Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Tolerance”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources
PubMed1998

Tolerance and cross-tolerance to the rate-suppressing effects of opioids in butorphanol-treated rats: influence of maintenance dose and relative efficacy at the mu receptor.

The purpose of the present investigation was to examine the development of tolerance to the rate-suppressing effects of mu and kappa opioids in rats administered either 3.0 (low) or 30 (high) mg/kg per day of butorphanol, an opioid with low relative efficacy at the mu receptor. The mu opioids butorphanol, buprenorphine, morphine, fentanyl and sufentanil, and the kappa opioid U50,488 dose-dependently suppressed responding under all conditions examined. In rats administered the low maintenance dose of butorphanol, tolerance developed to the effects of butorphanol, buprenorphine and morphine, but not to fentanyl and sufentanil. In rats administered the high maintenance dose, tolerance developed to all of the mu opioids examined. In both treatment groups, the degree to which tolerance developed was greater for butorphanol and buprenorphine than for morphine, fentanyl and sufentanil; and the degree to which tolerance developed to these mu opioids was greater in rats administered the high maintenance dose of butorphanol. The tolerance that developed to morphine, fentanyl and sufentanil was not altered when tested at both 23 and 47 h following the previous maintenance dose of butorphanol, suggesting that these changes were not due to any acute pharmacological interactions between butorphanol and the test compound (i.e., antagonism). Tolerance was also conferred to the kappa opioid U50,488 in both groups of rats, and in rats administered the high maintenance dose, this effect was obtained when tested 23 and 47 h following the previous maintenance dose of butorphanol. Physical dependence developed in rats administered the high maintenance dose of butorphanol, as evidenced by the development of enhanced sensitivity to the rate-suppressing effects of naloxone, and the finding that 30 mg/kg naloxone decreased body weight in a time-dependent manner. No physical dependence was apparent in rats administered the low maintenance dose of butorphanol. These data suggest that during chronic treatment with butorphanol, (1) greater degrees of tolerance are conferred to drugs possessing low efficacy at the mu opioid receptor, (2) tolerance is enhanced as the maintenance dose of the toleragen is increased, and (3) mu-opioid tolerance may be observed under conditions that do not produce mu-opioid dependence.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗
PubMed1984

Tolerance to N6-(L-phenylisopropyl) adenosine. Contribution of behavioral mechanisms and cross-tolerance profile.

The contribution of behavioral mechanisms to tolerance to N6-(L-phenylisopropyl)adenosine (L-PIA) was studied, along with the degree of cross-tolerance to other drugs active in the CNS. Rats were stabilized on a fixed-ratio of a 20 lever-pressing schedule for food reward and were then assigned to three daily-treatment groups. One group (saline-behavior associated) was injected with saline 15 min before the session, another (L-PIA-behavior associated) was injected with L-PIA (0.08 mg/kg) 15 min before the session and the last (L-PIA-behavior dissociated) was injected with L-PIA (0.08 mg/kg) immediately after the session. Tolerance developed to the decreasing effects of L-PIA on response rate in both groups, L-PIA-behavior associated and L-PIA-behavior dissociated. Behavioral mechanisms were thus not important in tolerance to L-PIA. In subsequent cross-tolerance tests, L-PIA-tolerant rats were cross-tolerant to the adenosine Al receptor agonist, N6-cyclohexyladenosine. The drugs 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), diazepam, pentobarbital, ketamine, clonidine, d-amphetamine and caffeine did not produce differential effects in L-PIA-tolerant and non-tolerant subjects; however, L-PIA-tolerant subjects were more sensitive to the suppressive effects of chlorpromazine on the response-rate.

Adenosine↗
PubMed1989

Catalepsy induced by combinations of ketamine and morphine: potentiation, antagonism, tolerance and cross-tolerance in the rat.

Previous studies demonstrated that both ketamine and morphine induced analgesia and catalepsy in the rat. Pre-treatment with ketamine produced cross-tolerance to morphine, whereas pretreatment with morphine did not induce cross-tolerance to ketamine but rather augmented the cataleptic response; this augmentation was attributed to residual morphine in the brain. The present studies explored the duration of the loss of righting reflex induced by sub-effective doses of ketamine and morphine, administered simultaneously. There was mutual potentiation between sub-effective doses of ketamine and morphine, but sub-effective doses of ketamine partly antagonized fully-effective doses of morphine. Latency to the loss of righting reflex, rigidity and behavior on recovery, reflected the relative predominance of ketamine or morphine in each combination. Naloxone inhibited the induced cataleptic effects. The degree and time course of development of tolerance to daily administration of sub-effective dose combinations of ketamine and morphine were similar. Rats, tolerant to ketamine-dominant combinations, were cross-tolerant to both drugs, while those tolerant to morphine-dominant combinations were cross-tolerant to morphine but showed either no cross-tolerance or an augmented response to ketamine. While the mutual potentiation, antagonism and tolerance suggest common mechanisms for the induced catalepsy, differences in latency, rigidity and behavior, asymmetry of cross-tolerance and a widely-different ID50 for naloxone would argue against an action at a single opioid site.

Animals↗
PubMed1986

Learned and pharmacologically-induced tolerance to ethanol and cross-tolerance to morphine and clonidine.

The development of tolerance to the inhibitory effect of ethanol on the tail-flick reflex was studied in the spinal rat. This preparation was used in order to avoid uncontrolled learning effects. Tolerance due to intoxicated practice (learned tolerance) and tolerance due to mere ethanol exposure (pharmacologically-induced tolerance) were studied in separate experiments. It was found that that learned tolerance to ethanol also caused tolerance to morphine and clonidine, whereas pharmacologically-induced tolerance did not have the same effect. The results challenge the concept of "behaviorally augmented tolerance" and suggest that learned and pharmacologically-induced tolerance involve different basal mechanisms in the CNS.

Animals↗
PubMed1963

Mechanisms of endotoxin tolerance. I. Relationship between tolerance and reticuloendothelial system phagocytic activity in the rabbit.

Pyrogenic tolerance following 7 daily intravenous injections of 2.0 microg/kg E. coli endotoxin in albino rabbits was associated with significant increases in RES phagocytic activity as measured with colloidal carbon. Nevertheless, 4 hours after RES blockade with thorotrast (3 ml/kg), the tolerant rabbits exhibited significantly lower fever indices following intravenous endotoxin challenge than did non-tolerant control animals despite comparably depressed capacities to clear carbon from the blood. Moreover, plasma from rabbits tolerant to endotoxin induced significant tolerance in normal rabbits prepared by thorotrast blockade without enhancing the depressed carbon clearance. This passive protection extended to heterologous endotoxins. Analysis of the data indicates that RES blockade does not abolish tolerance; rather blockade resets the reactivity to endotoxin in the normal and tolerant animal, rendering both exquisitely reactive, but permitting retention of the major portion of tolerance. Apparently the tolerant animal possesses a dual endotoxin defense system. One system is abolished by thorotrast; the other is in part humoral, accounts for the greater portion of tolerance, and is thorotrast-resistant. The nature of the humoral component is not defined but is consistent with that of an opsonin with high endotoxin specificity.

Animals↗
PubMed1985

T cell tolerance studied at the level of antigenic determinants. I. Latent reactivity to lysozyme peptides that lack suppressogenic epitopes can be revealed in lysozyme-tolerant mice.

Whether T cell tolerance represents direct inactivation of antigen-specific T cells via recognition of antigen plus major histocompatibility complex, or via T suppressor (Ts) cells, or a combination of these mechanisms, remains to be clarified. This problem was investigated using a novel approach based on the finding in several systems that T helper/proliferative (Th/Tp) cell-inducing antigenic determinants are dissociable from Ts cell-inducing determinants. Thus, peptide probes containing known sites that stimulate T proliferative activity, as well as peptides from distinct sites assumed to bear Ts-inducing determinants, were used in studying hen (chicken) eggwhite lysozyme (HEL)-tolerant mice. The clear prediction from clonal deletion model is that Th/Tp response potential to short peptides in the tolerant mouse would not exist, while regulatory suppression models predict the coexistence of antigen-reactive cells and antigen-specific regulatory cells that prevent their expression. Adult mice, treated with 2 mg HEL in saline, were tolerant to HEL in complete Freund's adjuvant (CFA). Latent T cell proliferative responses could be revealed to determinants within two HEL peptide probes, which lacked the amino-terminal region of the molecule. This responsiveness suggested two conclusions: first, Ts cells directed against the amino terminus of lysozyme exist in the tolerant genetic responder B10.A; second, these Ts regulate the activity of functional antigen-reactive T cells directed against epitopes elsewhere on the molecule, but only in the presence of the complete molecule, HEL. Examination of neonatally induced tolerance did not reveal any latent responsiveness, supporting the hypothesis that clonal deletion or anergy is the relevant mechanism in this situation. Possible reservations in these explanations of the two tolerant states, plus analysis of the more complex "split tolerance" resulting from 20 mg HEL in saline treatment in adults, are discussed. The approach of dissociation of proliferation-inducing determinants from suppression-inducing determinants clarifies our understanding of the tolerant state and holds promise for more definitive exploration of mechanisms of T cell tolerance.

Aging↗
PubMed1999

Expansion of neonatal tolerance to self in adult life: I. The role of a bacterial adjuvant in tolerance spread.

T cell neonatal tolerance to self evolves perturbation of the Th1/Th2 balance towards Th2-type self-specific T cells. In the current study we have demonstrated that a tolerant state could be extended to another encephalitogenic determinant only if the neonatally tolerizing determinant was co-administered in adult life with an emulsion of Mycobacterium tuberculosis (i.e. complete Freund's adjuvant). The mechanisms underlying tolerance elicitation and expansion were then explored by an in vitro system in which indirect suppression could be measured. Addition of a tolerizing epitope to splenic T cells from neonatally tolerized animals induced a marked suppression of the anti-MT response. This response could be restored by neutralizing antibodies to IL-4. In contrast, the neutralizing antibodies to IL-4 had no affect on the response of these cells to the tolerizing determinant. These findings are highly significant not only because they explore the important role of microbial antigens in neonatal tolerance, but also because they distinguish, for the first time, between tolerizing and tolerized T cells.

Aging↗
PubMed2006

A broad-scale analysis of population differentiation for Zn tolerance in an emerging model species for tolerance study: Arabidopsis halleri (Brassicaceae).

Although current knowledge about the overall distribution of zinc (Zn) tolerance in Arabidopsis halleri populations is scarce, the species is an emerging model for the study of heavy metal tolerance in plants. We attempted to improve this knowledge by testing the Zn tolerance of scattered European metallicolous (M) and nonmetallicolous (NM) populations of A. h. subsp. halleri and A. h. subsp. ovirensis in hydroponic culture. The occurrence of constitutive tolerance was unconditionally established in A. h. halleri and tolerance was extended to the subspecies ovirensis. M populations were the most tolerant but there was a continuous range of variation in tolerance from NM to M populations. Finally, relatively high levels of tolerance were detected in some NM populations, suggesting that enhanced tolerance could be present at high frequency in populations that have not experienced metal exposure. We used our results to argue the evolutionary dynamics and origin of Zn tolerance in A. halleri.

Arabidopsis↗
PubMed2000

Sedative tolerance accompanies tolerance to the analgesic effects of fentanyl in infant rats.

Iatrogenic tolerance and physical dependence have been documented in human neonates and infants infused with fentanyl or morphine i.v. to maintain continuous analgesia and sedation during extracorporeal membrane oxygenation (ECMO) and mechanical ventilation for the treatment of life-threatening pulmonary diseases. Using postnatal d 17 infant rats, the hypothesis was tested that sedative tolerance accompanies tolerance to fentanyl analgesia in the tail-flick test. Postnatal d 14 infant rats remained naive or received osmotic minipumps infusing saline (1 microL/h) or fentanyl citrate (60 microg x kg(-1) h(-1)). Seventy-two hours later, fentanyl's antinociceptive potency was reduced 3.1-fold in fentanyl-infused rats. Conscious sedation and deep sedation were examined with the cliff-avoidance and the righting-reflex procedures, respectively. Fentanyl-infused infants were tolerant to both the conscious and deep sedative effects of fentanyl. Another hypothesis tested was that very high receptor intrinsic activity opioids are less likely to produce tolerance, or to be cross-tolerant to other opioids. Dihydroetorphine is 5,000 to 10,000 times more potent than morphine. However, fentanyl-infused infant rats were cross-tolerant to the analgesic and sedative effects of dihydroetorphine. Interestingly, dihydroetorphine's analgesic efficacy was significantly reduced to a maximum analgesic efficacy (Emax) value of 40% maximum possible effect (MPE). Another concern was whether fentanyl tolerance would generalize to another class of sedatives, the benzodiazepines. This was especially relevant considering the widespread use of benzodiazepines like midazolam in ECMO and mechanical ventilation. Midazolam elicited no analgesia in the tail-flick test. Furthermore, fentanyl-tolerant rats were not cross-tolerant to the conscious or deep sedative effects of midazolam.

Analgesics, Opioid↗
PubMed2000

The maintenance of tolerance after successful immune tolerance induction in hemophilia A and B: the North American Registry. Factor VIII/IX Subcommittee of the International Society for Thrombosis and Hemostasis.

The North American Immune Tolerance Registry (NAITR) was initiated with the goal of determining, by questionnaire, immune tolerance (ITT) practices in hemophilia treatment centers in Canada and the United States. Sixty-eight centers (40%) responded. Of the 130 registry subjects with hemophilia A who completed ITT, 93 (72%) achieved tolerance. Of the 11 completed ITT courses in patients with hemophilia B, 4 (36%) were successful. Maintenance therapy was defined as any dotting factor regimen administered subsequent to the patient achieving the treating physician's criteria for successful immune tolerance. Seventy-five (81%) of 93 individuals in the hemophilia A cohort who successfully achieved tolerance were maintained on a regular (prophylactic factor VIII (FVIII) regimen for a variable time period post-ITT. The median dose used was 150 units/kg/week (range: 17-700). Forty-eight (64%) subjects remained tolerant while receiving regular doses of FVIII for a median observation period of 13 months (range 0-129 months). Of 27 patients whose maintenance therapy had been stopped, 17 (68%) remained tolerant over a median period of 19 months (range 1-54 months) and 9 relapsed. Among the relapses, 3 occurred after maintenance therapy was stopped; 6 were noted on prophylactic FVIII at a median time of 11 months (range 2-61 months). The definition of tolerance was reviewed for the 9 subjects who relapsed and was defined by a normal recovery and survival in only 1/9 patients. Among the 11 hemophilia B subjects in the cohort who completed tolerance, 4 had a successful outcome. Four individuals were placed on maintenance regimens of 25-100 units FIX/kg/day and all remained tolerant.

Factor IX↗
PubMed2004

[Molecular mechanism of morphine tolerance and biological approaches to resolve tolerance].

One of the major problems associated with the chronic use of morphine is tolerance. Repeated uses of morphine to relieve pain often cause patients to develop increasing resistance to the effects of the drugs, so that progressively higher doses are required to achieve the same analgesic effects. Acquired tolerance is thought to be different from dependence or addiction, but molecular mechanism underlying the development of tolerance is still unclear. Tolerance has been explained by desensitization of opioid receptor signaling and loss of functional receptors in the cell surface. The classical hypothesis was that phosphorylation and arrestin binding resulted in uncoupling of the receptor from G proteins, and reduced agonist efficacy. The receptor internalization would then result in fewer functional receptors at the cell surface. These events would cause so-called signaling desensitization. However, recent molecular biological studies have led researchers to revise the classical view of tolerance from observations that morphine does not always promote efficient receptor internalization. Among several key processes, the sequestration and subsequent internalization of the opioid receptor may play an important role for morphine tolerance. In fact, recent studies have suggested that receptor internalization can reduce tolerance. In addition, activation of the NMDA subtype of the glutamate receptor has been suggested as an anti-opioid system in the development of morphine tolerance. In this review, we focus on recent research progress on the morphine tolerance, and molecular biological and clinical approaches to resolve morphine tolerance.

Animals↗
PubMed1989

Autoimmune syndrome after induction of neonatal tolerance to alloantigens. CD4+ T cells from the tolerant host activate autoreactive F1 B cells.

The induction of transplantation tolerance to H-2b alloantigens in BALB/c (H-2d) mice by neonatal injection of (C57BL/6 x BALB/c)F1 spleen cells, produces an autoimmune lupus-like syndrome due to an activation of persisting F1 donor B cells. This syndrome is characterized by hypergammaglobulinaemia, high levels of anti-DNA antibodies, as well as by circulating immune complexes and glomerular deposits of Ig. The role of host T cells in this model was investigated by using athymic BALB/c nu/nu mice as recipients of normal (C57BL/6 x BALB.Igb)F1 spleen cells. In these "tolerized" BALB/c nu/nu mice, there was a persistence of F1 donor B cells but none of the autoimmune features were expressed, conversely to tolerized BALB/c nu/+ littermates. The injection of CD4+CD8- T lymphocytes from adult normal BALB/c mice in 3-wk-old tolerized BALB/c nu/nu mice triggered the appearance of all the autoimmune findings observed in euthymic tolerant mice. The autoantibodies were produced by persisting F1 donor B cells as shown by allotype analysis. More strikingly, a similar triggering of the autoimmune syndrome, including high titers of anti-DNA IgG antibodies and circulating immune complexes, was observed after injection of CD4+CD8- T cells from 2-wk-old tolerant BALB/c mice into "tolerized" BALB/c nu/nu mice. The anti-ssDNA antibodies were shown to bear only the Ighb allotype, indicating their exclusive origin from F1 donor B cells. These results imply that CD4+ T cells from the tolerant mice are necessary for the activation of autoreactive F1 B cells and for the development of the autoimmune syndrome occurring in this model. They also suggest that, although there is a marked depletion of H-2b-specific alloreactive CTL precursors in those neonatally tolerized mice, this state of tolerance can be associated with the persistence of H-2b-specific alloreactive CD4+ cells.

Animals↗
PubMedDate not supplied

Antinociceptive effect of diisopropylphosphofluoridate: development of tolerance and lack of cross-tolerance to morphine.

The irreversible cholinesterase inhibitor diisopropylphosphofluoridate (DFP) causes a naloxone-sensitive antinociceptive effect in laboratory animals. Chronic treatment of male mice with DFP (2 mg/kg/day for fourteen days) rendered the animals tolerant to its antinociceptive effect. Animals tolerant to DFP were cross-tolerant to the antinociception induced by the cholinergic agonists oxotremorine and nicotine, but no cross-tolerance with morphine was observed. Similarly, mice made tolerant to morphine were not cross-tolerant to DFP, nor were they cross-tolerant to oxotremorine and nicotine. Binding of muscarinic and nicotinic cholinergic ligands was significantly decreased in the brain of DFP-tolerant mice, due to a reduction in receptor density. No change was observed in the binding of [3H]-dihydromorphine to opiate receptors. None of these three binding sites was altered in mice tolerant to morphine. Although there is evidence of an involvement of endogenous opioids in the antinociceptive action of DFP, the lack of cross-tolerance between DFP and morphine suggests the existence of a more complex interaction between DFP and the cholinergic and opiate systems.

Analgesics↗
PubMed1997

Acute tolerance to spinally administered morphine compares mechanistically with chronically induced morphine tolerance.

The mechanistic similarity between acutely and chronically induced morphine tolerance has been previously proposed but remains largely unexplored. Our experiments examined the modulation of acutely induced tolerance to spinally administered morphine by agonists that affect the N-methyl-D-aspartate receptor and nitric oxide synthase systems. Antinociception was detected via the hot water (52.5 degrees C) tail flick test in mice. Intrathecal pretreatment with morphine (40 nmol) produced a 9.6-fold rightward shift in the morphine dose-response curve. This shift confirmed the induction of acute spinal morphine tolerance. Intrathecal copretreatment with the receptor antagonists (competitive and noncompetitive, respectively) dizolcipine (MK801, 3 nmol) or LY235959 (4 pmol) and morphine [40 nmol, intrathecally (i.t.)] attenuated acute tolerance to morphine measured 8 hr later. A 60-min pretreatment of 7-nitroindazole (6 nmol, i.t.), a selective neuronal NOS inhibitor, followed by administration of morphine (40 nmol, i.t.) blocked the induction of morphine tolerance. Intrathecal copretreatment with morphine (40 nmol, i.t.) and agmatine (4 nmol, i.t.), an imidazoline, receptor agonist and putative nitric oxide synthase inhibitor, almost completely abolished acute spinal morphine tolerance. The results of these experiments agree with previous reports using models of chronically induced morphine tolerance. This evidence supports the proposal that the mechanisms responsible for acute morphine tolerance parallel those underlying chronic morphine tolerance. This study attests to the powerful predictive value of acute induction as a model for morphine tolerance.

Agmatine↗
PubMed2000

Tolerance mechanism of the ethanol-tolerant mutant of sake yeast.

Several ethanol-tolerant mutants have been bred from industrial sake yeasts, but the mechanism of ethanol tolerance in these mutants has not been elucidated. After the determination of the entire genome sequence of Saccharomyces cerevisiae, various methods to monitor the whole-gene expression of the yeast have been developed. In this study, we used a commercially available nylon membrane on which virtually every gene of S. cerevisiae was spotted to compare expression profiles between the ethanol-tolerant mutant and its parent sake yeast to investigate the mechanism of ethanol tolerance in this mutant. As a result, we found that several genes were highly expressed only in the ethanol-tolerant mutant but not in the parent strain. These genes were known to be induced in cells that were exposed to various stresses, such as ethanol, heat, and high osmolarity, or at the stationary-phase but not at the log-phase. In the ethanol-tolerant mutant, the expression level of these stress-responsive genes was further increased after exposure to ethanol. We also found that substances such as catalase, glycerol and trehalose that may have protective roles under stressful conditions were accumulated in high amounts in the ethanol-tolerant mutant. The ethanol-tolerant mutant also exhibited resistance to other stresses including heat, high osmolarity and oxidative stress in addition to ethanol tolerance. These results indicate that the mutant exhibits multiple stress tolerance because of elevated expression of stress-responsive genes, resulting in accumulation of stress protective substances.

Journal Article↗
PubMed2003

Screening for tolerance to bavistin, a benzimidazole fungicide containing methyl benzimidazol-2-yl carbamate (MBC), in Beauveria bassiana: sequence analysis of the beta-tubulin gene to identify mutations conferring tolerance.

The entomopathogenic Beauveria bassiana is a potential biopesticide. Fungicide-tolerant isolates of this fungus would have an added advantage of being compatible with the conventional chemical methods of pest control. Therefore, 30 isolates of the fungus were screened for tolerance to bavistin a commonly used benzimidazole fungicide containing methyl benzimidazol-2-yl carbamate (MBC). Germination and growth bioassays in the presence of 0.05% bavistin were conducted for screening. Three tolerant isolates were identified, showing tolerance up to 2% bavistin. Mutation in the beta-tubulin gene is known to confer tolerance to MBC; the nine known mutation sites in the gene involved were sequenced in the tolerant strains. The beta-tubulin gene from codons 1-405 was amplified using two pairs of degenerate primers, designed for the conserved region of the beta-tubulin gene and sequenced. From the sequence suitable primers were designed for the regions flanking the nine known sites involved in mutations conferring MBC tolerance. The amplified products with these primers from the MBC-tolerant isolates were sequenced and in two (ARSEF 1315 and ARSEF 1316) a mutation was detected in the 198 codon resulting in replacement of glutamic acid with lysine. In the third tolerant isolate (ITCC 913) no mutation could be detected in any of the nine known sites conferring tolerance to MBC. To complete the sequencing of the beta-tubulin gene, the remaining part (from codon 405 onwards) of the gene was isolated by a three-prime gene walk. The gene sequence showed a close homology to fungal beta-tubulin genes with four introns.

Amino Acid Sequence↗
PubMed1993

Conversion of glyceryl trinitrate to nitric oxide in tolerant and non-tolerant smooth muscle and endothelial cells.

1. Exposure of smooth muscle cells (SMC) to glyceryl trinitrate (GTN, 75-600 microM) for 30 min led to a concentration-dependent increase in nitrite (NO2-), one of the breakdown products of nitric oxide (NO). This was not affected by 30 min pretreatment of the cells with 0.5 mM of sulphobromophthalein (SBP) an inhibitor of glutathione-S-transferase (GST), by metyrapone or SKF-525A inhibitors of cytochrome P450. These experiments were confirmed by organ bath studies using rabbit aortic strips denuded of endothelium and contracted with phenylephrine. Thus, a 30 min incubation of the strips with 0.5 mM SPB, metyrapone or SKF-525A did not affect the relaxations in response to GTN (10(-10)-10(-6) M). 2. Potentiation of the anti-platelet effect of GTN (44 microM) by endothelial cells (EC, 40 x 10(3) cells) was not affected by prior incubation of EC with SBP, metyrapone or SKF-525A (all at 0.5 mM). 3. Potentiation of the antiplatelet activity of GTN (11-352 microM) by small numbers of SMC (24 x 10(3) cells) or EC (40 x 10(3) cells) treated with indomethacin (10 microM) was attenuated when the SMC or EC were treated in culture with a high concentration of GTN (600 microM) for 18 h beforehand (referred to as 'tolerant' cells). In addition, tolerant SMC produced far less NO2- than non-tolerant SMC. 4. Exposure of non-tolerant SMC or EC (10(5) cells) to GTN (200 microM) for 3 min increased (3-4 fold) the levels of guanosine 3':5'-cyclic monophosphate (cyclic GMP). This increase was much less (< I fold) in the tolerant SMC or EC (105 cells). The basal levels of cyclic GMP were similar in normal or tolerant SMC or EC. Sodium nitroprusside (80 JAM) or atrial natriuretic factor (ANF, I0- M) increased the levels of cyclic GMP in normal or tolerant SMC or EC to the same extent.5 The anti-platelet effects of GTN (44 JM) were potentiated by the sulphydryl donor N-acetylcysteine(NAC, 0.5mM). Incubation of GTN (150-1200fJM) for 30min with NAC (0.1-1mM) led to aconcentration-dependent increase in N02- formation. The reduced ability of tolerant SMC or EC to potentiate the anti-platelet activity of GTN was restored by NAC (0.5 mM). These anti-aggregatory effects were abolished by concurrent co-incubation with oxyhaemoglobin (10 JM) indicating that they were due to NO release.6 Thus, in SMC or EC, metabolism of GTN to NO does not depend on glutathione-S-transferase or the cytochrome P450 system. Furthermore, when compared to normal cells, tolerant SMC or EC metabolize GTN to NO less effectively as assessed by the reduced capacity to potentiate the antiplatelet effects of GTN, to release NO2- and to increase the level of cyclic GMP. This decrease in NO formation shows that tolerance to GTN is mainly due to impaired biotransformation of GTN to NO. NAC, by directly forming NO from GTN, compensates for this failing mechanism.

Acetylcysteine↗
PubMed1989

Comparison of in vivo and in vitro activities of antibiotics with different modes of action against a tolerant and a non-tolerant Staphylococcus aureus strain.

The antibacterial efficacies of 4 antibiotics with different modes of action against a penicillin-tolerant and a non-tolerant strain of Staphylococcus aureus were investigated. For the in vitro studies the minimum inhibitory concentration (MIC) and the minimum bacterial concentration (MBC) were determined and short-term growth experiments at different antibiotic concentrations were performed. For the in vivo studies, antibacterial efficacy in an experimental infection in normal and granulocytopenic mice was evaluated. For erythromycin, rifampicin and ciprofloxacin, there was no difference in the MIC and MBC values for the 2 strains. Benzylpenicillin had an MBC value for the tolerant strains which was 256 times higher than the MIC; with the non-tolerant strain there was no difference. EC50 values, calculated from the in vitro short-term growth curves, gave similar results. Only benzylpenicillin exhibited a difference in activity against the tolerant strain, as reflected by the EC50 that was 290 times the EC50 for the non-tolerant strain. Studies in normal and granulocytopenic mice gave similar results: benzylpenicillin was 268 times less active against the tolerant strain than against the non-tolerant strain. Erythromycin, rifampicin and ciprofloxacin were 2-3 times less active against the tolerant strain than against the non-tolerant strain. The presence of granulocytes is important for the antibacterial effect of all antibiotics studied, since in the absence of granulocytes higher doses of the antibiotics are needed in order to obtain the same antibacterial effect as when granulocytes are present.

Agranulocytosis↗