Fate of ( 14 C) glycerol in the perfused cat brain

Glycerol Infusion Versus Mannitol for Cerebral Edema: A Systematic Review and Meta-analysis

PURPOSE

For the treatment of cerebral edema, the use of glycerol, an osmotic agent, as well as mannitol, is popular in Asia. However, the relative therapeutic benefit of glycerol remains unknown. The goal of this study was to investigate the comparative efficacy and safety of glycerol infusion versus mannitol infusion for cerebral edema.

METHODS

A systematic search was performed in PubMed, Cochrane Central Register of Controlled Trials, Web of Science, EMBASE, and Scopus for all eligible articles published before July 2020, with no restrictions on language. Two reviewers independently screened the articles, extracted data, and carefully assessed the quality of the evidence.

FINDINGS

Eight studies (6 clinical, 2 animal) were ultimately included in the qualitative analysis, and five were included in the quantitative analysis. Pooled analyses revealed nonsignificant differences in the successful control of cerebral edema (relative risk [RR], 0.97; 95% CI, 0.81-1.15). The combination therapy with glycerol led to a favorable trend in neurologic improvements. Regarding safety, glycerol was associated with a significantly lower risk of acute kidney injury (RR, 0.27; 95% CI, 0.11-0.69) and electrolyte disturbances (RR, 0.20; 95% CI, 0.06-0.64), as well as a lower possibility of rebound effects. No hemolysis was observed at the final follow-up.

IMPLICATIONS

Although the data are limited, compared with mannitol, glycerol shows a similar level of effectiveness, a more favorable safety profile, and promising neurologic improvement in individuals with cerebral edema. Additional research is needed to confirm these findings.

PROSPERO

CRD42020187702.

Comparative efficacy and safety of glycerol versus mannitol in patients with cerebral oedema and elevated intracranial pressure: A systematic review and meta-analysis

WHAT IS KNOWN AND OBJECTIVE

Glycerol is thought to be superior to mannitol in the treatment of cerebral oedema and elevated intracranial pressure (ICP), particularly with safety concerns. However, the current evidence remains insufficient. Therefore, we aimed to compare the efficacy and safety of glycerol versus mannitol in this meta-analysis.

METHODS

PubMed, EMBASE, Web of Science, CENTRAL, China National Knowledge Infrastructure, Wanfang Database, Chongqing VIP information, ClinicalTrials.gov, and the reference lists of relevant articles were searched for randomized controlled trials comparing glycerol and mannitol in patients with brain oedema and elevated ICP. Two investigators independently identified the articles, assessed the study quality and extracted data. Data analyses were performed using RevMan software.

RESULTS AND DISCUSSION

Thirty trials involving 3144 patients met our inclusion criteria. Pooled data indicated that glycerol and mannitol had comparable effectiveness in controlling cerebral oedema (RR, 1.00; 95% CI, 0.97 to 1.03; p = .97), but the risks of acute kidney injury and electrolyte disturbances were significantly lower with glycerol (RR, 0.21; 95% CI, 0.16 to 0.27 and RR, 0.23; 95% CI, 0.17 to 0.30, respectively) than mannitol. Moreover, there seemed to be a lower probability of rebound ICP after the withdrawal of glycerol. Neither haemolysis nor elevated blood glucose levels were observed in the glycerol group.

WHAT IS NEW AND CONCLUSION

Regarding the balance between efficacy and safety, glycerol could be an effective and more tolerable alternative therapy for cerebral oedema and elevated ICP than mannitol, especially for high-risk populations of renal failure.

Metabolic alterations in brain autopsies: proton NMR identification of free glycerol

Metabolic alterations in bovine brain homogenate were examined as a function of post mortem interval (PMI) using high-resolution proton NMR spectroscopy. In particular, while lactate, glutamate, glutamine, creatine and inositols as well as the total concentration of trimethyl-ammonium compounds remained constant, prominent changes due to the hydrolysis of N-acetylaspartate to acetate and aspartate as well as the decomposition of glycerophosphocholine into free choline and glycerol correlated linearly with the duration of PMI (3-195 h). The spectroscopic identification of the latter process was confirmed by proton NMR studies of model solutions as well as of extracts of mammalian brain showing high levels of free glycerol. Since the methylene resonances of glycerol overlap with the proton resonances of myo-inositol, care should be taken in the interpretation of both in vitro and in vivo brain spectra.

Glycerol oxidation in discrete areas of rat brain from young, adolescent, and adult rats

We have reported previously that the oxidation rate of [1,3-14C] glycerol to 14CO2 is lower in whole brain homogenates from neonatal rats and increases about 30% during the suckling period to adult levels. To determine whether there are developmental changes in glycerol oxidation in discrete regions of brain, we examined the oxidation of glycerol by homogenates of hypothalamus, cerebellum, brain stem, hippocampus, and cerebral cortex of young (4-6 days) and older (18-20 days) pups and adult (greater than 90 day) rats. The oxidation was measured at both low (0.2 mM) and high (2.0 mM) concentrations of glycerol, since the oxidation of glycerol by brain tissue has been shown to exhibit biphasic kinetics. At each age, and with both concentrations of glycerol, there were significant differences among the discrete brain regions. Although the rate of glycerol oxidation increased with age in most areas of brain, each brain region had a distinct developmental profile. In the hypothalamus, the oxidation of glycerol increased significantly between 4-6 days and adult levels at 18-20 days. The oxidation of glycerol was essentially the same in homogenates of cortex from young and older pups, but it was significantly increased in adults. In contrast with other brain regions, the oxidation of glycerol by brain stem was highest at 4-6 days and significantly decreased with age. The developmental profile of glycerol oxidation in hippocampus and cerebellum was particularly complex, since it increased with age at low, but not at high, concentrations of glycerol. This developmental pattern in hippocampus and cerebellum could be related to changes in the biphasic kinetics of glycerol oxidation and suggests that glycerol metabolism is different in these two areas of brain compared with other areas of brain.

Hydrophobicity and central nervous system agents: on the principle of minimal hydrophobicity in drug design

The problem of getting drugs across the so-called blood-brain barrier (BBB) has long been under extensive investigation; however, the other side of the problem, that of keeping drugs out of the central nervous system (CNS), has not been studied so intently. As we strive to make more and more refined drugs with fewer side effects, the problem of keeping drugs out of the CNS has possibly become more important than getting them in. The role of lipophilicity has long been recognized as being important in CNS penetration by chemicals, but we believe that not enough attention has been devoted to just exactly what is meant when it is said that "a lipophilic drug is needed for CNS penetration." How lipophilic? Can hydrophilic properties keep drugs out of the CNS? How hydrophilic should they be? There are other reasons for making drugs hydrophilic. Hydrophobic drugs, other factors being equal, are more inhibitory of biochemical systems than hydrophilic congeners. Evidence is beginning to show that cytochrome P450 is induced in direct proportion to hydrophobicity by a variety of compounds, and cytochrome P450 may produce modifications in lipophilic molecules in the body. Hydrophobic drugs are more slowly eliminated. This report discusses these problems in terms of the octanol-water (log P) hydrophobic scale. The principle is proposed that, without convincing evidence to the contrary, drugs should be made as hydrophilic as possible without loss of efficacy. Antihistamines are discussed in terms of what kind of hydrophobic-hydrophilic balance is best to avoid CNS-related problems.

Glycerol oxidation in rat brain: subcellular localization and kinetic characteristics

The oxidation of [1,3-14C] glycerol to 14CO was measured in slices, whole homogenates, and subcellular fractions of rat brain. In all of these tissue preparations, the Lineweaver-Burk plots of glycerol oxidation were biphasic, yielding two apparent Km and V values. Similar kinetic characteristics were obtained with brain homogenates from guinea pig, mouse, rabbit, monkey, and pig. In other tissues of the rat, including heart, kidney, liver, and skeletal muscle, the Lineweaver-Burk plots for glycerol oxidation were not biphasic but were linear. Heating the brain homogenates for five minutes at 5 degrees C caused a 50% decrease in the rate of oxidation of glycerol without a change in the biphasic double reciprocal plot. The addition of purified glycerol kinase (EC 2.7.1.30) to the homogenate caused an increase in the rate of oxidation and resulted in linear Lineweaver-Burke plot. Brain mitochondria were prepared by two different methods, both of which yielded an enrichment of glycerol oxidation. In contrast, the rate of glucose oxidation was higher in homogenates than in mitochondria, and glucose competed with glycerol as substrate only extramitochondrially. The effects of various metabolic inhibitors suggested the participation of intact, coupled mitochondria, of glycolytic enzymes, and of electron transport in the oxidation of glycerol. The data support the primary localization of glycerol oxidation in nonsynaptic mitochondria in brain and the presence in that organelle of enzymes of the Embden-Meyerhoff pathway or an as yet unidentified system for oxidizing this compound.

Phospholipid composition and [14C]glycerol incorporation into glycerolipids of toad retina and brain

The phospholipid composition as well as the in vivo [14C]glycerol uptake in lipids was found to be similar in the toad brain and retina. The choroid lipid labeling was markedly different. An in vitro time-course study of [14C]glycerol incorporation in toad retina lipids disclosed that under the conditions of these experiments: (1) retina is able to rapidly synthesize phosphatidic acid from the radioactive precursor; (2) the sequence phosphatidic acid-diacylglycerol-triacylglycerol operates; (3) a high rate of phosphatidylinositol de novo biosynthesis takes place; (4) phosphoglycerides of choline and of ethanolamine are also heavily labeled after a lag period; (5) in vivo labeling profiles resembled those obtained in vitro mainly regarding phosphatidylinositol biosynthesis; and (6) the presence of glycerol kinase in the CNS is suggested.

The involvement of lysophosphoglycerides in neurotransmitter release; the composition and turnover of phospholipids of synaptic vesicles of guinea-pig cerebral cortex and Torpedo electric organ and the effect of stimulation

(1) Crude synaptosomal fractions (P2) derived from guinea-pig cerebral cortex were incubated in the presence of 50 mM KCl in a Krebs-glucose medium. Torpedo marmorata electric organs were stimulated electrically in vivo at 5 pulses/sec for 30 min by electrodes placed on the electric lobe. Synaptic vesicles were isolated from each source and the phospholipid compositions analysed and compared with vesicles from unstimulated controls. (2) Lysophosphatidylcholine was the only lysophosphoglyceride demonstrable in the synaptic vesicles from either source and its low levels did not increase as a result of chemical or electircal stimulation. In each case there was a close similarity of the phospholipid distributions in the vesicles taken from control and stimulated samples. (3) Control experiments indicated extensive decreases in the acetylcholine content of the vesicles from the stimulated electric organ and smaller decreases in the acetylcholine content of the synaptic vesicles from stimulated crude synaptosomal fractions. These fractions were found to respire linearly in the presence of 10 mM glucose and the vesicle fractions were shown to have low levels of contaiminating membranes as judged by marker enzyme analyses. (4) Crude synaptosomal fractions from guinea-pig cerebral cortex were incubated in a Krebs-glucose medium with labelled fatty acids and [3H]glucose in the presence or absence of 50 mM KCl. Subsynaptosomal fractionation was carried out and specific radioactivities of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol were determined in fractions D (synaptic vesicles), E (microsomes) and H (disrupted synaptosomes). The release of neurotransmitter did not significantly enhance the labelling of phospholipids in any of the fractions studied as compared with phospholipids from unstimulated fractions. This was found after two incubation times and using [14C]oleate, [14C]arachidonate, [3H]palmitate and [3H]glucose.