Using [2H]water to quantify the contribution of de novo palmitate synthesis in plasma: enabling back-to-back studies.

An increased contribution of de novo lipogenesis (DNL) may play a role in cases of dyslipidemia and adipose accretion; this suggests that inhibition of fatty acid synthesis may affect clinical phenotypes. Since it is not clear whether modulation of one step in the lipogenic pathway is more important than another, the use of tracer methods can provide a deeper level of insight regarding the control of metabolic activity. Although [2H]water is generally considered a reliable tracer for quantifying DNL in vivo (it yields a homogenous and quantifiable precursor labeling), the relatively long half-life of body water is thought to limit the ability of performing repeat studies in the same subjects; this can create a bottleneck in the development and evaluation of novel therapeutics for inhibiting DNL. Herein, we demonstrate the ability to perform back-to-back studies of DNL using [2H]water. However, this work uncovered special circumstances that affect the data interpretation, i.e., it is possible to obtain seemingly negative values for DNL. Using a rodent model, we have identified a physiological mechanism that explains the data. We show that one can use [2H]water to test inhibitors of DNL by performing back-to-back studies in higher species [i.e., treat nonhuman primates with platensimycin, an inhibitor of fatty acid synthase]; studies also demonstrate the unsuitability of [13C]acetate.

In vivo monitoring of urea cycle activity with (13)C-acetate as a tracer of ureagenesis.

BACKGROUND: The hepatic urea cycle is the main metabolic pathway for detoxification of ammonia. Inborn errors of urea cycle function present with severe hyperammonemia and a high case fatality rate. Long-term prognosis depends on the residual activity of the defective enzyme. A reliable method to estimate urea cycle activity in-vivo does not exist yet. The aim of this study was to evaluate a practical method to quantify (13)C-urea production as a marker for urea cycle function in healthy subjects, patients with confirmed urea cycle defect (UCD) and asymptomatic carriers of UCD mutations. METHODS: (13)C-labeled sodium acetate was applied orally in a single dose to 47 subjects (10 healthy subjects, 28 symptomatic patients, 9 asymptomatic carriers). RESULTS: The oral (13)C-ureagenesis assay is a safe method. While healthy subjects and asymptomatic carriers did not differ with regards to kinetic variables for urea cycle flux, symptomatic patients had lower (13)C-plasma urea levels. Although the (13)C-ureagenesis assay revealed no significant differences between individual urea cycle enzyme defects, it reflected the heterogeneity between different clinical subgroups, including male neonatal onset ornithine carbamoyltransferase deficiency. Applying the (13)C-urea area under the curve can differentiate between severe from more mildly affected neonates. Late onset patients differ significantly from neonates, carriers and healthy subjects. CONCLUSION: This study evaluated the oral (13)C-ureagenesis assay as a sensitive in-vivo measure for ureagenesis capacity. The assay has the potential to become a reliable tool to differentiate UCD patient subgroups, follow changes in ureagenesis capacity and could be helpful in monitoring novel therapies of UCD.

Influence of regular exercise on gastric emptying in healthy men: a pilot study.

Functional gastrointestinal disorders (FGIDs), including functional dyspepsia (FD), are common chronic disorders even in the younger population. Physical activity is advocated for patients with FGIDs, although the evidence is insufficient. We investigated the association between the intensity of regular exercise and gastric emptying to determine the effect of physical activity on dyspeptic symptoms. Thirty healthy individuals were selected and divided into three groups (low, moderate, and high) using the index of total exercise intensity in a week. Gastric emptying was evaluated by the 13C-acetate breath test. Gastroesophageal reflux symptoms, dyspeptic symptoms, stool forms, scores of anxiety and depression, and scores of sleep quality were also compared. Baseline scores of gastroesophageal reflux symptoms, anxiety, depression, and sleep quality were not different among the three groups. Gastric emptying was significantly faster in low-intensity exercise group than the moderate-intensity exercise group. Although the presence of loose stool and alcohol consumption were also associated with the intensity of regular exercise, these variables were not confounders. In conclusion, the intensity of regular exercise was independently associated with gastric emptying in healthy individuals. These baseline data would be useful for consideration of an optimal exercise intervention for the treatment of FD.

The Evaluation of Gastric Emptying Using the 13 C-Acetate Breath Test in Neurologically Impaired Patients - A Focus on the Stomach Function and Morphology.

PURPOSE: We explored factors affecting gastric emptying in neurologically impaired (NI) patients using the 13C-acetate breath test. METHODS: Twenty-four NI patients were classified by the presence of gastroesophageal reflux disease (GERD), which was treated by fundoplication plus gastrostomy, or the absence of GERD, which was treated by gastrostomy alone, along with gastric malposition involving cascade stomach and organoaxial gastric volvulus (OGV). Gastric emptying parameters (GEPs), which were the emptying half time (T 1/2 , minute), the lag phase time (T lag , minute), and the gastric emptying coefficient (GEC), were measured before and after surgery. We evaluated the relationship between GEPs and GERD, gastric malposition, and surgical intervention. All data were expressed as the median (interquartile range). RESULTS: The T1/2 and GEC of patients with OGV were significantly worse than in those without OGV before surgery (T1/2 with OGV: 241.3 [154.9, 314.3] vs. T1/2 without OGV: 113.7 [105.2, 151.4], p = 0.01, GEC with OGV: 3.19 [2.46, 3.28] vs. GEC without OGV: 3.65 [3.24, 3.90], p = 0.02). GERD and cascade stomach were not associ ated with GEPs. The GEPs of all NI patients showed no significant difference between before and after surgery. The surgical change in T1/2 (ΔT 1/2 ) in the patients with OGV was significantly lower than in those without OGV (ΔT1/2 with OGV: -47.1 [-142.7, -22.1] vs. ΔT1/2 without OGV: -3.78 [-26.6, 12.0], p = 0.03). CONCLUSION: Stomach malposition, such as OGV, seems to affect gastric emptying and may be improved by surgi cal intervention.

Dietary Fibre and Organic Acids in Kiwifruit Suppress Glycaemic Response Equally by Delaying Absorption-A Randomised Crossover Human Trial with Parallel Analysis of 13C-Acetate Uptake.

Non-sugar components of kiwifruit reduce the amplitude of the glycaemic response to co-consumed cereal starch. We determined the relative contribution of different non-sugar kiwifruit components to this anti-glycaemic effect. Healthy participants (n = 9) ingested equal carbohydrate meals containing 20 g starch as wheat biscuit (WB, 30 g), and the sugar equivalent of two kiwifruit (KFsug, 20.4 g), either intrinsic or added as glucose, fructose and sucrose (2:2:1). The meals were WB+KFsug (control, no non-sugar kiwifruit components), WB + whole kiwifruit pulp (WB+KF), WB + neutralised kiwifruit pulp (WB+KFneut), WB + low-fibre kiwifruit juice (WB+KFjuice) and WB+KFsug + kiwifruit organic acids (WB+KFsug+OA). All meals were spiked with 100 mg sodium [1-13C] acetate to measure intestinal absorption. Each participant ingested all meals in random order. Blood glucose and breath 13CO2 were measured at ingestion and at 15 min intervals up to 180 min. Compared with WB+KFsug, whole kiwifruit pulp (WB+KF) almost halved glycaemic response amplitude (p < 0.001), reduced incremental area under the blood glucose response curve (iAUC) at 30 min (peak) by 50% (p < 0.001), and averted late postprandial hypoglycaemia. All other treatments suppressed response amplitude half as much as whole kiwifruit and averted acute hypoglycaemia, with little effect on iAUC. Effects on 13CO2 exhalation paralleled effects on blood glucose (R2 = 0.97). Dietary fibre and organic acids contributed equally to the anti-glycaemic effect of kiwifruit by reducing intestinal absorption rate. Kiwifruit flesh effectively attenuates glycaemic response in carbohydrate exchange, as it contains fructose, dietary fibre and organic acids.

pH Dependence of T2 for Hyperpolarizable 13C-Labelled Small Molecules Enables Spatially Resolved pH Measurement by Magnetic Resonance Imaging.

Hyperpolarized 13C magnetic resonance imaging often uses spin-echo-based pulse sequences that are sensitive to the transverse relaxation time T2. In this context, local T2-changes might introduce a quantification bias to imaging biomarkers. Here, we investigated the pH dependence of the apparent transverse relaxation time constant (denoted here as T2) of six 13C-labelled molecules. We obtained minimum and maximum T2 values within pH 1-13 at 14.1 T: [1-13C]acetate (T2,min = 2.1 s; T2,max = 27.7 s), [1-13C]alanine (T2,min = 0.6 s; T2,max = 10.6 s), [1,4-13C2]fumarate (T2,min = 3.0 s; T2,max = 18.9 s), [1-13C]lactate (T2,min = 0.7 s; T2,max = 12.6 s), [1-13C]pyruvate (T2,min = 0.1 s; T2,max = 18.7 s) and 13C-urea (T2,min = 0.1 s; T2,max = 0.1 s). At 7 T, T2-variation in the physiological pH range (pH 6.8-7.8) was highest for [1-13C]pyruvate (ΔT2 = 0.95 s/0.1pH) and [1-13C]acetate (ΔT2 = 0.44 s/0.1pH). Concentration, salt concentration, and temperature alterations caused T2 variations of up to 45.4% for [1-13C]acetate and 23.6% for [1-13C]pyruvate. For [1-13C]acetate, spatially resolved pH measurements using T2-mapping were demonstrated with 1.6 pH units accuracy in vitro. A strong proton exchange-based pH dependence of T2 suggests that pH alterations potentially influence signal strength for hyperpolarized 13C-acquisitions.

Tracking Metabolic Rewiring of Cancer Metabolism in Humans Using Isotope-Resolved NMR.

Altered metabolism is considered one of the hallmarks of cancer. The findings that malignant brain tumors and brain metastases utilize acetate as an alternative nutrient are relatively recent and offer new avenues for investigation of altered metabolism in human cancers. Here, we describe comprehensively the details of the 13C NMR-based isotopomer methodology to measure in vivo acetate utilization in brain tumor patients, including the contribution from acetate metabolism of peripheral tissues. Methods described in this chapter can be readily extended to other cancer types.

Autonomic dysfunction of gastric motility in major depression.

BACKGROUND: Patients suffering from major depressive disorder (MDD) often complain about somatic symptoms. Cardiac complaints have been examined predominantly. However, gastrointestinal complaints are also reported frequently and are associated with worse outcomes. The research concerning changes in gastric motility of these patients is rather sparse. The aim of our study was to determine dysfunction of gastric motility and gastrointestinal symptoms in MDD. The duration and severity of MDD were examined regarding its influence over gastric emptying. METHODS: Gastric emptying was determined by a 13C-acetate breath test in patients with MDD (n = 29) and healthy control subjects (n = 51). Prior to this, depressive illness was operationalized using external and self-assessment scales (HAMD-21, MADRS, BDI, CGI). Whether the severity or duration of MDD influenced the gastric emptying parameters was examined using Spearman's correlation. In addition, autonomic complaints were recorded by means of an ANS score. Each ANS score item was determined using a Mann-Whitney U or Kruskal-Wallis test concerning the gastric emptying parameters. RESULTS: There was a significant difference in the parameters of the maximum gastric emptying rate (Tmax) and gastric half emptying time T1/2b between patients with MDD and healthy control subjects (Tmax 66.21min vs 53.35min, p < 0.006, T1/2b 207.59min vs 133.27min, p < 0.005). There was a significant negative correlation between Tmax and the severity of MDD determined with the depression rating scales BDI (Spearman's rank - 0.521, p = 0.013) and HAMD-21 (r - 0.384, p = 0.048). No correlation was found between the duration of MDD and the maximum gastric emptying rate (r - 0.125, p = 0.519) and gastric half emptying time (r - 0.62, p = 0.749). CONCLUSION: Gastrointestinal motility is significantly impaired in patients with MDD compared to healthy control subjects. Autonomic complaints were indicated frequently in MDD patients. The duration of MDD had no influence over the time of gastric emptying. There was a significant negative correlation between the severity of MDD and Tmax, indicating that the Tmax was reached earlier with the progression of MDD. The slowing of gastric motility in MDD patients is likely a result of a dysfunction of the autonomic nervous system.

Measurement of 13 C turnover into glutamate and glutamine pools in brain tumor patients.

Malignant brain tumors are known to utilize acetate as an alternate carbon source in the citric acid cycle for their bioenergetics. 13 C NMR-based isotopomer analysis has been used to measure turnover of 13 C-acetate carbons into glutamate and glutamine pools in tumors. Plasma from the patients infused with [1,2-13 C]acetate further revealed the presence of 13 C isotopomers of glutamine, glucose, and lactate in the circulation that were generated due to metabolism of [1,2-13 C]acetate by peripheral organs. In the tumor cells, [4-13 C] and [3,4-13 C]glutamate and glutamine isotopomers were generated from blood-borne 13 C-labeled glucose and lactate which were formed due to [1,2-13 C[acetate metabolism of peripheral tissues. [4,5-13 C] and [3,4,5-13 C]glutamate and glutamine isotopomers were produced from [1,2-13 C]acetyl-CoA that was derived from direct oxidation of [1,2-13 C] acetate in the tumor. Major portion of C4 13 C fractional enrichment of glutamate (93.3 ± 0.02%) and glutamine (90.9 ± 0.03%) were derived from [1,2-13 C]acetate-derived acetyl-CoA.

Hepatic gluconeogenesis influences (13)C enrichment in lactate in human brain tumors during metabolism of [1,2-(13)C]acetate.

(13)C-enriched compounds are readily metabolized in human malignancies. Fragments of the tumor, acquired by biopsy or surgical resection, may be acid-extracted and (13)C NMR spectroscopy of metabolites such as glutamate, glutamine, 2-hydroxyglutarate, lactate and others provide a rich source of information about tumor metabolism in situ. Recently we observed (13)C-(13)C spin-spin coupling in (13)C NMR spectra of lactate in brain tumors removed from patients who were infused with [1,2-(13)C]acetate prior to the surgery. We found, in four patients, that infusion of (13)C-enriched acetate was associated with synthesis of (13)C-enriched glucose, detectable in plasma. (13)C labeled glucose derived from [1,2-(13)C]acetate metabolism in the liver and the brain pyruvate recycling in the tumor together lead to the production of the (13)C labeled lactate pool in the brain tumor. Their combined contribution to acetate metabolism in the brain tumors was less than 4.0%, significantly lower than the direct oxidation of acetate in the citric acid cycle in tumors.

On the validity of the (13) C-acetate breath test for comparing gastric emptying of different liquid test meals: a validation study using magnetic resonance imaging.

BACKGROUND: (13) C-acetate breath testing (BT) is applied to assess and compare gastric emptying of liquid meals. Gastric half-emptying times (t50 ) from BT show offsets compared to t50 values from γ-scintigraphy and ultrasonography. Linear transformations have been proposed to correct these offsets. This investigation critically validates the BT for the assessment of liquid gastric emptying by using simultaneously recorded meal and total gastric content volume emptying data from magnetic resonance imaging (MRI). METHODS: Data were collected during a recently published double-blind, randomized, cross-over MRI gastric emptying study of three (13) C-labeled enteral formulas differing in protein sources (PMID: 24699556). Breath testing-derived t50 was computed with the analysis methods commonly applied in gastric emptying research, i.e., the exponential-beta function and the Wagner-Nelson (WN) method, respectively. KEY RESULTS: Breath testing t50 values from exponential-beta function and WN method showed a positive and negative offset to MRI data, respectively. Linear regression detected low concordance between MRI and both BT methods revealing meal specific and emptying rate-dependent offsets. The WN method showed worse agreement and correlation with MRI emptying data. Breath testing rather reflected meal volume than total gastric content volume emptying. CONCLUSIONS & INFERENCES: This validation study indicates that the (13) C-acetate breath test may not be applied to compare gastric emptying of arbitrary liquid meals without prior validation by imaging methods. t50 values from BT are biased by (i) the properties of the meal and (ii) the selected method used for (13) CO2 exhalation analysis. No linear transformation common for all meals was applicable to correct the offsets between BT and MRI.

Evaluation of periesophageal nerve injury after pulmonary vein isolation using the (13)C-acetate breath test.

BACKGROUND: Pulmonary vein isolation (PVI) has become an important option for treating patients with atrial fibrillation (AF). Periesophageal nerve (PEN) injury after PVI causes pyloric spasms and gastric hypomotility. This study aimed to clarify the impact of PVI on gastric motility and assess the prevalence of gastric hypomotility after PVI. METHODS: Thirty consecutive patients with AF underwent PVI under luminal esophageal temperature (LET) monitoring. The (13)C-acetate breath test was conducted before and after the procedure for all patients (PVI group). Gastric emptying was evaluated using the time to peak concentration of (13)CO2 (T max). This test was also conducted in another 20 patients who underwent catheter ablation procedures other than PVI (control group). RESULTS: The number of patients with abnormal T max (≥75 min) increased from seven (23%) to 13 (43%) and from three (15%) to five (25%) after the procedure in the PVI group and control group, respectively. The mean T max was longer after PVI than before PVI (64±14 min vs. 57±15 min, p=0.006), whereas there was no significant difference before and after the procedure in the control group. However, no significant difference in ΔT max was observed between the two groups (p=0.27). No patients suffered from symptomatic gastric hypomotility. CONCLUSIONS: Asymptomatic gastric hypomotility occurred more often after PVI. However, the average impact of PVI on gastric motility was minimal.

Real-time cardiac metabolism assessed with hyperpolarized [1-(13) C]acetate in a large-animal model.

Dissolution-dynamic nuclear polarization (dissolution-DNP) for magnetic resonance (MR) spectroscopic imaging has recently emerged as a novel technique for noninvasive studies of the metabolic fate of biomolecules in vivo. Since acetate is the most abundant extra- and intracellular short-chain fatty acid, we focused on [1-(13) C]acetate as a promising candidate for a chemical probe to study the myocardial metabolism of a beating heart. The dissolution-DNP procedure of Na[1-(13) C]acetate for in vivo cardiac applications with a 3 T MR scanner was optimized in pigs during bolus injection of doses of up to 3 mmol. The Na[1-(13) C]acetate formulation was characterized by a liquid-state polarization of 14.2% and a T1Eff in vivo of 17.6 ± 1.7 s. In vivo Na[1-(13) C]acetate kinetics displayed a bimodal shape: [1-(13) C]acetyl carnitine (AcC) was detected in a slice covering the cardiac volume, and the signal of (13) C-acetate and (13) C-AcC was modeled using the total area under the curve (AUC) for kinetic analysis. A good correlation was found between the ratio AUC(AcC)/AUC(acetate) and the apparent kinetic constant of metabolic conversion, from [1-(13) C]acetate to [1-(13) C]AcC (kAcC ), divided by the AcC longitudinal relaxation rate (r1 ). Our study proved the feasibility and the limitations of administration of large doses of hyperpolarized [1-(13) C]acetate to study the myocardial conversion of [1-(13) C]acetate in [1-(13) C]acetyl-carnitine generated by acetyltransferase in healthy pigs.

Nissen fundoplication for gastroesophageal reflux: No deterioration of gastric emptying measured by C-acetate breath test.

AIM: To study the gastric emptying 30 days after laparoscopic Nissen fundoplication (NF) in gastroesophageal reflux. MATERIALS AND METHODS: Three patients were evaluated with (13)C-acetate breath test (ABT) performed pre and post-NF. The liquid test meal consisted of Racol(™) mixed with (13)C-acetate. RESULTS: In the patient without neurological impairment (NI), the preoperative t(½)ex and t lag were 0.900 and 0.510 hours, respectively. The postoperative t(½)ex and t lag were 0.959 and 0.586 hours, respectively. In one patient with NI, the preoperative t(½)ex and t lag were 1.828 and 1.092 hours, respectively. The postoperative t(½)ex and t lag were 2.081 and 1.025 hours, respectively. In the other patient with NI, the preoperative t(½)ex and t lag were 2.110 and 0.980 hours, respectively. The postoperative t(½)ex and t lag were 1.118 and 0.415 hours, respectively. CONCLUSIONS: Our findings suggest that (13)C-ABT parameters did not worsen in any of the children after laparoscopic NF.

Removal of ovarian hormones affects the ageing process of acetate metabolism.

BACKGROUND: Despite a close association between gastrointestinal motility and sex hormones, it has been unknown whether ovarian hormones affect absorption and metabolism of nutrients. The aim of this study is, therefore, to evaluate metabolism of acetate in rats with age and the influence of ovariectomy on its change. METHODS: Fourteen female rats of the F344 strain were used, and 13C-acetate breath test was performed at 2, 7 and 13 months of age. Seven rats were ovariectomized at three weeks of age (ovariectomy group) and the remaining seven rats were studied as control group. After 24-hr fasting, rats are orally administrated 1ml of water containing sodium (13)C-acetate (100mg/kg) and housed in an animal chamber. The expired air in the chamber is collected in a breath-sampling bag using a aspiration pump. The (13)CO(2) concentration is measured using an infrared spectrometer for 120 min and expressed as delta per mil. RESULTS: The breath (13)CO(2) excretion increased with time and peaked 30 min in control rats. In ovariectomized rats, thee peak time of (13)CO(2) excretion was prolonged to 40 min at 7 and 13 months of age. Cmax was significantly higher at 2 months of age but lower at 4 months of age in ovariectomized rats than in control rats. Those of two groups became equal at 7 months of age. CONCLUSIONS: From the viewpoint of acetate metabolism, removal of ovarian hormones might make rats to be precocious ones and accelerate ageing.

Influence of chronic ethanol consumption on extra-pancreatic secretory function in rat.

BACKGROUND: The usefulness of the typical direct methods involving duodenal intubation, such as the secretin and secretin-cholecystokinin tests, in the diagnosis of exocrine pancreatic dysfunction is widely accepted. However, these diagnostic tests tend to be avoided because of their technical complexity and the burden on patients. Recently, a simple breath test was developed for assessment of exocrine pancreatic function employing 13C-dipeptide [i.e., benzoyl-L-tyrosyl-[1-13C] alanine (Bz-Tyr-Ala)]. Although alcohol abuse causes pancreatic damage in humans, this has been unclear in rats. AIMS: The aim of the study is to evaluate the effect of ethanol exposure beginning at an early age on extra-pancreatic secretory function in rats. MATERIALS AND METHODS: Twelve female rats of the F344 strain aged 12 months were used. Seven rats were fed on a commercial mash food with 16% ethanol solution (Japanese Sake) as drinking-fluid since at 29 days of age (ethanol group). The remaining five rats were fed on a nutrient-matched isocaloric diet with water as drinking-fluid (control group). After 24-hr fasting, rats are orally administrated 1cc of water containing sodium 13C-dipeptide (5 mg/kg) and housed in an animal chamber. The expired air in the chamber is collected in a breath-sampling bag using a tube and aspiration pump. The 13CO2 concentration is measured using an infrared spectrometer at 10-min interval for 120 min and expressed as delta per mil. RESULTS: The breath 13CO2 level increased and peaked at 20 min in both two groups. In general, 13CO2 excretion peaked rapidly and also decreased sooner in ethanol rats than in control rats. The mean value of the maximal 13CO2 excretion is 34.7 per mil in ethanol rats, greater than in control rats (31.4 per mil), but the difference did not reach the statistically significance. CONCLUSION: Chronic ethanol feeding beginning at an early age does not affect extra-pancreatic secretory function in rats.