[3H]dihydroergotamine as a high-affinity, slowly dissociating radioligand for 5-HT1B binding sites in rat brain membranes: evidence for guanine nucleotide regulation of agonist affinity states.

[3H]Dihydroergotamine (DE) labels a population of binding sites in rat brain membranes with an affinity of approximately 70 pM in both hippocampus (maximal binding at saturation [Bmax] = 340 fmol/mg of protein) and cerebral cortex (Bmax = 250 fmol/mg of protein). Specific binding typically comprises about 97% of total binding at the Kd of the radioligand when nonspecific binding is determined in the presence of 100 nM unlabeled DE. Association kinetics at 37 degrees C are consistent with a uniform association rate constant for all sites labeled. Specific binding is completely reversible with addition of excess unlabeled DE, but dissociation does not proceed with simple first-order kinetics, suggesting the presence of more than one discrete binding site. Competition studies with selective drugs reveal alpha adrenergic, 5-HT1A and 5-HT1B components of [3H]DE specific binding. When phentolamine (500 nM) is included to block alpha receptors and DPAT (100 nM) or spiroxatrine (500 nM) is included to block 5-HT1A receptors, specific binding is exclusively to sites with drug affinities characteristic of 5-HT1B receptors. Under these 5-HT1B-selective conditions, [3H]DE binding is about 90% specific, with a Kd of about 50 to 60 pM and a Bmax of 96 fmol/mg of protein in hippocampus and 77 fmol/mg of protein in cortex. [3H]DE binding to 5-HT1B sites is very slowly dissociable, with a T1/2 of greater than 2 h at 37 degrees C. 5-HT1B antagonists and DE itself yield competition curves at [3H]DE-labeled 5-HT1B sites that are adequately fit assuming a single site in nonlinear regression analysis. Competition by the agonists 5-HT and RU 24969 at 5-HT1B sites are often best described by two site fits. Addition of 100 microM guanylyl 5'-imidodiphosphate appears to convert nearly all 5-HT1B sites to those having low affinity for agonists while having a much smaller effect on the binding of [3H]DE. This suggests that the 5-HT1B site exists in two interconvertable agonist affinity states and is yet another member of the G-protein-linked receptor family. In agreement with previous studies using other radioligands, no 5-HT1B sites can be detected in bovine, porcine or human hippocampus membranes.

Ascorbic acid prevents nonreceptor "specific" binding of [3H]-5-hydroxytryptamine to bovine cerebral cortex membranes.

[3H]-5-Hydroxytryptamine ([3H]-5-HT) decomposes rapidly when exposed to air in solution at physiological pH if antioxidants are not present. The decomposition products appear to bind to two saturable sites on brain membranes (apparent Kd values = 1-2 and 100-1000 nM). This binding mimics "specific" ligand/receptor binding in that it is inhibited by 10 microM unlabeled 5-HT. This inhibition is not competitive, but rather is due to the prevention of [3H]-5-HT breakdown by excess unlabeled 5-HT. Unlike genuine ligand/receptor binding, the binding of [3H]-5-HT breakdown products is essentially irreversible and does not display a tissue distribution consistent with binding to authentic 5-HT receptors. [3H]-5-HT decomposition can be eliminated by the inclusion of 0.05 to 5 mM ascorbic acid. At these concentrations ascorbic acid is not deleterious to reversible [3H]-5-HT binding. When [3H] 5-HT exposure to air occurs in the presence of brain membranes, the apparent antioxidant activity of brain membranes themselves affords protection against [3H]-5-HT degradation equal to ascorbic acid. This protection is effective below final [3H]-5-HT concentrations of 10 nM. Above 10 nM [3H]-5-HT, addition of ascorbic acid or other antioxidants is necessary to avoid the occurrence of additional low affinity (apparent Kd = 15-2000 nM) binding sites that are specific but nonetheless irreversible. When care is taken to limit [3H]-5-HT oxidation, the only reversible and saturable specific binding sites observed are of the 5-HT1 high affinity (Kd = 1-2 nM) type. Radioligand oxidation artifacts may be involved in previous reports of low affinity (Kd = 15-250 nM) [3H]-5-HT binding sites in brain membrane preparations.

Effect of aspirin and indomethacin on the formation of benzo(a)pyrene-induced pulmonary adenomas and DNA adducts in A/HeJ mice.

Previous results in various in vitro systems suggest that prostaglandin endoperoxide synthetase (PES) could serve as either an alternative or an additional enzyme to the cytochrome P-450-dependent monooxygenases for the formation of mutagenic, cell-transforming, and DNA-binding metabolites of carcinogens. To test this hypothesis in vivo, we examined the effect of PES inhibitors on benzo(a)pyrene (BP)-induced pulmonary adenoma and BP metabolite:DNA adduct formation in A/HeJ mice. Animals were treated with a dosage regimen of aspirin which inhibited PES but had no effect on the cytochrome P-450-dependent oxidation of 7,8-dihydrodihydroxybenzo(a)pyrene. Aspirin did not significantly alter the number of pulmonary adenomas per mouse at either dose of BP (6.0 and 3.0 mg per mouse, administered twice, 2 weeks apart). In addition, aspirin treatment did not depress the in vivo formation of BP metabolite:DNA adducts in lung or liver at either dose of BP (6.0 and 0.06 mg/mouse). The lower dose of BP was used in the adduct study to assess the effect of aspirin at a more environmental exposure level of BP. Treatment with indomethacin, another PES inhibitor, also did not reduce the pulmonary BP metabolite:DNA adduct levels. The failure of PES inhibitors to reduce the number of pulmonary adenomas and BP metabolite:DNA adduct levels suggests that cooxidation of BP during prostaglandin biosynthesis may not play a significant role in BP-induced pulmonary adenomas.

Dose-response relationships for the binding of benzo(a)pyrene metabolites to DNA and protein in lung, liver, and forestomach of control and butylated hydroxyanisole-treated mice.

In this study, the formation of benzo(a)pyrene (BP) metabolite:DNA adducts in lung, liver, and forestomach of control and butylated hydroxyanisole (BHA)-treated (5 mg/g diet) female A/HeJ mice was examined as a function of BP dose (p.o.), ranging from 2 to 1351 mumol/kg. The major identified adduct in each tissue at each dose was the (+)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BPDEI):deoxyguanosine adduct. A 7 beta,8 alpha-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene:deoxyguanosine adduct, a(-)-BPDEI:deoxyguanosine adduct, and an unidentified adduct were also observed. In lung and liver of untreated animals, the dose-response curves for BPDEI:DNA adduct levels were sigmoidal. In forestomach, there was no indication of saturation of DNA binding over the BP dose range examined. The dose-response curves became linear as BP dose approached zero and thus, no threshold dose existed below which binding of BPDEI to DNA did not occur, at least in lung, liver, and forestomach of these mice. In forestomach, the dose-response curve for BPDEI:DNA adducts in BHA-treated mice, 0.5% of diet for 2 weeks, was parallel to the curve for control animals and thus, the inhibition (45%) of adduct formation is independent of BP dose. In contrast, BHA treatment diminished the curvilinear nature of the dose-response curves for BPDE adducts in lung and liver. The inhibition of BPDEI:DNA adduct formation by BHA in lung and liver was dose dependent. The inhibition of lung (68%) and liver (82%) adduct formation was highest at a BP dose of 270 mumol/kg. As the BP dose approached zero, the inhibition of BPDEI:DNA adduct formation by BHA decreased with BP dose and approached values of approximately 40% (lung) and 55% (liver). The dose dependency of the binding of BP metabolites to protein was also examined. BPDEI:DNA adduct concentrations ranged from 2 to 10% of protein binding concentrations in liver of untreated animals, from 3 to 7% in forestomach, and from 5 to 7% in lung. The dose-response curves for protein binding of BP metabolites in lung and liver from BHA-treated animals were essentially parallel to those in control animals and thus, the inhibition of protein binding by BHA treatment had no dose dependency in these organs. No consistent BHA effect was observed on the amount of binding of BP metabolites to forestomach protein.

Inappropriate methods for the emergency determination of plasma paracetamol.

Methods for the estimation of plasma paracetamol which depend on acid hydrolysis to p-aminophenol without a prior extraction step also measure inactive metabolites which are present in high concentrations. The extent of the overestimate obtained with such methods was determined using 24 samples from patents after paracetamol overdosage. There was a positive error of between 40 and 700% compared with a high-performance liquid chromatography reference method which measured only unchanged paracetamol. These non-specific methods should not be used to determine the need for specific therapy in patients with paracetamol poisoning.