Dynamic measurements of cerebral pentose phosphate pathway activity in vivo using [1,6-13C2,6,6-2H2]glucose and microdialysis

Cerebral pentose phosphate pathway (PPP) activity has been linked to NADPH-dependent anabolic pathways, turnover of neurotransmitters, and protection from oxidative stress. Research on this potentially important pathway has been hampered, however, because measurement of regional cerebral PPP activity in vivo has not been possible. Our efforts to address this need focused on the use of a novel isotopically substituted glucose molecule, [1,6-13C2,6,6-2H2]glucose, in conjunction with microdialysis techniques, to measure cerebral PPP activity in vivo, in freely moving rats. Metabolism of [1,6-13C2,6,6-2H2]glucose through glycolysis produces [3-13C]lactate and [3-13C,3,3-2H2]lactate, whereas metabolism through the PPP produces [3-13C,3,3-2H2]lactate and unlabeled lactate. The ratios of these lactate isotopomers can be quantified using gas chromatography/mass spectrometry (GC/MS) for calculation of PPP activity, which is reported as the percentage of glucose metabolized to lactate that passed through the PPP. Following addition of [1,6-13C2,6,6-2H2]glucose to the perfusate, labeled lactate was easily detectable in dialysate using GC/MS. Basal forebrain and intracerebral 9L glioma PPP values (mean +/- SD) were 3.5 +/- 0.4 (n = 4) and 6.2 +/- 0.9% (n = 4), respectively. Furthermore, PPP activity could be stimulated in vivo by addition of phenazine methosulfate, an artificial electron acceptor for NADPH, to the perfusion stream. These results show that the activity of the PPP can now be measured dynamically and regionally in the brains of conscious animals in vivo.

Human cerebral osmolytes during chronic hyponatremia. A proton magnetic resonance spectroscopy study

The pathogenesis of morbidity associated with hyponatremia is postulated to be determined by the state of intracellular cerebral osmolytes. Previously inaccessible, these metabolites can now be quantitated by proton magnetic resonance spectroscopy. An in vivo quantitative assay of osmolytes was performed in 12 chronic hyponatremic patients (mean serum sodium 120 meq/liter) and 10 normal controls. Short echo time proton magnetic resonance spectroscopy of occipital gray and parietal white matter locations revealed dramatic reduction in the concentrations of several metabolites. In gray matter, myo-inositol was most profoundly reduced at 49% of control value. Choline containing compounds were reduced 36%, creatine/phosphocreatine 19%, and N-acetylaspartate 11% from controls. Similar changes were found in white matter. Recovery of osmolyte concentrations was demonstrated in four patients studied 8-14 wk later. These results are consistent with a reversible osmolyte reduction under hypoosmolar stress in the intact human brain and offer novel suggestions for treatment and monitoring of this common clinical event.

In vivo activity of glutaminase in the brain of hyperammonaemic rats measured by 15N nuclear magnetic resonance

The in vivo activity of phosphate-activated glutaminase (PAG) was measured in the brain of hyperammonaemic rat by 15N n.m.r. Brain glutamine was 15N-enriched by intravenous infusion of 15NH4+ until the concentration of [5-15N]glutamine reached 6.1 mumol/g. Further glutamine synthesis was inhibited by intraperitoneal injection of methionine-DL-sulphoximine, an inhibitor of glutamine synthetase, and the infusate was changed to 14NH4+ during observation of decrease in brain [5-15N]glutamine due to PAG and other glutamine utilization pathways. Progressive decrease in brain [5-15N]glutamine, PAG-catalysed production of 15NH4+ and its subsequent assimilation into glutamate by glutamate dehydrogenase were monitored in vivo by 15N n.m.r. Brain [5-15N]glutamine (15N enrichment of 0.35-0.50) decreased at a rate of 1.2 mumol/h per g of brain. The in vivo PAG activity, determined from the observed rate and the quantity of 15NH4+ produced and subsequently assimilated into glutamate and aspartate, was 0.9-1.3 mumol/h per g. This activity is less than 1.1% of the reported activity in vitro measured in rat brain homogenate at a 10 mM concentration of the activator Pi. Inhibition by ammonia (brain level 1.4 mumol/g) alone does not account for the observed low activity in vivo. The result strongly suggests that, in intact brain, PAG activity is maintained at a low level by a suboptimal in situ concentration of Pi and the strong inhibitory effect of glutamate. The observed PAG activity in vivo is lower than the reported in vivo activity of glutamate decarboxylase which converts glutamate into gamma-aminobutyrate (GABA). The result suggests that PAG-catalysed hydrolysis of glutamine is not the sole provider of glutamate used for GABA synthesis.

Oxidation therapy: the use of a reactive oxygen species-generating enzyme system for tumour treatment

Oxygen radicals induce cytotoxicity via a variety of mechanisms, including DNA damage, lipid peroxidation and protein oxidation. Here, we explore the use of a polyethylene glycol (PEG)-stabilised enzyme capable of producing reactive oxygen species (ROS), glucose oxidase (GO), for the purpose of harnessing the cytotoxic potential of ROS for treating solid tumours. PEG-GO (200 U), administered by two intratumoral injections 3 h apart, produced a significant growth delay in subcutaneous rat 9L gliomas as compared with control animals receiving heat-denatured PEG-GO. Rats were protected from systemic toxicity by subsequent i.v. administration of PEG-superoxide dismutase (PEG-SOD) and PEG-catalase. In vivo tumour metabolic changes, monitored using 31P magnetic resonance spectroscopy (31P-MRS) 6 h following initial administration of PEG-GO, revealed a 96 +/- 2% reduction in the ATP/Pi ratio and a 0.72 +/- 0.10 unit decline in intracellular pH. A 3-fold sensitisation of 9L glioma cells in vitro to hydrogen peroxide could be achieved by a 24 h preincubation with buthionine sulphoximine (BSO). This study suggests that oxidation therapy, the use of an intratumoral ROS-generating enzyme system for the treatment of solid tumours, is a promising area which warrants further exploration.

Subclinical hepatic encephalopathy: proton MR spectroscopic abnormalities

PURPOSE

To determine whether hydrogen-1 magnetic resonance (MR) spectroscopy of the brain allows detection of subclinical hepatic encephalopathy (SCHE).

MATERIALS AND METHODS

In a double-blind study, overt hepatic encephalopathy (HE) and SCHE (defined with clinical and neuropsychiatric tests) were compared by means of H-1 MR spectroscopic criteria--reduction in cerebral myo-inositol (< 2 standard deviations [SDs] from normal) and choline (< 2 SDs from normal) with or without increased cerebral glutamine (> 1 SD from normal)--in 20 patients with cirrhosis.

RESULTS

Concordance between MR spectroscopic and neuropsychiatric test results was 94% (kappa = 0.84). MR spectroscopy allowed diagnosis of SCHE in nine of nine patients (100%) and of HE in seven of eight (88%). Myo-inositol depletion alone had 80%-85% sensitivity for detection of HE and SCHE.

CONCLUSION

H-1 MR spectroscopy allows accurate diagnosis of SCHE, and the results suggest an important role for myo-inositol in psychomotor and visuopractic functions.

Metabolic loss of deuterium from isotopically labeled glucose

The isotopically substituted molecule (6-13C, 1, 6, 6-2H3)glucose was evaluated to determine whether metabolic 2H loss would prevent its use in quantitating pentose phosphate pathway (PPP) activity. PPP activity causes the C1 of glucose to be lost as CO2, while C6 can appear in lactate. 2H NMR analysis of the lactate produced from this glucose can distinguish (3-2H)-lactate (from C1 of glucose) from (3-13C, 3, 3-2H2)lactate (from C6 of glucose). 2H NMR spectroscopic analysis of medium containing (6-13C, 1, 6, 6-2H3)glucose after incubation with cultured rat 9L glioma cells suggested a 30.8 +/- 2.1% PPP activity as compared with 6.0 +/- 0.8% from separate, parallel incubations with (1-13C)glucose and (6-13C)glucose. Subsequent experiments with other isotopically labeled glucose molecules suggest that this discrepancy is due to selective loss of 2H from the C1 position of glucose, catalyzed by phosphomannose isomerase. Failure to consider 2H exchange from the C1 and C6 positions of glucose can lead to incorrect conclusions in metabolic studies utilizing this and other deuterated or tritiated glucose molecules.

In vitro and in vivo 13C and 31P NMR analyses of phosphocholine metabolism in rat glioma cells

In vivo magnetic resonance spectroscopy (MRS) has revealed that phosphomonoesters (PME) such as phosphocholine (PCho) and phosphoethanolamine (PEth) are elevated in tumors and rapidly proliferating tissues. The regulation of PME levels and their relationship to proliferation are not well known. In the present study, we investigated the regulation of PCho and PEth levels in rat glioma cells grown in vivo and in vitro using 31P and 13C MRS. However, the ability of cells to produce choline endogenously is variable. To fully understand regulation of PCho levels, it is necessary to characterize the activity of the endogenous pathway, if it exists. This was first investigated by following the metabolic fate of 13C-labeled methionine of 9L glioma tumors in vivo. Our results indicate that there is a significant amount of de novo choline synthesis in vivo. However, similar experiments performed in vitro using cells cultured in bioreactors indicated that glioma cells themselves are unable to synthesize choline de novo, suggesting that the in vivo results were due to the involvement of extra-tumoral organs, e.g., liver. Further in vitro experiments demonstrated that the uptake and phosphorylation of physiologically relevant concentrations of exogenous choline is very active in these systems. Thus, it appears that the exogenous pathway for PCho biosynthesis predominates and regulates PCho levels in glioma cells. Our results also demonstrate that PCho levels are lowest, and PEth levels are highest, in non-proliferating cells. These observations indicate that there is a decrease in the biosynthesis of PCho concomitant with a reduction in culture growth. The source of the increased PEth is, as yet, undefined.

Measurement of pentose phosphate-pathway activity in a single incubation with [1,6-13C2,6,6-2H2]glucose

The isotopically substituted molecule D-[1,6-13C2,6,6-2H2]glucose is introduced for measuring the relative activities of the pentose phosphate pathway (PPP) and glycolysis in a single incubation. PPP activity in cultured cells was determined by gas chromatography/mass spectrometric analysis of lactate produced by cells incubated with [1,6-13C2,6,6-2H2]glucose. Two other isotopes, [1,5,6-13C3]glucose and [6-13C,1,6,6-2H3]glucose, were not satisfactory for measurements of this activity. This method has four advantages over the traditional one in which 14CO2 production from [1-14C]glucose and [6-14C]glucose is compared: (1) repeated measurements can be made on a single set of cells, (2) only a single incubation is required, (3) extensive CO2 production by Krebs-cycle activity does not interfere with the measurements and (4) it is not necessary to measure the amount of glucose consumed in order to calculate relative activities of the PPP and glycolysis. Preliminary observation indicates that rat brain PPP activity may be measured in vivo with [1,6-13C2,6,6-2H2]glucose when combined with microdialysis techniques.

Abnormal cerebral metabolite concentrations in patients with probable Alzheimer disease

To establish whether recently described abnormalities of peak ratios are the result of changes in metabolite concentrations, quantitative 1H magnetic resonance spectroscopy was performed in 10 patients with Alzheimer disease (AD) and seven normal elderly. CSF volumes, metabolite T1 and T2 relaxation rates, ratios and concentrations of N-acetyl residues, creatine, choline residues, myo-inositol, glutamine plus glutamate (Glx), and glucose were obtained. Difference spectroscopy and quantitative assays showed a 50% increase in myo-inositol (6.4-9.8 mM; P < 0.005) and a decrease in N-acetyl in occipital gray matter. A reduction in beta, gamma-Glx and a significant increase in intracerebral [glucose], greater than attributable to CSF, were defined. Choline concentration increased with age, but was not elevated above normal in AD patients. These findings indicate the need for quantitative 1H MRS to substantiate metabolite ratios. The increased myo-inositol concentration in AD is demonstrated by these studies.

Oxidative stress in the central nervous system: monitoring the metabolic response using the pentose phosphate pathway

We propose that monitoring the activity of the pentose phosphate pathway (PPP) may provide an opportunity to obtain unique information regarding the metabolic response to oxidative stress since glutathione peroxidase activity is coupled, via glutathione reductase, to the PPP enzyme glucose-6-phosphate dehydrogenase. PPP activity was quantitated from data obtained from gas chromatography/mass spectrometry analysis of released lactate following metabolic degradation of (1,6-13C2,6,6-2H2)glucose. The feasibility of this approach for in vitro studies is shown using cultured rat 9L gliosarcoma cells, primary mixed cerebrocortical and primary astrocytic cultures and in vivo using intracerebral microdialysis. Exposure of 9L gliosarcoma cells to increasing concentrations of phenazine methosulfate, diamide and H2O2 correlated with increasing stimulation of the PPP, revealing the coupling of the PPP to the glutathione pathway. In all cultured cell types, the activity of the PPP was stimulated in a concentration-dependent fashion by exposure to H2O2. In primary mixed and purified astrocytic cultures, PPP activity was stimulated with H2O2 from 2.0 to 22.5 and from 5.9 to 66.7%, respectively. H2O2-induced neuronal injury was evident before saturation of the PPP occurred. H2O2 toxicity was attenuated when neurons were preincubated with the iron chelator, deferoxamine, and did not occur until saturation of the PPP. In vivo measurements of PPP activity in the conscious rat forebrain revealed basal levels of 4.5%, which was stimulated to 16.9 and 35.7% when 1 mM H2O2 and 500 microM phenazine methosulfate were added to the perfusion solution, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Scyllo-inositol depletion in hepatic encephalopathy

Cerebral myo-inositol depletion is found in patients with hepatic encephalopathy and can be implicated in the pathogenesis of hepatic encephalopathy. We measured scyllo-inositol, a stereoisomer of myo-inositol, in brain extracts from patients dying in hepatic coma using HPLC and high resolution 1H MRS. The cerebral scyllo-inositol concentration, determined by both methods, in patients without hepatic encephalopathy was 0.41 +/- 0.11 mmol/kg wet weight. It decreased by 73% and 76%, respectively, as measured by HPLC and 1H MRS, in patients with hepatic encephalopathy. These findings indicate that myo-inositol depletion in patients with hepatic encephalopathy is not due to enhanced conversion of myo-inositol to scyllo-inositol or inhibition of myo-inositol transport by scyllo-inositol, but rather to the reduced biosynthesis or transport of both inositols.

Phosphate energy metabolism during domoic acid-induced seizures

The effect of domoic acid-induced seizure activity on energy metabolism and on brain pH in mice was studied by continuous EEG recording and in vivo 31P nuclear magnetic resonance (NMR) spectroscopy. Mice were divided into ventilated (n = 6) and nonventilated (n = 7) groups. Baseline EEG was 0.1-mV amplitude with frequence of > 30-Hz and of 4-5 Hz. After intraperitoneal (i.p.) administration of domoic acid (6 mg/kg), electrographic spikes appeared at increasing frequency, progressing to high-amplitude (0.1-0.8 mV) continuous seizure activity (status epilepticus). In ventilated mice, the [31P]NMR spectra showed that high-energy phosphate levels and tissue pH did not change after domoic acid administration or during the intervals of spiking or status epilepticus. Nonventilated mice showed periods of EEG suppression accompanied by decreases in the levels of high-energy phosphate metabolites and in pH, corresponding to episodic respiratory suppression during the spiking interval. In all animals, status epilepticus was followed by a marked decrease in EEG amplitude that progressed rapidly to isoelectric silence. [31P]NMR spectra obtained after this were indicative of total energy failure and tissue acidosis. In a separate group of ventilated mice (n = 4), domoic acid-induced status epilepticus was accompanied initially by an increase in mean arterial blood pressure (MAP) that slowly returned to baseline level. Isoelectric silence was accompanied by a decrease in MAP to 75 +/- 8 mm Hg. These experiments suggest that domoic acid-induced seizures are not accompanied by an increase in substrate demand that exceeds supply.

Development of the human brain: in vivo quantification of metabolite and water content with proton magnetic resonance spectroscopy

Cerebral metabolite concentrations and water content were measured by means of localized proton magnetic resonance spectroscopy in 50 children, while metabolite peak ratios in short echo time spectra were evaluated in 173 examinations. Normative curves for normal development were established for two cerebral locations. The current report presents the first study of absolute metabolite concentrations and T1- and T2- relaxation as a function of age. Myo-inositol was found dominating the spectra at birth (12 mmoles/kg), while choline is responsible for the strongest peak in older infants (2.5 mmoles/kg). Creatine and N-acetyl groups are at significantly lower concentrations in the neonate than in the adult (Cr: 6, NA: 5 mmoles/kg). NA and Cr are determined by gestational age, whereas the concentration of ml correlates best with postnatal age. Quantitative 1H MRS is expected to be of particular value in diagnosis and monitoring of pathology in infants, since metabolite ratios are often misleading.

Decrease in cerebral inositols in rats and humans

Rats with portacaval shunts and humans with hepatic encephalopathy show severe myo-inositol depletion in the brain. The portacaval-shunted rat may therefore be a useful model for the investigation of neurochemical pathways containing myoinositol, which are modulated not only in hepatic encephalopathy but also in diabetes mellitus and Alzheimer's disease.

Direct 31P imaging in human limb and brain

OBJECTIVE

This article describes two methods for direct imaging of the 31P-metabolites phosphocreatine (PCr) and inorganic phosphate (Pi) and their application in human brain and muscle.

MATERIALS AND METHODS

All studies were performed on a 1.5 T whole-body GE Signa scanner, using bird cage resonators double-tuned to 31P and 1H. The pulse sequence was based on a multiple slice, multiple echo imaging sequence. Selection of the PCr signal was achieved either by selective excitation or by the extensive chemical shift artifact in read direction. Examinations were done in two diabetic patients, in four patients with cerebral neoplasms, and in several healthy subjects.

RESULTS

In calf muscle, images showed uniform distribution of PCr in normal muscle; deficits corresponded to bony structures and neurovascular bundles. Repeated exercise (dorsiflexion of the foot) led to selective loss of PCr in the anterior muscle compartment. A simultaneous increase in Pi appeared as a spatially distinct map. Diabetic patients showed more severe changes of PCr distribution in the calf muscle at rest and during much milder exercise. Direct imaging in the human brain with chemical shift selective excitation was completed in 5-30 min. In normal cerebral cortex, PCr was uniformly distributed around deficits marking the lateral ventricles. Tumors exhibiting moderate to severe depletion of PCr appeared as well defined deficits in the PCr image.

CONCLUSION

Direct imaging of PCr and Pi, with or without selective excitation of PCr, was effective in the human brain and limb. The methods described should lead to greatly improved fast phosphorus imaging. Clinical utility in peripheral ischemia and in localized energy deficits in the brain of patients with tumor, stroke, and other pathologies is anticipated.

15N n.m.r. measurement of the in vivo rate of glutamine synthesis and utilization at steady state in the brain of the hyperammonaemic rat

The rate of glutamine synthesis and utilization at steady state was measured in vivo in the brains of hyperammonaemic rats by 15N n.m.r. in combination with biochemical techniques. Rats were given an intravenous 15NH4+ infusion at the rate of 4.8 +/- 0.3 mmol/h per kg body wt. for 3.5 +/- 0.2 h, followed by 14NH4+ infusion at the same rate for an additional 5.1 h (chase period). During the chase period, blood ammonia (0.61 +/- 0.015 mumol/g), brain ammonia (2.9 +/- 0.3 mumol/g), glutamate (9.4 +/- 0.8 mumol/g) and glutamine (15N + 14N; 14.4 +/- 1.3 mumol/g) were at steady state. The rate of change in the cerebral [5-15N]glutamine concentration was measured in vivo by 15N n.m.r. at 20.27 MHz. To estimate 15N enrichment of precursor ammonia for glutamine synthetase (GS) in astrocytes which are interposed between cerebral capillaries and neurons, 15N enrichments of blood and brain ammonia were measured by gas chromatography-mass spectrometry. The in vivo rate of glutamine synthesis, which is equal to the rate of glutamine utilization at steady state, was estimated, from the observed rate of change in [5-15N]glutamine concentration and 15N enrichment of brain glutamine, to be 4.8 +/- 1.1 mumol/h per g of brain if 15N enrichment of ammonia at the site of GS in astrocytes is equal to that of blood-borne ammonia, and 13.0 +/- 3.9 mumol/h per g if it is equal to that measured for the whole brain. The observed GS activity in vivo in the brain of the hyperammonaemic rat is 2-5% of the reported optimum activity in vitro measured at enzyme-saturating concentrations of all substrates. The result suggests that substrates and/or cofactors other than ammonia kinetically limit GS activity in vivo. The g.c. chromatogram and mass spectrum of ammonia-derived N-trifluoroacetyl-dibutylglutamate (TAB-glutamate) are shown in Supplementary Publication SUP 50170 (4 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire, U.K., from whom copies can be obtained on the terms indicated in Biochem. J.

Alzheimer disease: depiction of increased cerebral myo-inositol with proton MR spectroscopy

To define altered metabolites in the brain of patients with probable Alzheimer disease (AD) of two brain regions, localized in vivo hydrogen-1 magnetic resonance (MR) spectroscopy was performed with a short echo time (30 msec) in 11 elderly patients and 10 healthy age-matched subjects. The patients had mild to moderate dementia, assessed with standard neuropsychological tests. Two abnormalities in the patients' cerebral cortex were defined: When compared with healthy subjects, the patients showed a 22% increase (P = .005) (approximately equal to 1.5 mmol/kg) in myo-inositol (MI) and an 11% decrease (P = .005) in residues of N-acetyl (NA), a putative neuronal marker. The elevation of MI in patients with mild to moderate AD suggests that abnormalities in the inositol polyphosphate messenger pathway occur early in the natural history of AD. The combination of high MI and low NA at examination with H-1 MR spectroscopy shows promise as an early diagnostic test for AD.

A 15N NMR study of in vivo cerebral glutamine synthesis in hyperammonemic rats

Rats were given intravenous 15NH4+ infusion at a rate of 2.2 or 5.5 mmol/h/kg body wt to induce hyperammonemia, as animal models of hepatic encephalopathy. Its effect on cerebral amino acid metabolism was studied in vivo by 15N NMR spectroscopy at 20.27 MHz for 15N. Cerebral [gamma-15N]glutamine (present at a tissue concentration of 4-9 mumol/g) and [alpha-15N]glutamate/glutamine (6 mumol/g) were clearly observed in living rats within 9-18 min. In portacaval-shunted rats, final cerebral [gamma-15N]glutamine concentrations were higher than those in controls after the same infusion period, presumably because decreased 15NH4+ removal in the liver led to increased 15NH3 diffusion into the astrocytes. In control rats, cerebral [gamma-15N]glutamine pool increased at a rate of 1.7 mumol/h/g when blood ammonia concentration was 0.8 mM. 15N enrichment in gamma-15N was 71%. From these observations, in vivo activity of glutamine synthetase in rat brain was estimated to be 3.5 mumol/h/g. Comparison with reported optimum in vitro activity suggests that in situ concentrations of some substrates and cofactors limit the activity of glutamine synthetase in vivo.

Magnetic resonance methods for measurement of disease progression in rheumatoid arthritis

Magnetic resonance imaging methods are described for measurement of disease activity in knee in rheumatoid arthritis patients. Measurements of cartilage thickness, joint effusion volume, and pannus volume have been made. The latter measurement relies on synthetic fractional enhancement images of the response to gadopentetate dimeglumine.

Enhancement of hyperthermia effect in vivo by amiloride and DIDS

PURPOSE

Intracellular pH is regulated mainly by Na+/H+ antiport and Cl-/HCO3- exchange through the cell membrane. Amiloride (3,5-diamino-6-chloro-N-(diaminomethylene)pyrazine carboxamide) is a diuretic drug that blocks Na+/H+ antiport and DIDS (4,4-diisothiocyanatostilbene-2,2'-disulfonic acid) is an inhibitor of Cl-/HCO3- exchange. We investigated the potency of these drugs to lower pHi and increase the thermosensitivity of tumors in vivo.

MATERIALS AND METHODS

The cytocidal effect of heat in combination with drug effect in vivo was studied using the in vivo-in vitro clonogenic assay method and the tumor growth delay method with SCK tumors, a mammary adenocarcinoma, on the hind limbs of A/J mice. The effects of amiloride and DIDS on tumor pHi and high energy phosphate levels were investigated using 31P-NMR.

RESULTS

We observed that amiloride or DIDS alone increased the effect of hyperthermia at 42.5 degrees C or 43.5 degrees C to suppress tumor growth. The thermosensitization was greater when the two drugs were combined. For example, hyperthermia at 43.5 degrees C alone resulted in a tumor growth delay of about 4 days. When 10 mg/kg amiloride or 25 mg/kg DIDS was injected prior to heating, the growth delay increased to about 6 days. When both drugs were injected prior to heating, a total growth delay of 8 days was obtained. In vivo-in vitro excision assays for cell survival demonstrated that these drugs enhanced the heat-induced tumor cell death. An i.p. injection of 10 mg/kg amiloride plus 25 mg/kg DIDS did not lower the tumor pHi over a 120 min interval. Heating the tumors at 42.5 degrees C for 1 hr significantly lowered the pHi and when the tumor-bearing mice were injected i.p with amiloride and DIDS, and the tumors were heated 1 hr later, the drop in pHi was greater relative to that by heating alone. Heating alone significantly lowered the tumor energy levels as indicated by PCr/Pi and beta-ATP/Pi ratios and an i.p. injection of 25 mg/kg amiloride prior to heating further reduced the energy status in the tumors.

CONCLUSION

Amiloride or its analogs and DIDS may be useful in increasing the therapeutic efficacy of hyperthermia treatments by enhancing the reduction in tumor pHi.

Proceedings of a National Cancer Institute workshop: MR spectroscopy and tumor cell biology

In December 1991, the National Cancer Institute held a workshop to evaluate the role of magnetic resonance (MR) spectroscopy in human cancer biology. The clinical and basic cancer research issues requiring use of MR spectroscopy, the advantages and limitations of MR spectroscopy, and future directions in MR spectroscopy of cancer were discussed. Consensus-building panels were formed on the following four topics: cell membrane biochemistry, tumor therapeutic response or drug resistance, appropriate model systems, and potential clinical applications of MR spectroscopy. The workshop members concluded that large prospective clinical studies as well as in vivo animal and human studies to define prognostic variables should be performed, with correlation between MR spectroscopic results and biochemical and physiologic features. Studies of phospholipid metabolism, the pharmacokinetics of anticancer agents, and effects of new cancer treatments on the tumor vasculature and normal tissues are needed.

Real-time study of the urea cycle using 15N n.m.r. in the isolated perfused rat liver

1. Isolated rat liver was perfused with 10 mM-15NH4Cl, 5 mM-lactate and 1 mM-ornithine, or with 3 mM-[15N]alanine and 1 mM-ornithine, in haemoglobin-free medium. The liver was physiologically stable for over 3 h and synthesized urea at the rate of 1.15 mumol.min-1.g of liver-1 (15NH4(+)-perfused) or 0.41 mumol.min-1.g-1 ([15N]alanine-perfused). 2. The perfused liver was continuously monitored by 15N n.m.r. spectroscopy at 20.27 MHz for 15N. Well-resolved 15N resonances of precursors and intermediates of the urea cycle, present at tissue concentrations of 0.2-3.0 mumol/g, were observed from the intact liver in 5-40 min of acquisition. Key metabolites in liver extract and the final perfusion medium were analysed by n.m.r. and by biochemical assays to determine fractional 15N enrichment and the total 15N recovery. 3. In 15NH4(+)-perfused liver (n = 6), 15N incorporation into glutamate and alanine (1.0-1.3 mumol/g), as well as progressive formation of [15N2]urea, was observed during the first 2 h of perfusion. In the second and third hour, hepatic concentrations of [omega-15N]citrulline and [omega,omega'-15N]argininosuccinate increased to n.m.r.-detectable levels (0.3-0.9 mumol/g). The [15N]aspartate pool was large in the absence of added ornithine, but on its addition was rapidly incorporated into argininosuccinate (n = 3). 4. In [15N]alanine-perfused liver, major metabolites were [15N]glutamate, [gamma-15N]glutamine and [15N]urea. Urea-cycle intermediates were undetectable. 5. The results suggest that, in intact liver provided with excess ammonia, low concentrations of cytosolic argininosuccinate synthetase and argininosuccinate lyase limited the rate of metabolite flux in the urea cycle. By contrast, in alanine-perfused liver at a physiological rate of urea synthesis, mitochondrial carbamoylphosphate synthetase was rate-limiting. 6. The potential utility of 15N n.m.r. for study of metabolite channelling through urea-cycle enzymes in intact liver is discussed.

Morphologic effects of hGRH gene expression on the pituitary, liver, and pancreas of MT-hGRH transgenic mice. An in situ hybridization analysis

Morphologic changes in the pituitary, liver, and pancreas of mice with the metallothionein-human growth hormone--releasing hormone (MT-hGRH) transgene were analyzed by in situ hybridization histochemistry (ISH). There was progression from somatotroph hyperplasia to neoplasia in pituitaries of transgenic mice. Pituitary neoplasms were present between 9 to 12 months of age in some mice. Magnetic resonance imaging (MRI) readily identified enlarged pituitaries in MT-hGRH transgenic mice. Serum mouse GH and hGRH levels were marked elevated in MT-hGRH transgenic mice. In situ hybridization histochemistry showed mRNA for hGRH in liver, pituitary, pancreas, spleen, and in most other tissues examined. Combined ISH and immunohistochemistry in the pituitary gland showed that some of the GH cells also produced hGRH, and ultrastructural immunohistochemical analysis of pituitaries showed that GH and hGRH were localized in the same cell and within the same secretory granules. Liver cells of MT-hGRH transgenic mice showed evidence of hypertrophy, and the pancreatic islets were hyperplastic with significant increases in the islet cell areas. The morphologic changes in the liver were distinctive enough to separate control littermates from MT-hGRH transgenic mice in all cases. The enlarged pancreatic islets had increased numbers of insulin-producing cells. Immunoreactive hGRH and hGRH mRNA were both localized in islet cells, and an intense hybridization signal of hGRH mRNA, but only weak staining for hGRH protein, were detected in the liver of transgenic mice. These results indicate that excessive hGRH production leads to distinct morphologic changes in various organs in MT-hGRH transgenic mice and that there is temporal progression from hyperplasia to adenomatous somatotrophs in pituitaries with chronic stimulation by hGRH that involves paracrine, endocrine, and autocrine mechanisms.