Sodium-23 and proton nuclear magnetic resonance imaging studies of carbon tetrachloride-induced liver damage in the rat

Magnetic Resonance Imaging and Spectroscopy: New Noninvasive In Vivo Approaches in Toxicology Research

Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) give anatomical and biochemical information about a human patient or animal in a non-invasive manner. This unique quality permits the study of toxicological responses of an organ within an intact animal and in a manner in which many fewer animals are needed than by conventional methods of investigation. The use of MRI and MRS in the study of hepatotoxicants, particularly bromobenzene and ethanol, is reviewed. Bromobenzene causes localised hepatic oedema and bioenergetic deterioration; these changes were followed with time by 1H MRI and 31P MRS, respectively. Phosphocholine levels in the liver were found to increase dramatically during bromobenzene-induced damage, possibly related to an intracellular control mechanism in response to tissue damage. The ability of the bromobenzene-challenged liver to metabolise a fructose load was followed by dynamic 31P MRS. Chronic ethanol administration damages the liver. This toxicological process results in the accumulation of fat in the liver, which was followed by fat-selective 1H MRI. When ethanol is no longer administered to the subject, the fatty infiltration subsides, and this process was followed over 16 days in the same animal using fat-selective 1H MRI. Chronic ethanol renders the liver in situ more susceptible to hypoxic injury and less likely to recover afterwards, as shown by 31P MRS.

In vivo assessment of microcystin-LR-induced hepatotoxicity in the rat using proton nuclear magnetic resonance (1H-NMR) imaging

Microcystin-LR (MCLR)-induced hepatotoxicity was assessed in vivo in male Sprague-Dawley rats (150-350 g) using magnetic resonance imaging (MRI). Following the intraperitoneal administration of MCLR (LD(50)), a region of damage, characterised by increased signal intensity on T(2)-weighted images, was seen proximal to the hepatic portal vein in the liver. Similarly, increased signal intensity was seen in the chemical-shift selective images (CSSI) of water frequency, proximal to the hepatic portal vein in the liver. This indicates that the increased signal intensity observed in the T(2)-weighted images was due to an increased amount of magnetic resonance (MR) visible protons in the tissue which represents an oedematous response. Image analysis of regions of apparent damage around the hepatic portal vein indicated a statistically significant increase in signal intensity in this region. Mitochondrial swelling and lipid inclusions were observed by transmission electron microscopy (TEM) in samples obtained from the oedematous regions of the liver using spatial coordinates from the magnetic resonance (MR) images. Massive haemorrhagic necrosis and nuclear swelling were observed by light microscopy in the centrilobular regions of the lobules.

Quantitation of intracellular [Na+] in vivo by using TmDOTP5- as an NMR shift reagent and extracellular marker

A method is presented to measure the absolute concentration of intracellular Na+ ([Na+]i) in vivo by using interleaved 23Na- and 31P-nuclear magnetic resonance (NMR) spectroscopy and TmDOTP5- as shift reagent and chemical marker of tissue extracellular space (ECS). The technique was used to determine [Na+]i and relative ECS in livers of control rats (21 +/- 3 and 0.11 +/- 0.02 mM, respectively) and in rats exposed to carbon tetrachloride (103 +/- 29 and 0.23 +/- 0.03 mM, respectively). The NMR measurements were confirmed independently on excised tissue samples by using atomic absorption spectroscopy. The results confirm that TmDOTP5- can be used as a combined cation shift reagent and ECS marker, thereby allowing quantitation of [Na+]i in vivo by NMR.

Studies on bromobenzene-induced hepatotoxicity using in vivo MR microscopy with surgically implanted RF coils

Using surgically implanted RF coils at 300 MHz, three-dimensional microscopic MR images of rat liver were obtained in vivo to follow the development of pathology induced by bromobenzene exposure. Formalin fixed specimens of liver from these animals were also imaged using in vitro MR microscopy, followed by conventional optical microscopy. All MR images were acquired using a spin-warp pulse sequence with TR = 950 ms and TE = 23 ms. The in vivo images were reconstructed as 256(2) x 32 arrays with a voxel size of (50 microns)2 x 219 microns, while the in vitro images were reconstructed as 256(2) x 128 arrays, giving an isotropic resolution at (39 microns)3. Based on results from six animals, we have found in all animals exposed to bromobenzene, image intensity decreased in specific hepatic tissue regions. These regions were well correlated to low signal intensity areas observed in in vitro MR images at higher resolution. Conventional optical microscopy indicated that the low signal intensity regions corresponded to areas of necrosis. The decrease in signal intensity is consistent with increased local diffusion coefficients as a result of necrosis. This study demonstrates that MR microscopy with implanted RF coils can be successfully used to follow tissue pathological changes in living tissues.

Use of 1H/23Na and 1H/31P double frequency tuned birdcage coils to study in vivo carbon tetrachloride-induced hepatotoxicity in rats

In vivo 1H and 23Na magnetic resonance imaging (MRI) and 31P magnetic resonance spectroscopy (MRS) techniques were used to study CCl4-induced acute hepatotoxicity in rats in situ. One or two hours following exposure to CCl4, a localized edematous region was detected in the liver by 1H MRI. The CCl4-induced edema was localized in a region surrounding the hepatic portal vein. With the use of a 23Na/1H double frequency tuned bird-cage imaging coil an increase in Na+ ion flux was also observed in the same region as the edematous region detected by 1H-MRI. Pretreatment with alpha-phenyl-tert-butyl nitrone (PBN), a free radical spin trap, 30 min prior to CCl4 exposure, was found to reduce the CCl4-induced edematous response in the liver observed in either 1H or 23Na-NMR images. Inhibition of the CCl4-induced edematous response in rat liver by PBN demonstrates that free radical intermediates, arising from the metabolism of CCl4, are possibly the key causal agents in the initiation of the edematous response. In addition, with the use of a 31P/1H double frequency tuned bird-cage imaging/spectroscopy coil, localized 31P spectra (ISIS) were obtained from the regions of CCl4-induced "tissue damage" observed in the 1H-MRI images. The most notable changes observed from the 31P spectra were an increase in inorganic phosphate (Pi) and a decrease in hepatocytosolic pH in the CCl4-treated rat livers in comparison to saline-treated control livers.

Proton magnetic resonance imaging and phosphorus-31 magnetic resonance spectroscopy studies of bromobenzene-induced liver damage in the rat

Respiratory-gated proton magnetic resonance imaging was used to study the response of the rat liver in situ to bromobenzene, a classic hepatotoxicant. A localized region of high proton signal intensity in the perihilar region of the liver was seen 24-48 hr after an intraperitoneal injection of bromobenzene. Localized proton magnetic resonance spectra from within this region indicated that the increased proton signal intensity was not due to accumulation of fat in the liver, but primarily due to a longer T2 for the proton resonance of water. This is consistent with acute edema in this localized region. In vivo 31P magnetic resonance spectroscopy studies of the same rat livers in situ were performed. Spectroscopic conditions were determined whereby localized, quantitative 31P spectra could be obtained. Using these methods, 10 mmol/kg bromobenzene was found after 24 hr to cause a number of statistically significant (p less than 0.05) effects: a decrease in adenosine 5'-triphosphate levels from 4.1 +/- 0.5 to 3.0 +/- 0.5 mM, a decrease in phosphodiester levels from 11.3 +/- 0.9 to 9.3 +/- 0.7 mM and an increase in the phosphomonoesters from 3.0 +/- 0.4 to 5.5 +/- 1.2 mM (mean +/- standard deviation). High resolution in vitro 31P spectra of perchloric acid extracts of these rat livers showed that the increased phosphomonoester resonance was due to a selective 4.3-fold increase in phosphocholine. Thus, our in vivo and in vitro 31P magnetic resonance spectra are consistent with the hypothesis that a phosphatidylcholine-specific phospholipase C (generating phosphocholine and diacylglycerol) is activated during tissue damage. Both the imaging and spectroscopy results obtained with bromobenzene closely resemble CCl4-induced liver changes previously reported, and may reflect a generalized response of the liver to any acutely acting toxic chemical.