Effects of anesthesia on renal function and metabolism in rats assessed by hyperpolarized MRI

β-Hydroxybutyrate Attenuates Painful Diabetic Neuropathy via Restoration of the Aquaporin-4 Polarity in the Spinal Glymphatic System

Waste removal is essential for maintaining homeostasis and the normal function of the central nervous system (CNS). The glymphatic system based on aquaporin-4 (AQP4) water channels on the endfeet of astrocytes is recently discovered as the excretion pathway for metabolic waste products of CNS. In the CNS, α-syntrophin (SNTA1) directly or indirectly anchors AQP4 in astrocyte membranes facing blood vessels. Studies have indicated that β-hydroxybutyrate (BHB) can raise the expression of SNTA1 and thus restoring AQP4 polarity in mice models with Alzheimer’s disease. The study aims to evaluate the neuroprotective mechanism of BHB in rats with painful diabetic neuropathy (PDN). PDN rats were modeled under a high-fat and high-glucose diet with a low dose of streptozotocin. Magnetic resonance imaging (MRI) was applied to observe the clearance of contrast to indicate the functional variability of the spinal glymphatic system. Mechanical allodynia was assessed by paw withdrawal threshold. The expressions of SNTA1 and AQP4 were tested, and the polarity reversal of AQP4 protein was measured. As demonstrated, PDN rats were manifested with deceased contrast clearance of the spinal glymphatic system, enhanced mechanical allodynia, lower expression of SNTA1, higher expression of AQP4, and reversed polarity of AQP4 protein. An opposite change in the above characteristics was observed in rats being treated with BHB. This is the first study that demonstrated the neuroprotective mechanism of BHB to attenuate PDN via restoration of the AQP4 polarity in the spinal glymphatic system and provides a promising therapeutic strategy for PDN.

Metabolism of hyperpolarised [1-13 C]pyruvate in awake and anaesthetised rat brains

The use of hyperpolarised ¹³ C pyruvate for nononcological neurological applications has not been widespread so far, possibly due to delivery issues limiting the visibility of metabolites. First proof-of-concept results have indicated that metabolism can be detected in human brain, and this may supersede the results obtained in preclinical settings. One major difference between the experimental setups is that preclinical MRI/MRS routinely uses anaesthesia, which alters both haemodynamics and metabolism. Here, we used hyperpolarised [1-¹³ C]pyruvate to compare brain metabolism in awake rats and under isoflurane, urethane or medetomidine anaesthesia. Spectroscopic [1-¹³ C]pyruvate time courses measured sequentially showed that pyruvate-to-bicarbonate and pyruvate-to-lactate labelling rates were lower in isoflurane animals than awake animals. An increased bicarbonate-to-lactate ratio was observed in the medetomidine group compared with other groups. The study shows that hyperpolarised [1-¹³ C]pyruvate experiments can be performed in awake rats, thus avoiding anaesthesia-related issues. The results suggest that haemodynamics probably dominate the observed pyruvate-to-metabolite labelling rates and area-under-time course ratios of referenced to pyruvate. On the other hand, the results obtained with medetomidine suggest that the ratios are also modulated by the underlying cerebral metabolism. However, the ratios between intracellular metabolites were unchanged in awake compared with isoflurane-anaesthetised rats.

Robust arterial spin labeling MRI measurement of pharmacologically induced perfusion change in rat kidneys

Kidney diseases such as acute kidney injury, diabetic nephropathy and chronic kidney disease (CKD) are related to dysfunctions of the microvasculature in the kidney causing a decrease in renal blood perfusion (RBP). Pharmacological intervention to improve the function of the microvasculature is a viable strategy for the potential treatment of these diseases. The measurement of RBP is a reliable biomarker to evaluate the efficacy of pharmacological agents' actions on the microvasculature, and measurement of RBP responses to different pharmacological agents can also help elucidate the mechanism of hemodynamic regulation in the kidney. Magnetic resonance imaging (MRI) with flow-sensitive alternating inversion recovery (FAIR) arterial spin labeling (ASL) has been used to measure RBP in humans and animals. However, artifacts caused by respiratory and peristaltic motions limit the potential of FAIR ASL in drug discovery and kidney research. In this study, the combined anesthesia protocol of inactin with a low dose of isoflurane was used to fully suppress peristalsis in rats, which were ventilated with an MRI-synchronized ventilator. FAIR ASL data were acquired in eight axial slices using a single-shot, gradient-echo, echo-planar imaging (EPI) sequence. The artifacts in the FAIR ASL RBP measurement due to respiratory and peristaltic motions were substantially eliminated. The RBP responses to fenoldopam and L-NAME were measured, and the increase and decrease in RBP caused by fenoldopam and L-NAME, respectively, were robustly observed. To further validate FAIR ASL, the renal blood flow (RBF) responses to the same agents were measured by an invasive perivascular flow probe method. The pharmacological agent-induced responses in RBP and RBF are similar. This indicates that FAIR ASL has the sensitivity to measure pharmacologically induced changes in RBP. FAIR ASL with multislice EPI can be a valuable tool for supporting drug discovery, and for elucidating the mechanism of hemodynamic regulation in kidneys.

Hyperpolarized Carbon (13C) MRI of the Kidney: Experimental Protocol

Alterations in renal metabolism are associated with both physiological and pathophysiologic events. The existing noninvasive analytic tools including medical imaging have limited capability for investigating these processes, which potentially limits current understanding of kidney disease and the precision of its clinical diagnosis. Hyperpolarized 13C MRI is a new medical imaging modality that can capture changes in the metabolic processing of certain rapidly metabolized substrates, as well as changes in kidney function. Here we describe experimental protocols for renal metabolic [1-13C]pyruvate and functional 13C-urea imaging step-by-step. These methods and protocols are useful for investigating renal blood flow and function as well as the renal metabolic status of rodents in vivo under various experimental (patho)physiological conditions.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This experimental protocol is complemented by two separate chapters describing the basic concept and data analysis.

Hyperpolarised 13C-MRI metabolic and functional imaging: an emerging renal MR diagnostic modality

Magnetic resonance imaging (MRI) is a well-established modality for assessing renal morphology and function, as well as changes that occur during disease. However, the significant metabolic changes associated with renal disease are more challenging to assess with MRI. Hyperpolarized carbon-13 MRI is an emerging technique which provides an opportunity to probe metabolic alterations at high sensitivity by providing an increase in the signal-to-noise ratio of 20,000-fold or more. This review will highlight the current status of hyperpolarised 13C-MRI and its translation into the clinic and how it compares to metabolic measurements provided by competing technologies such as positron emission tomography (PET).

High Intrarenal Lactate Production Inhibits the Renal Pseudohypoxic Response to Acutely Induced Hypoxia in Diabetes

Intrarenal hypoxia develops within a few days after the onset of insulinopenic diabetes in an experimental animal model (ie, a model of type-1 diabetes). Although diabetes-induced hypoxia results in increased renal lactate formation, mitochondrial function is well maintained, a condition commonly referred to as pseudohypoxia. However, the metabolic effects of significantly elevated lactate levels remain unclear. We therefore investigated in diabetic animals the response to acute intrarenal hypoxia in the presence of high renal lactate formation to delineate mechanistic pathways and compare these findings to healthy control animals. Hyperpolarized ¹³C-MRI and blood oxygenation level–dependent ¹H-MRI was used to investigate the renal metabolism of [1-¹³C]pyruvate and oxygenation following acutely altered oxygen content in the breathing gas in a streptozotocin rat model of type-1 diabetes with and without insulin treatment and compared with healthy control rats. The lactate signal in the diabetic kidney was reduced by 12%–16% during hypoxia in diabetic rats irrespective of insulin supplementation. In contrast, healthy controls displayed the well-known Pasteur effect manifested as a 10% increased lactate signal following reduction of oxygen in the inspired air. Reduced expression of the monocarboxyl transporter-4 may account for altered response to hypoxia in diabetes with a high intrarenal pyruvate-to-lactate conversion. Reduced intrarenal lactate formation in response to hypoxia in diabetes shows the existence of a different metabolic phenotype, which is independent of insulin, as insulin supplementation was unable to affect the pyruvate-to-lactate conversion in the diabetic kidney.

Glucose metabolism in brown adipose tissue determined by deuterium metabolic imaging in rats

BACKGROUND/OBJECTIVES

Brown adipose tissue (BAT) has gained growing interest as a potential target for treatment of obesity. Currently, the most widely used technique/method for in vivo measurements of BAT activity in humans is ¹⁸FDG PET/CT. To supplement these investigations novel radiation-free methods are warranted. Deuterium metabolic imaging (DMI) is a novel modality that combines magnetic resonance spectroscopic (MRS) imaging with deuterium-labelled glucose (²H-glucose). This allows for spatio-temporal and metabolic imaging beyond glucose uptake. We aimed to evaluate if DMI could discriminate glucose metabolism in BAT of cold-acclimatised and thermoneutral rats.

SUBJECTS/METHODS

Male Sprague-Dawley rats were housed in a cold environment (9 °C, n = 10) or at thermoneutrality (30 °C, n = 11) for 1 week. For imaging rats were anaesthetized, received a ²H-glucose (1 M, 1.95 g/kg) bolus and DMI was acquired at baseline followed by 20 min time intervals up to 2 h. Furthermore, Dixon MRI was performed for anatomical determination of the interscapular BAT (iBAT) depot along with dynamic contrast enhanced (DCE) MRI to evaluate perfusion.

RESULTS

²H-glucose signal was higher in cold-acclimatised rats compared with thermoneutral rats (p ≤ 0.001) indicating an overall increase in glucose uptake and metabolism. This was in line with a lower fat/water threshold, higher perfusion and increased UCP1 mRNA expression in iBAT (ninefold increment) of cold-acclimatised rats compared with thermoneutral rats.

CONCLUSIONS

We find that DMI can discriminate cold-acclimatised and thermoneutral BAT in rats. This is the first study to evaluate BAT activity by DMI, which may open up for the use of the non-radioactive DMI method for BAT measurements in humans.

Renal effects of general anesthesia from old to recent studies

Various types of anesthesia are being utilized to maintain physiologically secured surgical conditions. Nearly all categories of general anesthesia are characterized by various perioperative and postoperative complications. These shortcomings are important aspects that need to be considered by the anesthesiologist and surgeon before administration of these compounds. The renal effects of anesthesia play an important role in understanding possible systemic changes due to the fact that the kidney has a direct or indirect impact on nearly all the systems of the body. Various studies have been conducted to find out changes in renal parameters and its systemic effects upon administration of the anesthesia and its postoperative repercussions. Besides that, the impaired renal function might have an impact on the excretion of anesthetic metabolites, which can lead to long-term dysfunction. Patients with a previous history of disease ought to be brought under consideration because these chemicals can ameliorate pre-existent symptoms. This review is intended to discuss the early and latest studies based on the effects of general anesthesia on the renal system.

Hyperpolarized [1-13 C] pyruvate as a possible diagnostic tool in liver disease

Introduction of hyperpolarized magnetic resonance in preclinical studies and lately translation to patients provides new detailed in vivo information of metabolic flux in organs. Hyperpolarized magnetic resonance based on 13 C enriched pyruvate is performed without ionizing radiation and allows quantification of the pyruvate conversion products: alanine, lactate and bicarbonate in real time. Thus, this methodology has a promising potential for in vivo monitoring of energetic alterations in hepatic diseases. Using 13 C pyruvate, we investigated the metabolism in the porcine liver before and after intravenous injection of glucose. The overall mean lactate to pyruvate ratio increased significantly after the injection of glucose whereas the bicarbonate to pyruvate ratio was unaffected, representative of the levels of pyruvate entering the tricarboxylic acid cycle. Similarly, alanine to pyruvate ratio did not change. The increased lactate to pyruvate ratio over time showed an exponential correlation with insulin, glucagon and free fatty acids. Together, these data, obtained by hyperpolarized 13 C magnetic resonance spectroscopy and by blood sampling, indicate a hepatic metabolic shift in glucose utilization following a glucose challenge. Our findings demonstrate the capacity of hyperpolarized 13 C magnetic resonance spectroscopy for quantifying hepatic substrate metabolism in accordance with well-known physiological processes. When combined with concentration of blood insulin, glucagon and free fatty acids in the blood, the results indicate the potential of hyperpolarized magnetic resonance spectroscopy as a future clinical method for quantification of hepatic substrate metabolism.

Hyperpolarized [1-13 C]pyruvate MRI can image the metabolic shift in cardiac metabolism between the fasted and fed state in a porcine model

PURPOSE

Owing to its noninvasive nature, hyperpolarized MRI may improve delineation of myocardial metabolic derangement in heart disease. However, consistency may depend on the changeable nature of cardiac metabolism in relation to whole-body metabolic state. This study investigates the impact of feeding status on cardiac hyperpolarized MRI in a large animal model resembling human physiology.

METHODS

Thirteen 30-kg pigs were subjected to an overnight fast, and 5 pigs were fed a carbohydrate-rich meal on the morning of the experiments. Vital parameters and blood samples were registered. All pigs were then scanned by hyperpolarized [1-¹³ C]pyruvate cardiac MRI, and results were compared between the 2 groups and correlated with circulating substrates and hormones.

RESULTS

The fed group had higher blood glucose concentration and mean arterial pressure than the fasted group. Plasma concentrations of free fatty acids (FFAs) were decreased in the fed group, whereas plasma insulin concentrations were similar between groups. Hyperpolarized MRI showed that fed animals had increased lactate/pyruvate, alanine/pyruvate, and bicarbonate/pyruvate ratios. Metabolic ratios correlated negatively with FFA levels.

CONCLUSION

Hyperpolarized MR can identify the effects of different metabolic states on cardiac metabolism in a large animal model. Unlike previous rodent studies, all metabolic derivatives of pyruvate increased in the myocardium of fed pigs. Carbohydrate-rich feeding seems to be a feasible model for standardized, large animal hyperpolarized MRI studies of myocardial carbohydrate metabolism.

13C Pyruvate Transport Across the Blood-Brain Barrier in Preclinical Hyperpolarised MRI

Hyperpolarised MRI with Dynamic Nuclear Polarisation overcomes the fundamental thermodynamic limitations of conventional magnetic resonance, and is translating to human studies with several early-phase clinical trials in progress including early reports that demonstrate the utility of the technique to observe lactate production in human brain cancer patients. Owing to the fundamental coupling of metabolism and tissue function, metabolic neuroimaging with hyperpolarised [1-13C]pyruvate has the potential to be revolutionary in numerous neurological disorders (e.g. brain tumour, ischemic stroke, and multiple sclerosis). Through the use of [1-13C]pyruvate and ethyl-[1-13C]pyruvate in naïve brain, a rodent model of metastasis to the brain, or porcine brain subjected to mannitol osmotic shock, we show that pyruvate transport across the blood-brain barrier of anaesthetised animals is rate-limiting. We show through use of a well-characterised rat model of brain metastasis that the appearance of hyperpolarized [1-13C]lactate production corresponds to the point of blood-brain barrier breakdown in the disease. With the more lipophilic ethyl-[1-13C]pyruvate, we observe pyruvate production endogenously throughout the entire brain and lactate production only in the region of disease. In the in vivo porcine brain we show that mannitol shock permeabilises the blood-brain barrier sufficiently for a dramatic 90-fold increase in pyruvate transport and conversion to lactate in the brain, which is otherwise not resolvable. This suggests that earlier reports of whole-brain metabolism in anaesthetised animals may be confounded by partial volume effects and not informative enough for translational studies. Issues relating to pyruvate transport and partial volume effects must therefore be considered in pre-clinical studies investigating neuro-metabolism in anaesthetised animals, and we additionally note that these same techniques may provide a distinct biomarker of blood-brain barrier permeability in future studies.