated muscle. The PPARγ coactivator-1 (PGC-1) coactivators serve as inducible boosters of downstream transcription factors that control the expression of genes involved in mitochondrial energy transduction, ATP synthesis, and biogenesis. PGC-1 gain-of-function and loss-of-function studies targeting two PGC-1 family members, PGC-1α and PGC-1ß, have provided solid evidence that these factors are both necessary and sufficient for perinatal mitochondrial biogenesis and maintenance of high-capacity mitochondrial function in postnatal heart. In humans, during the development of heart failure owing to hypertension or obesity-related diabetes, the activity of the PGC-1 coactivators, and several downstream target transcription factors, is altered. Gene targeting studies in mice have demonstrated that loss of PGC-1α and PGC-1ß in heart leads to heart failure. Interestingly, the pattern of dysregulation within the PGC-1 transcriptional regulatory circuit distinguishes the heart disease caused by hypertension from that caused by diabetes. This transcriptional regulatory cascade and downstream metabolic pathways should be considered as targets for novel etiology-specific therapeutics aimed at the early stages of heart failure.

Tetrazolium reduction methods for assessment of substrate oxidation and strain differentiation among mycoplasmas, with particular reference to Mycoplasma bovigenitalium and some members of the Mycoplasma mycoides cluster.

AIMS: To apply a rapid nitroblue tetrazolium (NBT) reduction assay of substrate metabolism by mycoplasmas that would help to differentiate Mycoplasmas. METHODS AND RESULTS: Growth, substrate preferences and tetrazolium reduction were assessed for 18 strains of Mycoplasma bovigenitalium and Mycoplasma ovine serogroup 11. NBT reduction was detectable in 1 h with 10(8) CFU ml(-1). Use of alpha-ketobutyrate, lactate and pyruvate to support growth and NBT reduction were correlated: pyruvate was preferred and lactate was used by only four of the 18 strains. Selected members of the Mycoplasma mycoides cluster were also assessed and monotetrazoles tested as alternatives to NBT. The NBT method was applied to a further 19 species. CONCLUSIONS: This simple and reproducible method requires only small amounts of cells, enabling routine assessment of substrate use within 1 h, and the rapid assignment of numerous mycoplasmas to one of six physiological groups. The four physiological groups of M. bovigenitalium and Mycoplasma serogroup 11 strains were indistinguishable from each other, which supports the view that these belong to the same species. SIGNIFICANCE AND IMPACT OF THE STUDY: Strain-specific substrate-utilization patterns by mycoplasmas can be obtained rapidly and reliably. The method has potential as a large-scale semi-automated procedure to monitor numerous strains and substrates simultaneously.

Isolation and immunological detection of Mycoplasma ovipneumoniae in sheep with atypical pneumonia, and lack of a role for Mycoplasma arginini.

Mycoplasma ovipneumoniae NCTC 10151(T) and four new isolates from UK sheep flocks were compared. Only glucose and pyruvate were used as energy sources by the five strains: glucose was the best energy source for the type strain, pyruvate supported better growth of the new strains. Whole cell protein patterns and antigenic profiles showed high similarity between all five strains. The new isolates fell into two groups in ELISA tests. Serum samples from 30 pneumonic sheep were assessed for M. ovipneumoniae infection and Mycoplasma arginini co-infection. Fourteen (out of 30) serum samples were positive for M. ovipneumoniae both by ELISA and immunoblotting. Twelve antigenic proteins of M. ovipneumoniae were detected in infected serum samples: the antigen patterns were unique, with between one and at least seven occurring in any one sample. All serum samples were designated as negative for M. arginini antibodies by both ELISA and immunoblotting.

Spread-space spread-spectrum technique for secure multiplexing.

A novel technique for multiplexing complex images is proposed in which each image may be demultiplexed only if a set of random encryption keys is known. The technique utilizes the ability of the double random phase encoding method to spread a signals' energy in both the space and the spatial frequency domains in a controlled manner. To multiplex, images are independently encrypted with different phase keys and then superimposed by recording sequentially on the same material. Each image is extracted by using the particular key associated with it. During decryption the energy from the other images is further spread, making it possible to minimize its effects by using suitable filters. Wigner analysis is applied to the technique, and numerical results are presented.

Evaluation of the potential developmental toxicity of cyclododecatriene (CDDT).

The potential maternal and developmental toxicity of cyclododecatriene (CDDT) was assessed in rats. Groups of 22 time-mated female Crl:CD (SD) BR rats were exposed by inhalation (whole-body, 6 h/day) to either 0 (control), 10, 25, or 67 ppm CDDT over days 6-20 of gestation (days 6-20 G); the day of copulation plug detection was designated day 0 G. The dams were euthanized on day 21 G, and their abdominal and thoracic viscera were examined grossly. The fetuses were weighed, sexed, and examined for external, visceral, and skeletal alterations. Evidence of maternal toxicity was seen at 25 and 67 ppm. There were compound-related reductions in maternal body weight and food consumption parameters as well as increased occurrences of wet and stained fur at these exposure levels. Developmental toxicity evident as reduced mean fetal weight and delayed skeletal ossification was seen only at 67 ppm. There was no evidence of either maternal or developmental toxicity at 10 ppm. Thus, the no-observed-effect level (NOEL) for maternal toxicity was 10 ppm, and the NOEL for developmental toxicity was 25 ppm. Because developmental toxicity was observed only after exposures that also produced signs of maternal toxicity, CDDT was not considered to be a selective developmental toxicant in the rat.

Inhalation toxicity of methylglutaronitrile in rats.

Methylglutaronitrile (MGN) is a high-boiling (263 degrees C) solvent/intermediate used in the fiber industry. Twenty male rats per group were exposed nose-only to condensation aerosol/vapor concentrations of approximately either 5, 25, or 200 mg/m3 of MGN for 6 h/day, 5 days/week over a 4-week period. Ten rats/group were sacrificed one day after the final exposure and the remaining rats after a four-week recovery period. No effects were observed in clinical observations during the exposure period, but body-weight depression was observed in the 200 mg/m3 group. The 200 mg/m3 group showed minimal decreases in red blood cell count, hemoglobin, and hematocrit values accompanied by increases in reticulocytes. There were no other effects observed in clinical or pathologic evaluations in the study. A neurobehavioral battery of tests (including grip strength, functional observational battery, and motor activity tests) given at the end of the exposure and recovery periods showed no MGN effects. During the 4-week recovery, body weights in the 200 mg/m3 group returned to normal and the hematologic findings in all groups were normal. Based on the above findings of body weight depression at 200 mg/m3, the no-observed-adverse-effect level (NOAEL) for this study was considered to be 25 mg/m3.

Alkanesulfonate degradation by novel strains of Achromobacter xylosoxidans, Tsukamurella wratislaviensis and Rhodococcus sp., and evidence for an ethanesulfonate monooxygenase in A. xylosoxidans strain AE4.

Novel isolates of Achromobacter xylosoxidans, Tsukamurella wratislaviensis and a Rhodococcus sp. are described. These grew with short-chain alkanesulfonates as their sole source of carbon and energy. T. wratislaviensis strain SB2 grew well with C(3)-C(6) linear alkanesulfonates, isethionate and taurine, Rhodococcus sp. strain CB1 used C(3)-C(10) linear alkanesulfonates, taurine and cysteate, but neither strain grew with ethanesulfonate. In contrast, A. xylosoxidans strain AE4 grew well with ethanesulfonate, making it the first bacterium to be described which can grow with this compound. It also grew with unsubstituted C(3)-C(5) alkanesulfonates and isethionate. Hydrolysis was excluded as a mechanism for alkanesulfonate metabolism in these strains; and evidence is given for a diversity of uptake and desulfonatase systems. We provide evidence for an initial monooxygenase-dependent desulfonation in the metabolism of ethanesulfonate and propanesulfonate by A. xylosoxidans strain AE4.

p38 mitogen-activated protein kinase activates peroxisome proliferator-activated receptor alpha: a potential role in the cardiac metabolic stress response.

The expression of enzymes involved in fatty acid beta-oxidation (FAO), the principal source of energy production in the adult mammalian heart, is controlled at the transcriptional level via the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha). Evidence has emerged that PPARalpha activity is activated as a component of an energy metabolic stress response. The p38 mitogen-activated protein kinase (MAPK) pathway is activated by cellular stressors in the heart, including ischemia, hypoxia, and hypertrophic growth stimuli. We show here that PPARalpha is phosphorylated in response to stress stimuli in rat neonatal cardiac myocytes; in vitro kinase assays demonstrated that p38 MAPK phosphorylates serine residues located within the NH(2)-terminal A/B domain of the protein. Transient transfection studies in cardiac myocytes and in CV-1 cells utilizing homologous and heterologous PPARalpha target element reporters and mammalian one-hybrid transcription assays revealed that p38 MAPK phosphorylation of PPARalpha significantly enhanced ligand-dependent transactivation. Cotransfection studies performed with several known coactivators of PPARalpha demonstrated that p38 MAPK markedly increased coactivation specifically by PGC-1, a transcriptional coactivator implicated in myocyte energy metabolic gene regulation and mitochondrial biogenesis. These results identify PPARalpha as a downstream effector of p38 kinase-dependent stress-activated signaling in the heart, linking extracellular stressors to alterations in energy metabolic gene expression.

Inhalation toxicity of cyclooctadiene in rats.

Groups of 20 male Crl:CDBR rats each were exposed, whole-body, for six hours/day, for a total of nine exposures over a two-week period to concentrations of 52, 150, or 500 ppm of 1,5-cyclooctadiene vapor. A control group of 20 male rats was exposed simultaneously to houseline air. Ten rats per group were used for standard toxicological evaluations and ten rats per group for neurotoxicity testing. In the standard toxicology group, at the end of the exposure period, blood and urine samples were collected for clinical analyses, and five rats per group were sacrificed for pathologic examination. After a two-week recovery period, the surviving rats in the standard groups were also given clinical and pathological examinations. The neurotoxicity group was given a functional observational battery (FOB) test and motor activity evaluations after the fourth and ninth exposures. In addition, six of ten neurotoxicity rats per exposure group were given neuropathology evaluations at the end of the exposure period. In rats exposed to 500 ppm of 1,5-cyclooctadiene there was an absence of alerting response toward the end of the daily six-hour exposures. These rats appeared to recover within 1/2 hour after exposure. This effect was not observed in the other test groups. The FOB evaluation showed an increase in the number of rats found sleeping in the 500 and 150 ppm groups compared to controls after the last exposure, but there were no treatment-related effects in the motor activity evaluation. Since there were no other neurobehavioral findings and no toxicity findings in the 150 ppm group, the sleeping behavior in the 150 ppm group was considered insufficient evidence of an adverse effect. Clinical laboratory evaluation of the 500 ppm group showed urinary pH decreases at the end of the exposure period but not after the two-week recovery period. There were no other toxicologically important changes in urine analysis, hematologic, or blood chemistry evaluations attributable to the test compound. Histologic effects were found in the nose and kidneys of rats in the 500 ppm group. There was a mild degeneration/necrosis of nasal olfactory epithelium observed immediately after the exposure period and a mild degeneration/regeneration in this area observed after the two-week recovery. In addition, there were increased kidney weights in the 500 ppm group immediately after exposure along with increased hyaline droplets in the kidneys. These effects were reversible after the two-week recovery period. There were no significant nasal or kidney effects observed in the 150 and 52 ppm test groups, and no other organ weight or histological effects attributable to the test compound observed in the standard toxicology groups at either evaluation time. The neuropathologic evaluation showed only one minor lesion in one 500 ppm-group rat and this was not considered to be attributable to exposure to 1,5-cyclooctadiene. Based on the decreased alerting response observed in rats during exposure at 500 ppm, and on the effects observed in the nose, kidney, and urine in rats at this concentration, the no-observed-adverse-effect (NOAEL) level in this study was considered to be 150 ppm.

22q13 deletion syndrome.

We have recently collected clinical information on 37 individuals with deletion of 22q13 and compared the features of these individuals with 24 previously reported cases. The features most frequently associated with this deletion are global developmental delay, generalized hypotonia, absent or severely delayed speech, and normal to advanced growth. Minor anomalies include dolicocephaly, abnormal ears, ptosis, dysplastic toenails, and relatively large hands. As with many terminal deletions involving pale G-band regions, the deletion can be extremely subtle and can go undetected on routine cytogenetic analysis. In fact, 32% of the individuals in our study had previous chromosome analyses that failed to detect the deletion. Eight of 37 individuals had deletion of 22q13 secondary to an unbalanced chromosome translocation. In the newborn, this deletion should be considered in cases of hypotonia for which other common causes have been excluded. In the older child, this syndrome should be suspected in individuals with normal growth, profound developmental delay, absent or delayed speech, and minor dysmorphic features. We recommend high-resolution chromosome analysis and fluorescence in situ hybridization studies, or molecular analysis to exclude this diagnosis.

Hypoxia inhibits the peroxisome proliferator-activated receptor alpha/retinoid X receptor gene regulatory pathway in cardiac myocytes: a mechanism for O2-dependent modulation of mitochondrial fatty acid oxidation.

Hypoxia triggers a cascade of cellular energy metabolic responses including a decrease in mitochondrial oxidative flux. To characterize gene regulatory mechanisms by which mitochondrial fatty acid oxidative capacity is diminished in response to hypoxia, cardiac myocytes in culture were exposed to long-chain fatty acids (LCFA) under normoxic or hypoxic conditions. Hypoxia prevented the known LCFA-induced accumulation of mRNA encoding muscle carnitine palmitoyltransferase I (M-CPT I), an enzyme that catalyzes the rate-limiting step in mitochondrial fatty acid oxidation (FAO). Under hypoxic conditions, myocytes exhibited significant accumulation of intracellular neutral lipid consistent with reduced CPT I activity and diminished FAO capacity. Transient transfection experiments demonstrated that the hypoxia-mediated blunting of M-CPT I gene expression occurs at the transcriptional level, is localized to an LCFA/peroxisome proliferator-activated receptor alpha (PPARalpha)/retinoid X receptor (RXR) response element within the M-CPT I gene promoter, and is PPARalpha-dependent. DNA-protein binding studies demonstrated that exposure to hypoxia reduces PPARalpha/RXR binding activity. Immunoblotting studies demonstrated that whereas hypoxia had no effect on nuclear levels of PPARalpha protein, nuclear and cellular RXRalpha levels were reduced. Hypoxia also diminished the 9-cis-retinoic acid-mediated activation of a reporter containing an RXR homodimer response element. These results demonstrate that hypoxia deactivates PPARalpha by reducing the availability of its obligate partner RXR.

PPARalpha deficiency reduces insulin resistance and atherosclerosis in apoE-null mice.

PPARalpha is a ligand-dependent transcription factor expressed at high levels in the liver. Its activation by the drug gemfibrozil reduces clinical events in humans with established atherosclerosis, but the underlying mechanisms are incompletely defined. To clarify the role of PPARalpha in vascular disease, we crossed PPARalpha-null mice with apoE-null mice to determine if the genetic absence of PPARalpha affects vascular disease in a robust atherosclerosis model. On a high-fat diet, concentrations of atherogenic lipoproteins were higher in PPARalpha(-/-)apoE(-/-) than in PPARalpha(+/+)apoE(-/-) mice, due to increased VLDL production. However, en face atherosclerotic lesion areas at the aortic arch, thoracic aorta, and abdominal aorta were less in PPARalpha-null animals of both sexes after 6 and 10 weeks of high-fat feeding. Despite gaining as much or more weight than their PPARalpha(+/+)apoE(-/-) littermates, PPARalpha(-/-)apoE(-/-) mice had lower fasting levels of glucose and insulin. PPARalpha-null animals had greater suppression of endogenous glucose production in hyperinsulinemic clamp experiments, reflecting less insulin resistance in the absence of PPARalpha. PPARalpha(-/-)apoE(-/-) mice also had lower blood pressures than their PPARalpha(+/+)apoE(-/-) littermates after high-fat feeding. These results suggest that PPARalpha may participate in the pathogenesis of diet-induced insulin resistance and atherosclerosis.

Restoration of insulin-sensitive glucose transporter (GLUT4) gene expression in muscle cells by the transcriptional coactivator PGC-1.

Muscle tissue is the major site for insulin-stimulated glucose uptake in vivo, due primarily to the recruitment of the insulin-sensitive glucose transporter (GLUT4) to the plasma membrane. Surprisingly, virtually all cultured muscle cells express little or no GLUT4. We show here that adenovirus-mediated expression of the transcriptional coactivator PGC-1, which is expressed in muscle in vivo but is also deficient in cultured muscle cells, causes the total restoration of GLUT4 mRNA levels to those observed in vivo. This increased GLUT4 expression correlates with a 3-fold increase in glucose transport, although much of this protein is transported to the plasma membrane even in the absence of insulin. PGC-1 mediates this increased GLUT4 expression, in large part, by binding to and coactivating the muscle-selective transcription factor MEF2C. These data indicate that PGC-1 is a coactivator of MEF2C and can control the level of endogenous GLUT4 gene expression in muscle.

Characterization of altered myocardial fatty acid metabolism in patients with inherited cardiomyopathy.

Inherited defects in myocardial long-chain fatty acid metabolism are increasingly recognized as a cause of cardiomyopathy and sudden death in children. To evaluate whether the phenotypic expression of these genetic diseases could be delineated using positron emission tomography (PET), 11 patients with inherited defects in fatty acid metabolism were evaluated and results were compared with those of 6 nonaffected siblings. Myocardial perfusion, myocardial oxygen consumption (MVO2), and long-chain fatty acid metabolism were determined noninvasively with PET using quantitative mathematical models. There were no differences in haemodynamics, perfusion, MVO2 or plasma substrate levels between groups. Patients with defects in enzymes of fatty acid beta-oxidation (acyl-CoA dehydrogenase and 3-hydroxyacyl-CoA dehydrogenase deficiencies) (n = 5) had diminished myocardial palmitate oxidation compared with healthy siblings (3.2 +/- 3.0 vs. 13.0 +/- 5.6 nmol/g per min, p < 0.03) and a decrease in the percentage of MVO2 accounted for by palmitate (2% +/- 3% vs. 9% +/- 5%, p < 0.04). In these patients, extracted palmitate was shunted into a slow-turnover compartment (predominantly reflecting esterification to triglycerides) with expansion of palmitate in that pool (185 +/- 246 compared with 27 +/- 67 nmol/g in healthy siblings,p < 0.02). In contrast, myocardium of patients with carnitine deficiency (n = 6) (all on oral carnitine therapy) had normal palmitate extraction but expansion of the interstitial/cytosolic fatty acid pool (617 +/- 399 vs. 261 +/- 73 nmol/g in healthy siblings, p < 0.04), suggesting different mechanisms for handling upstream fatty acyl intermediates. Thus, PET can be used to noninvasively assess abnormal myocardial handling of fatty acids in patients with inherited defects of metabolism. This approach should be useful in the assessment of altered myocardial fatty acid metabolism associated with cardiomyopathy as well as for evaluating the efficacy of therapeutic interventions in affected patients.

Acute inhalation exposure to cyclohexane and schedule-controlled operant performance in rats: comparison to d-amphetamine and chlorpromazine.

Adult male rats pressed a lever on a multiple fixed ratio-fixed interval (FR20-FI120 sec) schedule of food presentation, and after attaining a stable baseline subjects received an acute inhalation exposure to cyclohexane vapor (0 ppm, 500 ppm, 2000 ppm, or 7000 ppm) for 6 hr. During the operant session that began 30 min after termination of exposure, FR running rate for the 7000 ppm group decreased 11% relative to performance on the previous day. FR post-reinforcement pause duration and the rate and pattern of FT performance were unaffected. Cyclohexane exposures of 500 or 2000 ppm had no detectable effects. No enduring effects of cyclohexane occurred up to 2 weeks after exposure. An independent set of rats, trained under nominally identical conditions, received various doses (i.p.) of d-amphetamine (AMPH) or chlorpromazine (CPZ) at 1-2 week intervals. Effective doses of AMPH decreased FR running rate, decreased FR post-reinforcement pause duration and increased FI rate of response. AMPH also decreased the FI index of curvature, indicating a change from an accelerating rate during the FI to a more constant rate. Effective doses of CPZ decreased FR rate, increased FR pause duration, decreased FI rate, and decreased FI index of curvature. Thus, schedule-controlled operant procedures that were sensitive to the effects of psychoactive drugs were able to identify only a minor and transient effect of the highest concentration (7000 ppm) of cyclohexane vapor on operant performance.

Inhalation developmental toxicity and reproduction studies with cyclohexane.

The reproductive and developmental toxicity of cyclohexane was assessed in a two-generation reproduction study with Crl:CD BR rats and in developmental toxicity studies with Crl:CD BR rats and Hra:(NZW)SPF rabbits. The animals were exposed whole-body to atmospheric concentrations of 0, 500, 2000, or 7000 ppm cyclohexane. In the two-generation reproduction study, parental effects included statistically significantly lower mean body weight, overall mean body weight gain, and overall mean food efficiency for P1 and F1 females of the 7000 ppm level and statistically significantly lower mean body weight for F1 males of that level. Adult rats exposed to 2000 ppm cyclohexane and above exhibited a transient diminished or absent response to a sound stimulus while in the chambers during exposure. Mean pup weight was statistically significantly lower than control from lactation day 7 throughout the remainder of the 25-day lactation period for both F1 and F2 7000 ppm litters. Changes observed at 500 ppm were either considered not to be compound related or not adverse. Therefore, the systemic-toxicity no-observed-effect level (NOEL) was 500 ppm and the reproductive NOEL was 2000 ppm. The reproductive NOEL was based solely on the decreased pup weights in both the F1 and F2 generations observed at 7000 ppm. In the developmental toxicity studies, only the rats showed evidence of maternal toxicity. For rats in the 7000 ppm group, statistically significant reductions were observed in overall maternal body weight gain and overall maternal food consumption for the treatment period. Rats exposed to 2000 ppm cyclohexane and above again exhibited a transient diminished or absent response to a sound stimulus while in the chambers during exposure. Therefore, for rats, the maternal no-observed-effect level (NOEL) was 500 ppm. In the rabbit developmental toxicity study, no compound-related maternal effects were observed at concentration levels of 7000 ppm and below. Therefore, the maternal NOEL for rabbits was 7000 ppm. No compound-related evidence of developmental toxicity was observed at any test concentration in either species. Therefore, the developmental NOEL for both species was 7000 ppm, the highest concentration tested.

Proposal for the reclassification of Thiobacillus novellus as Starkeya novella gen. nov., comb. nov., in the alpha-subclass of the Proteobacteria.

Thiobacillus novellus is a facultatively chemolithoautotrophic and methylotrophic, Gram-negative, rod-shaped sulfur bacterium, shown by 16S rRNA gene sequence analysis to be a member of the alpha-2 subclass of the Proteobacteria. As such, it must be excluded from the genus Thiobacillus, whose species are members of the beta-Proteobacteria. It closest phylogenetic neighbour appears to be Ancylobacter, from which it is distinct morphologically and in some physiological characteristics. It is distinct physiologically and biochemically in a number of diagnostic features from Paracoccus versutus, in the alpha-3 subclass of the Proteobacteria and does not appear to be sufficiently closely related to any other genus of the alpha-Proteobacteria to be reassigned to a known genus. The new genus and species name Starkeya novella is proposed for T. novellus. The type strain is ATCC 8093T (= NCIMB 10456T = NCIMB 9113T = DSM 506T = IAM 12100T = IFO 12443T = CCM 1077T).

Peroxisome proliferator-activated receptor gamma coactivator-1 promotes cardiac mitochondrial biogenesis.

Cardiac mitochondrial function is altered in a variety of inherited and acquired cardiovascular diseases. Recent studies have identified the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) as a regulator of mitochondrial function in tissues specialized for thermogenesis, such as brown adipose. We sought to determine whether PGC-1 controlled mitochondrial biogenesis and energy-producing capacity in the heart, a tissue specialized for high-capacity ATP production. We found that PGC-1 gene expression is induced in the mouse heart after birth and in response to short-term fasting, conditions known to increase cardiac mitochondrial energy production. Forced expression of PGC-1 in cardiac myocytes in culture induced the expression of nuclear and mitochondrial genes involved in multiple mitochondrial energy-transduction/energy-production pathways, increased cellular mitochondrial number, and stimulated coupled respiration. Cardiac-specific overexpression of PGC-1 in transgenic mice resulted in uncontrolled mitochondrial proliferation in cardiac myocytes leading to loss of sarcomeric structure and a dilated cardiomyopathy. These results identify PGC-1 as a critical regulatory molecule in the control of cardiac mitochondrial number and function in response to energy demands.