Timing of Regadenoson-induced Peak Hyperemia and the Effects on Coronary Flow Reserve

Background

Regadenoson is used to induce hyperemia in cardiac imaging, facilitating diagnosis of ischemia and assessment of coronary flow reserve (CFR). While the regadenoson package insert recommends administration of radionuclide tracer 10-20 seconds after injection, peak hyperemia has been observed at approximately 100 seconds after injection in healthy volunteers undergoing cardiovascular magnetic resonance imaging (CMR). It is unclear when peak hyperemia occurs in a patient population.

Objectives

The goal of this study was to determine time to peak hyperemia after regadenoson injection in healthy volunteers and patients, and whether the recommended image timing in the package insert underestimates CFR.

Methods

Healthy volunteers (n=15) and patients (n=25) underwent stress CMR, including phase-contrast imaging of the coronary sinus at rest and multiple timepoints after 0.4 mg regadenoson injection. Coronary sinus flow (ml/min) was divided by resting values to yield CFR. Smoothed, time-resolved curves for CFR were generated with pointwise 95% confidence intervals.

Results

CFR between 60 and 120 seconds was significantly higher than CFR at 30 seconds after regadenoson injection (p < 0.05) as shown by non-overlapping 95% confidence intervals for both healthy volunteers (30 s, [2.8, 3.4]; 60 s, [3.8, 4.4]; 90 s, [4.1, 4.7]; 120 s, [3.6, 4.3]) and patients (30 s, [2.1, 2.5]; 60 s, [2.6, 3.1]; 90 s, [2.7, 3.2]; 120 s, [2.5, 3.1]).

Conclusion

Imaging at 90 seconds following regadenoson injection is the optimal approach to capture peak hyperemia. Imaging at 30 seconds, which is more aligned with the package insert recommendation, would yield an underestimate of CFR and confound assessment of microvascular dysfunction.

Dietary Macronutrient Composition Differentially Modulates the Remodeling of Mitochondrial Oxidative Metabolism during NAFLD

Diets rich in fats and carbohydrates aggravate non-alcoholic fatty liver disease (NAFLD), of which mitochondrial dysfunction is a central feature. It is not clear whether a high-carbohydrate driven ‘lipogenic’ diet differentially affects mitochondrial oxidative remodeling compared to a high-fat driven ‘oxidative’ environment. We hypothesized that the high-fat driven ‘oxidative’ environment will chronically sustain mitochondrial oxidative function, hastening metabolic dysfunction during NAFLD. Mice (C57BL/6NJ) were reared on a low-fat (LF; 10% fat calories), high-fat (HF; 60% fat calories), or high-fructose/high-fat (HFr/HF; 25% fat and 34.9% fructose calories) diet for 10 weeks. De novo lipogenesis was determined by measuring the incorporation of deuterium from D₂O into newly synthesized liver lipids using nuclear magnetic resonance (NMR) spectroscopy. Hepatic mitochondrial metabolism was profiled under fed and fasted states by the incubation of isolated mitochondria with [¹³C₃]pyruvate, targeted metabolomics of tricarboxylic acid (TCA) cycle intermediates, estimates of oxidative phosphorylation (OXPHOS), and hepatic gene and protein expression. De novo lipogenesis was higher in the HFr/HF mice compared to their HF counterparts. Contrary to our expectations, hepatic oxidative function after fasting was induced in the HFr/HF group. This differential induction of mitochondrial oxidative function by the high fructose-driven ‘lipogenic’ environment could influence the progressive severity of hepatic insulin resistance.

Branched chain amino acids and carbohydrate restriction exacerbate ketogenesis and hepatic mitochondrial oxidative dysfunction during NAFLD

Mitochondrial adaptation during non-alcoholic fatty liver disease (NAFLD) include remodeling of ketogenic flux and sustained tricarboxylic acid (TCA) cycle activity, which are concurrent to onset of oxidative stress. Over 70% of obese humans have NAFLD and ketogenic diets are common weight loss strategies. However, the effectiveness of ketogenic diets toward alleviating NAFLD remains unclear. We hypothesized that chronic ketogenesis will worsen metabolic dysfunction and oxidative stress during NAFLD. Mice (C57BL/6) were kept (for 16-wks) on either a low-fat, high-fat, or high-fat diet supplemented with 1.5X branched chain amino acids (BCAAs) by replacing carbohydrate calories (ketogenic). The ketogenic diet induced hepatic lipid oxidation and ketogenesis, and produced multifaceted changes in flux through the individual steps of the TCA cycle. Higher rates of hepatic oxidative fluxes fueled by the ketogenic diet paralleled lower rates of de novo lipogenesis. Interestingly, this metabolic remodeling did not improve insulin resistance, but induced fibrogenic genes and inflammation in the liver. Under a chronic "ketogenic environment," the hepatocyte diverted more acetyl-CoA away from lipogenesis toward ketogenesis and TCA cycle, a milieu which can hasten oxidative stress and inflammation. In summary, chronic exposure to ketogenic environment during obesity and NAFLD has the potential to aggravate hepatic mitochondrial dysfunction.

Impact of Branched Chain Amino Acid Supplementation on Hepatic Mitochondrial Metabolism in Mice with Non-alcoholic Fatty Liver Disease (P08-136-19)

Objectives

Elevated circulating branched chain amino acid (BCAA) levels are correlated with the development of insulin resistance and type 2 diabetes mellitus in humans. On the other hand, BCAA supplementation has been shown to be beneficial in improving insulin resistance during chronic liver disease. Furthermore, there is recent evidence of significant crosstalk between BCAA and mitochondrial lipid metabolism. Considering the central role of dysfunctional mitochondrial lipid metabolism in diet-induced obesity and non-alcoholic fatty liver disease (NAFLD), our objective was to determine the impact of dietary BCAA supplementation on hepatic mitochondrial function, in a diet-induced mouse model of NAFLD. We hypothesized that the dietary supplementation of BCAA, together with a high fat diet, will exacerbate hepatic mitochondrial dysfunction during NAFLD.

Methods

C57BL/6NJ mice were fed with a control (10% kcal fat), high fat (HF; 60% kcal fat), or HF diet supplemented with BCAA (BA; 60% kcal fat; 1.5X BCAA) diet for 16 weeks. Livers from these mice were used for mitochondrial isolation, total liver and mitochondrial protein estimation, determination of amino acids and Kreb's cycle intermediates by mass spectrometry, and gene expression profiles.

Results

While the liver weights (g ± SEM) of HF fed mice (2.6 ± 0.36) were significantly higher than the control mice (1.5 ± 0.19), BCAA supplemented mice had significantly lower liver weights than the HF fed mice (1.8 ± 0.28). Hepatic mitochondrial protein content (µg/g liver ± SEM) was enriched in BCAA supplemented mice compared to their HF diet fed counterparts (BA, 3858 ± 476; HF, 2635 ± 394; P ˂ 0.05). Many of the organic acid intermediates of the Kreb's cycle were significantly lower in the liver of HF fed mice. Interestingly, BCAA supplementation (BA) with HF feeding restored hepatic organic acid intermediates to similar levels observed in control mice. Further, HF and BA feeding, both downregulated lipogenic gene expression (e.g., Fasn, Scd1, Acly) in the liver.

Conclusions

Our results suggest that BCAA supplementation enhanced hepatic mitochondrial biogenesis and oxidative metabolism in the liver of mice with NAFLD. High-fat diets which significantly suppressed the rates of de novo lipogenesis, could have provided the BCAAs, a metabolic milieu favorable for the induction of mitochondrial activity.

Funding Sources

National Institutes of Health (R01).