The effects of general anaesthetics on glucose and phosphate transport across the membrane of the human erythrocyte

Local Anesthetics and Antipsychotic Phenothiazines Interact Nonspecifically with Membranes and Inhibit Hexose Transporters in Yeast

Action mechanisms of anesthetics remain unclear because of difficulty in explaining how structurally different anesthetics cause similar effects. In Saccharomyces cerevisiae, local anesthetics and antipsychotic phenothiazines induced responses similar to those caused by glucose starvation, and they eventually inhibited cell growth. These drugs inhibited glucose uptake, but additional glucose conferred resistance to their effects; hence, the primary action of the drugs is to cause glucose starvation. In hxt(0) strains with all hexose transporter (HXT) genes deleted, a strain harboring a single copy of HXT1 (HXT1s) was more sensitive to tetracaine than a strain harboring multiple copies (HXT1m), which indicates that quantitative reduction of HXT1 increases tetracaine sensitivity. However, additional glucose rather than the overexpression of HXT1/2 conferred tetracaine resistance to wild-type yeast; therefore, Hxts that actively transport hexoses apparently confer tetracaine resistance. Additional glucose alleviated sensitivity to local anesthetics and phenothiazines in the HXT1m strain but not the HXT1s strain; thus, the glucose-induced effects required a certain amount of Hxt1. At low concentrations, fluorescent phenothiazines were distributed in various membranes. At higher concentrations, they destroyed the membranes and thereby delocalized Hxt1-GFP from the plasma membrane, similar to local anesthetics. These results suggest that the aforementioned drugs affect various membrane targets via nonspecific interactions with membranes. However, the drugs preferentially inhibit the function of abundant Hxts, resulting in glucose starvation. When Hxts are scarce, this preference is lost, thereby mitigating the alleviation by additional glucose. These results provide a mechanism that explains how different compounds induce similar effects based on lipid theory.

Sodium valproate inhibits glucose transport and exacerbates Glut1-deficiency in vitro

Anticonvulsant sodium valproate interferes with brain glucose metabolism. The mechanism underlying such metabolic disturbance is unclear. We tested the hypothesis that sodium valproate interferes with cellular glucose transport with a focus on Glut1 since glucose transport across the blood-brain barrier relies on this transporter. Cell types enriched with Glut1 expression including human erythrocytes, human skin fibroblasts, and rat astrocytes were used to study the effects of sodium valproate on glucose transport. Sodium valproate significantly inhibited Glut1 activity in normal and Glut1-deficient erythrocytes by 20%-30%, causing a corresponding reduction of Vmax of glucose transport. Similarly, in primary astrocytes as well as in normal and Glut1-deficient fibroblasts, sodium valproate inhibited glucose transport by 20%-40% (P < 0.05), accompanied by an up to 60% downregulation of GLUT1 mRNA expression (P < 0.05). In conclusion, sodium valproate inhibits glucose transport and exacerbates Glut1 deficiency in vitro. Our findings imply the importance of prudent use of sodium valproate for patients with compromised Glut1 function.

The effects of phenytoin and its metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin on cellular glucose transport

Glucose is the principal fuel for brain metabolism and its movement across the blood-brain barrier depends on Glut1. Impaired glucose transport to the brain may have deleterious consequences. For example, Glut1 deficiency syndrome (Glut1DS) is the result of heterozygous loss of function Glut1 mutation leading to energy failure of the brain and subsequently, epileptic encephalopathy. To preserve the integrity of the energy supply to the brain in patients with compromised glucose transport function, consumption of compounds with glucose transport inhibiting properties should be avoided. Phenytoin is a widely used anticonvulsant that affects carbohydrate metabolism. In this study, the hypothesis that phenytoin and its metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH) affect cellular glucose transport was tested. With a focus on Glut1, the effects of phenytoin and HPPH on cellular glucose transport were studied. Glucose uptake assay measuring the zero-trans influx of radioactive-labeled glucose analogues showed that phenytoin and HPPH did not exert immediate effects on erythrocyte Glut1 activity or glucose transport in Hs68 control fibroblasts, Glut1DS primary fibroblasts isolated from two patients, or in rat primary astrocytes. Prolonged exposure to the two compounds could stimulate glucose transport by up to 30-60% over the control level (p <0.05) in Hs68 and Glut1DS fibroblasts as well as in rat astrocytes. The stimulation of glucose transport by HPPH was dose-dependent and accompanied by an up-regulation of GLUT1 mRNA expression (p <0.05). In conclusion, phenytoin and HPPH do not compromise cellular glucose transport. Prolonged exposure to these compounds can modify carbohydrate homeostasis by up-regulating glucose transport in both normal and Glut1DS conditions in vitro.

GLUT1-deficiency: barbiturates potentiate haploinsufficiency in vitro

Barbiturates are known to inhibit glucose transport mediated by the facilitative sugar transporter GLUTI. We have studied such inhibition in children with GLUT1-deficiency. Zero-trans influx of 14C-labeled 3-O-methyl glucose (3OMG) into erythrocytes of patients (n = 3) was 35% of controls (n = 6). Preincubation with 10 mM phenobarbital or pentobarbital reduced patients' 30MG influx to 17%. In patients and controls, preincubation with barbiturates significantly decreased Vmax in a dose-dependent manner (for pentobarbital, IC50 = 0.84 mM, patient 2). The apparent Km in individuals remained largely unchanged. Three-OMG influx without preincubation resulted in a stronger inhibition at lower barbiturate concentrations. The patients' data are discussed in the light of individual missense mutations (patient 1: R126L and K256V; patient 2: T310I; patient 3: S66F) in the GLUTI gene. In conclusion, in controls and patients with GLUT1-deficiency barbiturates interact with GLUT1, lowering its intrinsic activity. The use of barbiturates in this condition for anesthesia or as anticonvulsants could therefore potentially aggravate the existing glucose transport defect and may put these patients at increased risk.

Inhibition of erythrocyte membrane ATPases with antisickling and anaesthetic substances and ionophoric antibiotics

A study has been carried out into the effects of clinically important antisickling and anaesthetic substances and ionophoric antibiotics on the activities of (Na+, K+)- and (Ca+2, Mg2+)-ATPases of the human erythrocyte membrane. In general, these drugs, with the exception of nystatin, inhibit both types of enzymic activities but with varying degrees of efficacy. (Ca2+, Mg2+)-ATPases was more sensitive to the lipophilic anaesthetics and (Na+,K+)-ATPase to the ionophoric antibiotic, amphotericin B. These results are explained in the light of the partition coefficients of these drugs in erythrocyte membranes, their effects on the fluidity of the erythrocytes membranes, the changes they induce in the permeability properties of erythrocytes and the subsequent effect of procaine on sickling of erythrocytes, and their potential interaction with specific membrane components.

Barbiturates inhibit hexose transport in cultured mammalian cells and human erythrocytes and interact directly with purified GLUT-1

Barbiturates reduce cerebral blood flow, metabolism, and Glc transfer across the blood-brain barrier. The effect of barbiturates on hexose transport in cultured mammalian cell lines and human erythrocytes was studied. Pentobarbital inhibits [3H]-2-dGlc uptake in 3T3-C2 murine fibroblasts by approximately 95% and approximately 50% at 10 and 0.5 mM, respectively. Uptake of [3H]-2-dGlc is linear with time in the presence or absence of pentobarbital, and the percent inhibition is constant. This suggests that hexose transport, not phosphorylation, is inhibited by barbiturates. Inhibition by pentobarbital of hexose transport in 3T3-C2 cells is rapid (< 1 min), is not readily reversible, is not altered by the presence of albumin [1% (w/v)], and is independent of temperature (4-37 degrees C) and the level of cell surface GLUT-1. The IC50's for inhibition of hexose transport in 3T3-C2 cells by pentobarbital, thiobutabarbital, and barbital are 0.8, 1.0, and 4 mM, respectively. This is consistent with both the Meyer-Overton rule and the pharmacology of barbiturates. Neither halothane (< or = 10 mM) nor ethanol [< or = 0.4% (v/v)] significantly inhibits hexose transport. Inhibition by pentobarbital (0.5 mM) of [3H]-2-dGlc uptake by 3T3-C2 cells decreases the apparent Vmax (approximately 50%) but does not alter the apparent Km (approximately 0.5 mM). Inhibition of hexose transport by barbiturates, but not ethanol [< or = 0.4% (v/v)], is also observed in human erythrocytes and four other cultured mammalian cell lines. Pentobarbital quenches (Qmax approximately 75%) the intrinsic fluorescence of purified and reconstituted GLUT-1 (Kd approximately 3 mM). Quenching is independent of Glc occupancy, is unchanged by mild proteolytic inactivation, and does not appear to directly involve perturbations of the lipid bilayer. We propose that barbiturates can interact directly with GLUT-1 and inhibit the intrinsic activity of the carrier. Glc crosses the blood-brain barrier primarily via the GLUT-1 of the endothelial cells of cerebral capillaries. Partial inhibition of this process by barbiturates may be of significance to cerebral protection.

Perturbation of the fluidity of the erythrocyte membrane with ionophoric antibiotics and lipophilic anaesthetics

The fluidity of the rat erythrocyte membrane was evaluated by measurement of excimer fluorescence of an intra-molecular forming fluorophore, 1,3-di(1-pyrenyl)propane. The polyene ionophoric antibiotics, amphotericin B and nystatin, were found to fluidize the erythrocyte membrane, as assessed by the increase in the excimer/monomer fluorescence intensity ratio, by 42 and 13%, respectively, compared with control samples. In contrast, of the peptide ionophoric antibiotics, valinomycin demonstrated about twice the effect which gramicidin A had on depressing the fluidity of the erythrocyte membrane. On the other hand, the general lipophilic anaesthetics, propanidid and althesin, led to an increase, by 70 and 32%, respectively, while the local anaesthetic, procaine, led to a decrease by 20%, in the fluidity of the erythrocyte membrane. These results were explained in the light of the partition coefficients determined for these drugs in decane and native membranes, their affinities for specific membrane components and the changes which they induce in the permeability properties of erythrocyte and other biological membranes.

Inorganic phosphate transport across the red blood cell membrane: the effect of exposure to hyperoxia

Red cell structure and function were examined in rats exposed to 80% oxygen: 20% nitrogen for five days and compared with red cells from air-breathing animals. Evidence of red cell destruction was noted in the animals exposed to hyperoxia. Osmotic fragility was found to be reduced in red cells from oxygen exposed rats (p less than 0.05), also, red cell haemolysis in hydrogen peroxide was noted to be greater (p less than 0.001) in these animals. Inorganic phosphate ion transport was significantly reduced (p less than 0.001) in the red cells from rats receiving oxygen for 5 days. The results demonstrate a decreased ability of the red cell to transport anions and suggest an impairment of the normal movement of water across the membrane following exposure to oxygen.

The opsin family of proteins

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Glucose transport into human erythrocytes treated with phospholipase A2 or C

Phospholipase A2 induced crenation of human erythrocytes and decreased glucose transport activity (influx rate) by 40% when 51% of phosphatidylcholine (PC) in the membrane was hydrolyzed. On the other hand, phospholipase C induced invagination of the cells and negligibly affected the glucose transport in the case of 21% hydrolysis of the PC. By altering the pH of the medium for suspending cells treated with phospholipase A2 from 7.4 to 6.0, cell shape was changed from clear crenation to slight invagination, but glucose transport activity was not affected. Cells that were treated with phospholipase A2 and then washed with albumin to remove free fatty acids produced in the cell membrane showed an almost normal cell shape and slightly higher glucose transport activity than did untreated cells. The ratios of beta-D-glucose transport rate to alpha-D-glucose transport rate in untreated cells, cells treated with phospholipase A2 and cells treated with phospholipase C were 1.13, 1.04, and 1.20, respectively. These results demonstrate that the drastic morphological change (invagination or crenation) induced by the treatment with phospholipases bears no clear relationship to the activity of glucose transport and suggest that the increase in the volume of the outer half of the lipid bilayer might reduce the rate of glucose transport across the human erythrocyte membrane and change the anomeric preference of glucose transport.

n-Alkanols and halothane inhibit red cell anion transport and increase band 3 conformational change rate

The effects of halothane and n-alkanols on band 3, the anion-exchange protein of the red cell membrane, have been characterized by radioactive sulfate exchange and equilibrium and kinetic binding of a fluorescent anion transport inhibitor, 4,4'-dibenzamido-2,2'-stilbenedisulfonic acid (DBDS), with fluorescence and stopped-flow techniques. Ethanol, butanol, hexanol, heptanol, octanol, and decanol inhibit radioactive sulfate efflux from red blood cells in a dose-dependent manner with an average Hill coefficient of 1.3 +/- 0.1. Over a 10(4)-fold range of buffer concentrations, the calculated membrane alkanol concentrations at which anion transport rates are reduced by 50% are 100-200 mM. At 100-300 mM membrane concentrations, halothane and the n-alkanols increase the apparent rate of DBDS binding to band 3 2-3-fold. Analysis of kinetic and equilibrium DBDS binding data shows that these drugs increase the rate of the DBDS-induced conformational change in the DBDS-band 3 complex. Equilibrium DBDS binding studies reveal differences between the actions of short-chain alkanols (ethanol and butanol) and those of long-chain alkanols (hexanol and longer). Short-chain alkanols reduce the equilibrium affinity of DBDS for band 3, while long-chain alkanols have no effect on equilibrium DBDS binding. The results for halothane and long-chain alkanols suggest a nonspecific, lipid-mediated mechanism of anesthetic action, which may be coupled to protein inactivation by an increase in the rate of protein conformational changes resulting in nonfunctional states. The results for short-chain alkanols indicate that they have the same nonspecific actions as the long-chain alkanols but also have specific effects on the stilbene binding site of band 3.

Phosphorylation of ovine rhodopsin. Identification of the phosphorylated sites

Light-dependent phosphorylation of sheep opsin was obtained in purified discs to which was added a partially purified preparation of rhodopsin kinase. A maximum ratio of 1.8 mol of phosphate/mol of rhodopsin bleached was obtained. Perturbing the lipid bilayer did not alter the phosphorylation ratio. Dephosphorylation in both segments and discs was only achieved when the supernatant fraction from a retina homogenate was added. Complete dephosphorylation required the presence of the detergent dodecyltrimethylammonium bromide in the incubation medium. Treatment of phosphorylated disc membranes with Staphylococcal aureus V8 proteinase generated two membrane-bound fragments, only one of which (V8-S, Mr 12 000) was labelled, together with a soluble seven-residue peptide that contained [32P]phosphoserine. Peptide sequencing, together with subdigestion procedures, localized the phosphorylation sites to serine residues at positions 334, 338 and 343 in the whole sequence and threonine residues at positions 335 and 336.