Biotransformation of glyceryl trinitrate and isosorbide dinitrate in vascular smooth muscle made tolerant to organic nitrates

It has been proposed that organic nitrates are prodrugs and biotransformation to a pharmacologically active metabolite (i.e., nitric oxide) must occur before the onset of vasodilation. If this postulated mechanism is correct, tolerance to organic nitrate-induced vasodilation might involve decreased biotransformation of organic nitrates by vascular smooth muscle. In this study, biotransformation of isosorbide dinitrate (ISDN) and glyceryl trinitrate (GTN) was estimated by measuring isosorbide mononitrate (ISMN) and glyceryl dinitrate (GDN), respectively, rather than the nitrate anion, because of a more sensitive method for measurement of ISMN and GDN. To test this hypothesis, isolated rabbit aortic strips (RAS) were made tolerant in vitro by incubation with 500 microM GTN or ISDN for 1 h. After a washout period and submaximal contraction with phenylephrine, the tissues were incubated with either 2.0 microM [14C]ISDN or 0.5 microM [14C]GTN for 2 min. ISDN- or GTN-induced relaxation of RAS was monitored and tissue parent drug and metabolite contents were determined by thin-layer chromatography and liquid scintillation spectrometry. ISDN- and GTN-induced relaxation of RAS and the metabolite concentrations were significantly less for both GTN- and ISDN-tolerant tissue compared with nontolerant tissue. These results are consistent with the hypothesis that organic nitrate biotransformation is required for organic nitrate-induced vasodilation.     

Effect of in vivo nitrate tolerance on hypersensitivity to NO donors after NO-synthase blockade

Animals treated with nitric oxide synthase (NOS) inhibitors exhibit marked hypersensitivity to the blood pressure lowering effects of exogenous nitric oxide (NO) donors. We used this model as a sensitive index to evaluate the relative importance of reduced biotransformation of glyceryl trinitrate (GTN) to NO in the development of nitrate tolerance. NOS-blockade hypertension using N(G)-nitro-L-arginine methyl ester (L-NAME) caused a marked enhancement of the mean arterial pressure (MAP) decrease mediated by GTN in nontolerant rats. However, even large doses of GTN were unable to change the MAP in GTN-tolerant, NOS-blockade hypertensive animals. In contrast, the MAP responses to the spontaneous NO donor sodium nitroprusside (SNP) were completely unaltered in either tolerant rats or tolerant NOS-blockade hypertensive animals, indicating that NO-dependent vasodilatory mechanisms remain intact despite the development of GTN tolerance. The MAP-lowering effects of GTN in NOS-blockade hypertensive animals were restored 48 h after cessation of chronic GTN exposure. These alterations in the pharmacodynamic response to GTN during tolerance development and reversal were associated with parallel changes in the pattern of GTN metabolite formation, suggesting that the activity of one or more enzymes involved in nitrate metabolism was altered as a consequence of chronic GTN exposure. These findings suggest that the vasodilation resulting from the vascular biotransformation of GTN to NO (or a closely related species) is severely compromised in nitrate-tolerant animals, and that although other mechanisms may contribute to the vascular changes observed following the development of GTN tolerance, decreased GTN bioactivation is likely the most important.     

In vivo and in vitro studies of a putative inhibitor of cyclic guanosine 3',5'-monophosphate production.

Our main objective was to test the efficacy of 6-anilino-5,8-quinolinedione (LY83583) in vivo, a putative inhibitor of cyclic guanosine 3',5'-monophosphate (cGMP) production. If the drug proved capable of lowering plasma, vascular, and kidney levels of cGMP and of inhibiting the hypotensive effect of sodium nitroprusside and methacholine, then LY83583 could be of potential use in exploring the contribution of cGMP to cardiovascular and renal physiology. We found that when administered to trained conscious rats, LY83583 (1-mg/kg bolus, followed by a 2-hr infusion of 3 mg/kg.hr) decreased plasma cGMP concentration by 36% (P less than 0.01). Doubling the dosage of drug (2-mg/kg bolus, 6 mg/kg.hr) decreased plasma cGMP by 46% (P less than 0.05). We next measured tissue levels of cGMP ex vivo from rats that had received LY83583 or vehicle for 2 hr. The cGMP content of aortic segments when LY83583 was infused at the low dose, or renal cortical tissue when LY83583 was infused at both doses, was not significantly different from the cGMP content of tissue from rats that had received vehicle. LY83583 in doses up to 10-mg/kg bolus, followed by 6 mg/kg.hr infusion also failed to attenuate the hypotensive response to sodium nitroprusside or methacholine in conscious rats. Last, we tested whether, in our hands, LY83583 could reduce cGMP of aortic segments and kidney cortical slices in vitro. We found that after 10 min of incubation, 10(-5) M LY83583 decreased intracellular cGMP by approximately 65% and 50% in aortic and kidney tissues, respectively. In order to ascertain whether LY83583 lowered cGMP by stimulating phosphodiesterase, we incubated tissues with 10(-4) M 3-isobutyl-1-methylxanthine to inhibit the enzyme. In the presence of 3-isobutyl-1-methylxanthine LY83583 still exerted an inhibitory effect on cGMP production by aortic and kidney tissues. In conclusion, although LY83583 is a useful agent to lower renal and vascular tissues levels of cGMP in vitro, its efficacy in vivo seems doubtful.     

Effect of malathion and radiation separately and jointly upon rat enzymes in vivo.

The effect of malathion (50 mg/kg) and radiation (900R) separately and jointly upon phosphatases and succinic dehydrogenase has been studied in female albino rats. Malathion and radiation when given separately, at the dose levels used in this study, appear to have no effect on acid and alkaline phosphatase of kidney, heart and brain. In preirradiated rats given malathion a significant decrease in phosphatases was observed in kidney and brain tissue after as little as 2 h and the values persisted for up to 24 h. No significant change was observed in heart tissue. At the dose level of 50 mg/kg of malathion, no significant change in succinic dehydrogenase was observed; however, radiation caused a significant increase in succinic dehydrogenase of brain and kidney tissue. In preirradiated rats given malathion the effect appeared to be enhanced in time to a certain degree. One could therefore speculate that simple non-specific stress caused by malathion adds to a similar effort of radiation by reducing phosphatases and enhancing the succinic dehydrogenase enzyme.     

Possible involvement of nitroglycerin converting step in nitroglycerin tolerance.

Nitroglycerin (GTN) produces a dilation of vascular smooth muscle by releasing NO through a putative GTN-converting step. However, the response to GTN is markedly attenuated after prolonged or repeated exposure, resulting in tolerance. We investigated the mechanisms of GTN tolerance, employing exogenous and endogenous NO in rat aorta. In endothelium-denuded rat aortic strips, the GTN-induced relaxation response was attenuated by preceding exposure to either GTN or sodium nitroprusside (SNP). In contrast, the SNP-induced relaxation response was not affected by this protocol of GTN or SNP preexposure. Preincubation of aortic strips with lipopolysaccharide (LPS) +/- L-arginine for 12 h also caused attenuation of GTN-induced responses such as relaxation, cGMP production and nitrite/nitrate formation. The attenuating effect of LPS abolished in aortic strips co-incubated with LPS and cycloheximide or N(G)-nitro-L-arginine. These results suggest that GTN tolerance is predominantly associated with the reduction of NO release from GTN, which is caused through inhibition of a GTN-converting step due to preceding exposure to NO itself.     

In vivo dose response relationship between physostigmine and cholinesterase activity in RBC and tissues of rats

Dose response of physostigmine (Phy) was studied in rat using various doses (25-500 micrograms/kg i.m.). Rats were sacrificed 15 min after Phy administration. Blood and tissues were analyzed for ChE activity by radiometric method and Phy concentration by HPLC method. A comparison of ChE values in different tissues of rats indicated that ChE activity was highest in brain (7.11 mumol/min/g) and least in diaphragm (0.67 mumol/min/g). The enzyme activity was eleven times more in brain as compared to diaphragm. Phy produced a dose-dependent inhibition of ChE in RBC (18-42%), brain (23-35%) and diaphragm (25-35%) from 50 to 200 micrograms/kg, then ChE inhibition was plateaued from 200 to 500 micrograms/kg in these tissues. A dose related ChE inhibition was seen in heart (16-50%) and thigh muscle (8-53%) from 50 to 500 micrograms/kg. Phy concentration increased linearly from 50 to 400 micrograms/kg in plasma, brain, heart and thigh muscle. These results indicate that ChE inhibition is linear up to 200 micrograms/kg in RBC, 150 micrograms/kg in brain and 300 micrograms/kg in heart. This linearity is not consistent in other tissues.     

Acetate supplementation attenuates lipopolysaccharide-induced neuroinflammation

Glyceryl triacetate (GTA), a compound effective at increasing circulating and tissue levels of acetate was used to treat rats subjected to a continual 28 day intra-ventricular infusion of bacterial lipopolysaccharide (LPS). This model produces a neuroinflammatory injury characterized by global neuroglial activation and a decrease in choline acetyltransferase immunoreactivity in the basal forebrain. During the LPS infusion, rats were given a daily treatment of either water or GTA at a dose of 6 g/kg by oral gavage. In parallel experiments, free-CoA and acetyl-CoA levels were measured in microwave fixed brains and flash frozen heart, liver, kidney and muscle following a single oral dose of GTA. We found that a single oral dose of GTA significantly increased plasma acetate levels by 15 min and remained elevated for up to 4 h. At 30 min the acetyl-CoA levels in microwave-fixed brain and flash frozen heart and liver were increased at least 2.2-fold. The concentrations of brain acetyl-CoA was significantly increased between 30 and 45 min following treatment and remained elevated for up to 4 h. The concentration of free-CoA in brain was significantly decreased compared to controls at 240 min. Immunohistochemical and morphological analysis demonstrated that a daily treatment with GTA significantly reduced the percentage of reactive glial fibrillary acidic protein-positive astrocytes and activated CD11b-positive microglia by 40-50% in rats subjected to LPS-induced neuroinflammation. Further, in rats subjected to neuroinflammation, GTA significantly increased the number of choline acetyltransferase (ChAT)-positive cells by 40% in the basal forebrain compared to untreated controls. These data suggest that acetate supplementation increases intermediary short chain acetyl-CoA metabolism and that treatment is potentially anti-inflammatory and neuroprotective with regards to attenuating neuroglial activation and increasing ChAT immunoreactivity in this model.     

Antagonism of ethanol-induced decrease in rat brain cGMP concentration by histidyl-proline diketopiperazine, a thyrotropin releasing hormone metabolite.

Intraperitoneal administration of thyrotropin releasing hormone (50 μmol/kg) produced an approximately 2-fold increase in rat brain cGMP concentration within 15 min. Histidyl-proline diketopiperazine, a metabolite of thyrotropin releasing hormone, produced a similar effect, but the response was faster and shorter-lasting. Intraperitoneal administration of ethanol (1.5 g/kg) decreased brain cGMP concentration approximately 50% within 10–15 min; thyrotropin releasing hormone or histidyl-proline diketopiperazine, injected 5 min after ethanol, antagonized the ethanol-induced decrease in cGMP. Antagonism of the ethanol-induced decrease in the cGMP level required 10 μmol/kg of thyrotropin releasing hormone but was observed with 5 μmol/kg of histidyl-proline diketopiperazine. These data suggest that the metabolic conversion of thyrotropin releasing hormone to histidylproline diketopiperazine might explain the previous observation that thyrotropin releasing hormone elevated the level of brain cGMP and antagonized the ethanolinduced decrease in brain cGMP concentration.     

Anoxia-induced changes in reactive oxygen species and cyclic nucleotides in the painted turtle

The Western painted turtle survives months without oxygen. A key adaptation is a coordinated reduction of cellular ATP production and utilization that may be signaled by changes in the concentrations of reactive oxygen species (ROS) and cyclic nucleotides (cAMP and cGMP). Little is known about the involvement of cyclic nucleotides in the turtle’s metabolic arrest and ROS have not been previously measured in any facultative anaerobes. The present study was designed to measure changes in these second messengers in the anoxic turtle. ROS were measured in isolated turtle brain sheets during a 40-min normoxic to anoxic transition. Changes in cAMP and cGMP were determined in turtle brain, pectoralis muscle, heart and liver throughout 4 h of forced submergence at 20–22°C. Turtle brain ROS production decreased 25% within 10 min of cyanide or N₂-induced anoxia and returned to control levels upon reoxygenation. Inhibition of electron transfer from ubiquinol to complex III caused a smaller decrease in [ROS]. Conversely, inhibition of complex I increased [ROS] 15% above controls. In brain [cAMP] decreased 63%. In liver [cAMP] doubled after 2 h of anoxia before returning to control levels with prolonged anoxia. Conversely, skeletal muscle and heart [cAMP] remained unchanged; however, skeletal muscle [cGMP] became elevated sixfold after 4 h of submergence. In liver and heart [cGMP] rose 41 and 127%, respectively, after 2 h of anoxia. Brain [cGMP] did not change significantly during 4 h of submergence. We conclude that turtle brain ROS production occurs primarily between mitochondrial complexes I and III and decreases during anoxia. Also, cyclic nucleotide concentrations change in a manner suggestive of a role in metabolic suppression in the brain and a role in increasing liver glycogenolysis.     

Metrifonate and dichlorvos: Effects of a single oral administration on cholinesterase activity in rat brain and blood

Cholinesterase activities in rat forebrain, erythrocytes, and plasma were assessed after a single oral administration of metrifonate or dichlorvos. In 3-month-old rats, the dichlorvos (10 mg/kg p.o.)-induced inhibition of cholinesterase reached its peak in brain after 15–45 min and after 10–30 min in erythrocytes and plasma. Cholinesterase activity recovered rapidly after the peak of inhibition, but did not reach control values in brain and erythrocytes within 24 h after drug administration. The recovery of plasma cholinesterase activity, in contrast, was already complete 12 h after dichlorvos treatment. Metrifonate (100 mg/kg p.o.) had qualitatively similar inhibition kinetics as dichlorvos, albeit with a slightly delayed onset. Peak values were attained 45–60 min (brain) and 20–45 min (blood), after drug administration. Apparently complete recovery of cholinesterase activity was noted in both tissues 24 h after treatment. The dose-dependence of drug-induced inhibition of cholinesterase in rat blood and brain was determined at the time of maximal inhibition, i.e., 30 min after dichlorvos treatment and 45 min after metrifonate treatment. The oral ED₅₀ values obtained for dichlorvos were 8 mg/kg for brain and 6 mg/kg for both erythrocyte and plasma cholinesterase. The corresponding oral ED₅₀ values for metrifonate were 10 to 15 times higher, i.e., 90 mg/kg in brain and 80 mg/kg in erythrocytes and plasma. In rats deprived of food for 18 h before drug treatment, the corresponding ED₅₀ values for metrifonate were 60 and 45 mg/kg, respectively, indicating an about two-fold higher sensitivity of fasted rats to metrifonate-induced cholinesterase inhibition compared to non-fasted rats. Compared to 3-month-old rats, 19-month-old rats showed a higher sensitivity towards metrifonate and dichlorvos. At the time of maximal inhibition, there was a strong correlation between the degree of cholinesterase inhibition in brain and blood. These results demonstrate that single oral administration of metrifonate and dichlorvos induces an inhibition of blood and brain cholinesterase in the conscious rat in a dose-dependent and apparently fully reversible manner. While the efficiency of a given dose of inhibitor may vary with the satiety status or age of the animal, the extent of brain ChE inhibition can be estimated from the level of blood ChE activity.     

Utilization of Plasma Fatty Acid in Rat Brain: Distribution of [14C]Palmitate Between Oxidative and Synthetic Pathways

Radioactivity within individual brain compartments was determined from 5 min to 44 h after intravenous injection of [14C]palmitate into awake Fischer-344 rats, aged 21 days or 3 months. Total radioactivity peaked broadly between 15 min and 1 h after injection, declined rapidly between 1 and 2 h, and then more slowly. In 3-month-old rats, the lipid and protein brain fractions were maximally labeled within 15 min after [14C]palmitate injection, then retained approximately constant label for up to 2 days. Radioactivity in the aqueous brain fraction comprised mainly radioactive glutamate and glutamine, and peaked at 45 min, when it comprised 48% of total brain radioactivity, then decreased to 27% of the total at 4 h, 15% at 20 h, and 10% at 44 h. Percent distribution of radioactivity within the different brain compartments, 4 h after intravenous injection of [14C]palmitate, was similar in 21-day-old and 3-month-old rats, despite higher net brain uptake in the younger animals. The results indicate that about 50% of plasma [14C]palmitate that enters the brain of adult rats is incorporated rapidly into stable protein and lipid compartments. The remaining [14C]palmitate enters the aqueous fraction after beta-oxidation, and is slowly lost. At 4 h after injection, 73% of brain radioactivity is within the stable brain compartments; this fraction increases to 86% by 20 h.     

Endothelial modulation and tolerance development in the vasorelaxant responses to nitrate of rabbit aorta

We investigated the endothelial modulations in nitrate tolerance in isolated rabbit aorta. Nitrate tolerance was induced by a 72-h treatment with transdermal nitroglycerin (NTG, 0.4 mg/h) in conscious rabbits, which was verified by a 20-fold increase in the EC50 values [NTG tolerance (6.1 +/- 0.8) x 10(-7) M vs control (3.0 +/- 0.6) x 10(-8) M]. The relaxations to NTG in tolerant and nontolerant aortic strips were enhanced when their endothelia were denuded [E(-)]. In the presence of endothelium [E(+)], NTG-tolerant vessels were not tolerant to acetylcholine (ACh), which can release endothelial nitric oxide (NO), exogenous NO or 8-bromo (Br)-cGMP. In NTG-tolerant and nontolerant vessels with endothelium, concentration-response curves for NO were the same as those in endothelium-absent tolerant vessels. In both NTG-tolerant and nontolerant vessels, treatment with superoxide dismutase (SOD, 20 units/ml), an O2-. scavenger, unaffected the responses to NTG reduced in the presence of endothelium, but treatment with NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), an NO synthase (NOS) inhibitor, reversed these reductions. Thus, our data did not indicate that an increased endothelial superoxide O2-. production contributes to nitrate tolerance. Our study suggested that (i) an impaired biotransformation process from NTG to NO is responsible for the occurrence of nitrate tolerance and (ii) vascular response to NTG enhanced by endothelial removal is related to blocked endothelial NO release.     

Histopathological and hemodynamic studies supporting hypoxia and vascular disruption as explanation to phenytoin teratogenicity.

The limb plates and craniofacial regions in rabbit fetuses were examined shortly after the last dose of phenytoin on day 16 after daily administration by gavage with either 150 mg/kg on days 14-16 or 300 mg/kg on days 15-16. Both treatment regimens resulted in similar changes. Histologically, the digital areas of the limb plates showed extensive edema and dilated blood vessels within 2 h. After 8 h, vascular disruption occurred with hemorrhages. At 24-48 h after dosing, mesenchymal necrosis and, on some occasions, amputation of digits was observed. In the craniofacial region, well-defined superficial hemorrhage was seen in the frontal and nasal region at 8 h. Histologically, subectodermal hemorrhage caused by vascular disruption and microfocal mesenchymal necrosis was observed. At 48 h, some fetuses showed severe diffuse intracranial and superficial hemorrhage, resulting in massive tissue damage, also in the central nervous system (CNS). Maternal heart rate, blood pressure, PO2, and PCO2 were also measured in awake pregnant rabbits 6 h after the last dose on day 16 after daily administration with 150 mg/kg during gestational days 14-16. An attempt was also made to measure fetal heart rate in anesthetized rabbits. The maternal heart rate and blood pressure decreased with about 15% in phenytoin-treated animals, resulting in a decrease in PO2 (approximately 15%) and an increase in PCO2 (approximately 15%). A decrease in fetal heart rate was also registered. The results thus indicate that phenytoin exerts its teratogenic effects by inducing fetal hypoxia, leading to vascular disrupture and necrosis of existing and developing structures.     

Examination of Physiological Function and Biochemical Disorders in a Rat Model of Prolonged Asphyxia-Induced Cardiac Arrest followed by Cardio Pulmonary Bypass Resuscitation

Background

Cardiac arrest induces whole body ischemia, which causes damage to multiple organs particularly the heart and the brain. There is clinical and preclinical evidence that neurological injury is responsible for high mortality and morbidity of patients even after successful cardiopulmonary resuscitation. A better understanding of the metabolic alterations in the brain during ischemia will enable the development of better targeted resuscitation protocols that repair the ischemic damage and minimize the additional damage caused by reperfusion.

Method

A validated whole body model of rodent arrest followed by resuscitation was utilized; animals were randomized into three groups: control, 30 minute asphyxial arrest, or 30 minutes asphyxial arrest followed by 60 min cardiopulmonary bypass (CPB) resuscitation. Blood gases and hemodynamics were monitored during the procedures. An untargeted metabolic survey of heart and brain tissues following cardiac arrest and after CPB resuscitation was conducted to better define the alterations associated with each condition.

Results

After 30 min cardiac arrest and 60 min CPB, the rats exhibited no observable brain function and weakened heart function in a physiological assessment. Heart and brain tissues harvested following 30 min ischemia had significant changes in the concentration of metabolites in lipid and carbohydrate metabolism. In addition, the brain had increased lysophospholipid content. CPB resuscitation significantly normalized metabolite concentrations in the heart tissue, but not in the brain tissue.

Conclusion

The observation that metabolic alterations are seen primarily during cardiac arrest suggests that the events of ischemia are the major cause of neurological damage in our rat model of asphyxia-CPB resuscitation. Impaired glycolysis and increased lysophospholipids observed only in the brain suggest that altered energy metabolism and phospholipid degradation may be a central mechanism in unresuscitatable brain damage.     

Tissue distribution and antithrombotic activity of unlabeled or 14C-labeled porcine intestinal mucosal heparin following administration to rats by the oral route

Distribution and antithrombotic activity of orally administered unfractionated porcine heparin were studied. [14C]Heparin was prepared by de-N-acetylation of porcine mucosal heparin followed by re-N-acetylation, using [14C]acetic anhydride. [14C]Heparin and (or) cold heparin (60 mg/kg) were administered by stomach tube to male Wistar rats. Blood, all levels of gut and gut contents, liver, lung, spleen, kidney, and aortic and vena caval endothelium were collected under deep anesthesia at 3, 6, 15, 30, and 60 min and 4 and 24 h (6 rats/group) after administration. Urine and feces were collected at 24 h, using metabolic cages. In three additional rats, drugs were administered in gelatin capsules. Tissues listed above and tongue, esophagus, trachea, brain, heart, thymus, bile ducts, vena caval and aortic walls, ureters, bladder, samples of muscle, skin, hair, and bone marrow were collected at 24 h. Radioactivity and chemical heparin, measured by agarose gel electrophoresis, were observed in all tissues examined as well as gut washes, plasma, urine, and feces. Radiolabel recovered was confirmed to be heparin by autoradiograms of gradient polyacrylamide electrophoretic gels. [14C]Heparin and chemical heparin in gut tissue suggest a transit time of 4 h. Porcine or bovine heparin (7.5 mg/kg), administered by stomach tube, decreased the incidence of thrombosis induced by applying 10% formalin in 65% methanol to the exposed jugular vein of rats. Heparin isolation from non-gut tissue, endothelium, urine, and plasma and the observed antithrombotic effect are consistent with oral bioavailability.     

The frequency of gonadotropin-releasing hormone secretion regulates expression of alpha and luteinizing hormone beta-subunit messenger ribonucleic acids in male rats

The influence of GnRH pulse frequency on LH subunit mRNA concentrations was examined in castrate, testosterone-replaced male rats. GnRH pulses (25 ng/pulse) or saline to controls, were given via a carotid cannula at intervals of 7.5-240 min for 48 h. alpha and LH beta mRNA concentrations were 109 +/- 23 and 30 +/- 5 pg cDNA bound/100 micrograms pituitary DNA, respectively, in saline controls. GnRH pulse intervals of 15, 30, and 60 min resulted in elevated alpha and LH beta mRNAs (P less than 0.01) and maximum responses (4-fold, alpha; 3-fold, LH beta) were seen after the 30-min pulses. Acute LH release to the last GnRH pulse was seen after the 15-, 30-, and 60-min pulse intervals. In contrast, LH subunit mRNAs were not increased and acute LH release was markedly impaired after the rapid (7.5 min) or slower (120 and 240 min) pulse intervals. Equalization of total GnRH dose/48 h using the 7.5- and 240-min intervals did not increase LH subunit mRNAs to levels produced by the optimal 30-min interval. These data indicate that the frequency of the pulsatile GnRH stimulus regulates expression of alpha and LH beta mRNAs in male rats. Further, GnRH pulse frequencies that increase subunit mRNA concentrations are associated with continuing LH responsiveness to GnRH.     

YC-1 enhances the responsiveness of tolerant vascular smooth muscle to glyceryl trinitrate

A major limitation of the use of organic nitrates in cardiovascular medicine is the development of tolerance, which has been attributed, in part, to a decrease in their metabolic activation in the vascular smooth muscle cell. Recently, 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1) was shown to potentiate vascular smooth muscle responsiveness to glyceryl trinitrate (GTN), sodium nitroprusside, and the nitric oxide donor NOC 18, in organic nitrate-naive vascular smooth muscle. We used GTN-tolerant rabbit aortic rings (RARs) to test the hypothesis that a non-vasorelaxant concentration of YC-1 enhances the ability of the prototypical organic nitrate GTN to relax vascular smooth muscle and elevate intravascular cGMP under conditions of GTN tolerance. Treatment with YC-1 (3 microM) produced a left shift of the GTN concentration-response curve and decreased the EC50 value for GTN-induced relaxation in both GTN-tolerant and non-tolerant RARs (P < 0.05). Intravascular cGMP elevation induced by GTN was enhanced in the presence of YC-1 in GTN-tolerant and non-tolerant RARs (P < 0.05). These observations indicate that YC-1, or similarly acting drugs, may be useful in overcoming the tolerance that develops during sustained GTN therapy, and that its mechanism may involve enhanced cGMP formation.     

Circadian Phase-Dependent Protein and Tissular Binding of Three Local Anesthetics (Bupivacaine, Etidocaine, and Mepivacaine) in Mice: A Possible Mechanismof Their Chronotoxicokinetics?

The aim of this study was to investigate the possible influence of the time of administration on bupivacaine (B), etidocaine (E), and mepivacaine (M) protein and tissue (brain and heart) binding. For each anesthetic agent, a single dose of B (20 mg/kg), E (40 mg/kg), or M (60 mg/kg) was administered intraperito-neally at 10:00,16:00,22:00, and 04:00 h. Blood and tissue samples were collected 15 min after drug administration. This study documents significant circadian variations in protein and tissue binding of the three local anesthetic agents. We did not demonstrate a temporal relationship between the respective free and tissue levels. Thus, the temporal variations of free plasma, brain, and heart levels do not seem to be involved in the temporal changes of induced mortality.     

Circadian Phase-Dependent Protein and Tissular Binding of Three Local Anesthetics (Bupivacaine,Etidocaine, and Mepivacaine) in Mice: A Possible Mechanismof Their Chronotoxicokinetics?

The aim of this study was to investigate the possible influence of the time of administration on bupivacaine (B), etidocaine (E), and mepivacaine (M) protein and tissue (brain and heart) binding. For each anesthetic agent, a single dose of B (20 mg/kg), E (40 mg/kg), or M (60 mg/kg) was administered intraperito-neally at 10:00,16:00,22:00, and 04:00 h. Blood and tissue samples were collected 15 min after drug administration. This study documents significant circadian variations in protein and tissue binding of the three local anesthetic agents. We did not demonstrate a temporal relationship between the respective free and tissue levels. Thus, the temporal variations of free plasma, brain, and heart levels do not seem to be involved in the temporal changes of induced mortality.