The basis of the histamine assay in skin.

1. The assay of wheal response to histamine injected sub-epidermally has been reviewed. Several discrepant claims were found in earlier work. 2. The saline control injection may evoke a wheal, which if present should be allowed for in the assay procedure. 3. There was no evidence for a limb dominance effect. 4. Using a 3.26 mM (1 mg ml-1) standard solution of histamine acid phosphate (1.18 mM as free base) and 0.1 ml injections, the straight central portion of the assay curve was found to be between 0.3 and 2.3 x 10(-3) mM, as free base, with an ED50 of about 0.018 mM histamine base.     

Enzymatic histamine degradation by human skin.

1. Degradation of histamine by homogenized human skin in vitro, in the presence of the cofactor S-adenosyl methionine, indicates the presence of the histamine metabolizing enzyme histamine-N-methyl transferase in human skin. Under the experimental conditions described, no significant histamine degradation by diamine oxidase was observed. 2. The enzyme activity is temperature-sensitive with an optimum at 37 degrees C. The enzyme is stable in in intact excised skin at -20 degrees C, but unstable in homogenized skin at this temperature. 3. Little or no enzyme activity is present in mid- or deep dermis, but the distribution of the enzyme between superficial papillary dermis and epidermis is uncertain. 4. The presence of a potent histamine degrading mechanism raises the possibility that histamine-N-methyl transferase activity may be an important modulating factor in histamine-mediated skin disorders.     

Sensory responses of human skin to synthetic histamine analogues and histamine.

The potential for itch production in human skin of the synthetic analogues of histamine, 2-methyl histamine (an H1-receptor agonist) and 4-methyl histamine and dimaprit (H2-receptor agonists) has been studied in vivo and compared with histamine. Itch thresholds for 2-methyl histamine were consistently much higher than for histamine (P < 0.001). The H1-receptor antagonist chlorpheniramine raised the itch thresholds to 2-methyl histamine and histamine significantly (P < 0.001). Pruritus was not obtained with either 4-methyl histamine or dimaprit. No evidence of synergism between 2-methyl histamine and either 4-methyl histamine or dimaprit was found. The results suggest that histamine-induced pruritus is mediated in part through the H1-receptor and in part via an additional (but probably non-H2) mechanism.     

Characterization of neuropeptide-induced histamine release from human dispersed skin mast cells.

1. Human skin mast cells, unlike other human mast cells so far studied, released histamine in a concentration-related manner in response to substance P, vasoactive intestinal peptide (VIP) and somatostatin (1 microM to 30 microM). In contrast, eledoisin, physalaemin, neurokinin A, neurokinin B, calcitonin gene-related peptide (CGRP), neurotensin, bradykinin and Lys-bradykinin induced negligible histamine release. 2. The low histamine releasing activity of physalaemin, eledoisin, neurokinin A and neurokinin B relative to substance P suggests that the human skin mast cell activation site is distinct from the tachykinin NK-1, NK-2 or NK-3 receptors described in smooth muscle. 3. The relative potencies of substance P and its fragments SP2-11, SP3-11, SP4-11 and SP1-4 in releasing histamine from human skin mast cells suggests that both the basic N-terminal amino acids and the lipophilic C-terminal portion of substance P are essential for activity. 4. Peptide-induced histamine release, like that induced by compound 48/80, morphine and poly-L-lysine, is rapid, reaching completion in 10-20 s, is largely independent of extracellular calcium but requires intact glycolysis and oxidative phosphorylation. 5. The substance P analogue, [D-Pro4,D-Trp7,9,10] SP4-11 (SPA), not only reduced substance P-induced histamine release in a concentration-related manner but also inhibited that induced by VIP, somatostatin, compound 48/80, poly-L-lysine and morphine but not anti-IgE. 6. The similar characteristics of histamine release induced by substance P, VIP, somatostatin, compound 48/80, poly-L-lysine and morphine suggest that they share a common pathway of activation-secretion coupling distinct from that of IgE-dependent activation. Furthermore, the ability of human skin mast cells to respond to basic non-immunological stimuli including neuropeptides may reflect a specialised function for these cells.     

The importance of bradykinin and histamine in the skin response to antigen.

On three separate occasions 12 atopic subjects were injected intradermally with two doses of antigen and one of saline as control. Pretreatment with terfenadine 60 mg orally significantly inhibited the flare response to both the lower dose of antigen and to saline (P less than 0.05). Ingestion of enalapril 5 mg orally 3 h before increased the flare response to both doses of antigen. Neither enalapril nor terfenadine affected the weal response when compared with placebo. Both endogenous histamine and bradykinin appear to be released during the intradermal flare response but are not important in the weal reaction to antigen.     

The inhibition of histamine formation in vivo

Histamine formation was measured by the [¹⁴C]histamine content of the stomach following injection of [¹⁴C]histidine into rats. Carbonyl reagents (e.g. oxyamines) which inactivate pyridoxal phosphate, the co-enzyme of histidine decarboxylase, were the most effective inhibitors of histamine formation in vivo. Although the oximes prepared from imidazol-4(5)-ylmethoxyamine and pyridoxal or pyridoxal phosphate cannot inactivate the co-enzyme, they proved sufficiently potent to merit further study as inhibitors of histamine formation in vivo.     

Inhibitory effect of beta-adrenergic stimulants on the histamine reaction in human skin. I. Studies with fenoterol.

In 20 volunteers histamine inhibition by fenoterol (Th 1165 a) was studied. The wheal and erythema reaction caused by intracutaneous application of 5 mug histamine can be inhibited by applying fenoterol in doses from 100--400 mug in form of a metered aerosol on the skin 5 min before the injection of histamine. In this study the dose-reaction effect yielded on ED50 of 140 mug fenoterol.     

Vascular reactions to histamine and compound 48/80 in human skin: suppression by a histamine H2-receptor blocking agent.

1 The ability of a specific competitive histamine H2-receptor antagonist, cimetidine, to inhibit vascular responses to histamine in human skin provides new evidence that skin blood vessels possess histamine H2 receptors. 2 Simultaneous systemic administration of cimetidine and chlorpheniramine (an H1-receptor antagonist) was more effective than either drug alone in inhibition of the erythematous reaction both to exogenous histamine, and endogenous histamine secreted by skin mast cells in response to compound 48/80. 3 These results suggest that combined therapy of histamine-mediated skin diseases included urticaria and dermatitis using a combination of H1- and H2-histamine receptor antagonists may be more effective than either class of drug alone.     

Responses of human skin blood vessels to synthetic histamine analogues

1 The vascular responses of human skin to two synthetic analogues of histamine, 2-methyl histamine (an H₁-receptor agonist) and 4-methyl histamine (an H₂-receptor agonist) have been studied in vivo.

2 Both compounds evoked dose-related erythema, 2-methyl histamine but not 4-methyl histamine causing erythema mediated by an axon reflex, thus suggesting that the axon reflex and direct vasodilator action of histamine are due to H₁ and H₂ actions respectively.

3 The H₁-receptor antagonist chlorpheniramine inhibited erythema due to 2-methyl histamine. Cimetidine, an H₂-receptor antagonist, had no effect on the reaction to 2-methyl histamine. In contrast, the erythema reaction to 4-methyl histamine was suppressed by both cimetidine and chlorpheniramine.

4 Although both histamine analogues caused wealing, this was not dose-related within the dose range used, and neither chlorpheniramine nor cimetidine caused detectable suppression of wealing responses to either histamine analogue.

5 These results lend further support to the view that human skin blood vessels possess H₂ as well as H₁ receptors.

     

Inhibitory effect of beta-adrenergic stimulants on the histamine reaction of the human skin. II. Studies with several derivatives in regard to the dosage used and reaction depending on time.

In 56 volunteers the inhibitory effects of the beta-adrenergic stimulants fenoterol, salbutamol, isoproterenol, orciprenaline, terbutaline, epinephrine and phenylephrine were studied. There was a difference between the substances in regard to the applied dose and duration of inhibitory effect. The latter parameter particularly depended on the hydrosolubility of the individual substances. Fenoterol and salbutamol had a very low solubility and a very strong and long-lasting effect. The possibly stronger adhesive power of these drugs in the tissue according to their poor solubility and the site of action in the reactive systems of the skin are discussed giving preference to the vascular system over the mast-cells and cAMP.     

Phosphopyridoxal cyclic compounds with histamine and histidine. 6: The formation of phosphopyridoxal cyclic compounds with histamine and histidine in the presence of biological material.

We have studied the dynamics of cyclic compound formation between histamine or histidine and pyridoxal 5'-phosphate (Hi-PLP or His-PLP) in incubates of rat gastric mucosa histidine decarboxylase (HD), rat intestinal diamine oxidase (DAO) or homogenates of either rat liver, intestine or gastric mucosa. For gastric mucosa HD, liver and gastric mucosa homogenates, the rate of cyclization was slightly decreased; however, the rate was significantly inhibited with intestinal DAO or intestinal homogenate. Binding of PLP by tissue components was measured; free PLP was bound abundantly by rat intestinal DAO and by rat intestinal homogenate. A possible mechanism by which intestinal tissues inhibit cyclic compound formation is discussed.     

Pathways of carbamazepine bioactivation in vitro: II. The role of human cytochrome P450 enzymes in the formation of 2-hydroxyiminostilbene.

Conversion of the carbamazepine metabolite, 2-hydroxycarbamazepine, to the potentially reactive species, carbamazepine iminoquinone (CBZ-IQ), has been proposed as a possible bioactivation pathway in the pathogenesis of carbamazepine-induced hypersensitivity. Generation of CBZ-IQ has been proposed to proceed through the intermediate, 2-hydroxyiminostilbene (2-OHIS); however, data suggested that 2-hydroxycarbamazepine is oxidized by cytochromes P450 (P450s) directly to CBZ-IQ, followed by NADPH-mediated reduction to 2-OHIS. In vitro studies were conducted to identify the P450s responsible for converting 2-hydroxycarbamazepine to 2-OHIS and to determine functional consequences of this bioactivation pathway. Formation of 2-OHIS in human liver microsomes (HLMs) was consistent with monophasic, Michaelis-Menten kinetics. The sample-to-sample variation in the rate of 2-OHIS formation correlated significantly (r2 > or = 0.706) with CYP3A4/5 and CYP2B6 activities in a panel of HLMs (n = 10). Studies with a panel of cDNA-expressed enzymes revealed that CYP3A4 preferentially catalyzed 2-OHIS formation; CYP3A4 formed 2-OHIS at a rate >10 times that of other enzymes capable of forming 2-OHIS (CYP1A1, CYP2C19, and CYP3A7). Inhibitors of CYP3A enzymes markedly impaired 2-OHIS formation in HLMs, whereas inhibitors of other P450s resulted in < or = 20% inhibition. Although CYP3A4 was primarily responsible for converting 2-hydroxycarbamazepine to 2-OHIS, neither 2-hydroxycarbamazepine, 2-OHIS, nor CBZ-IQ caused time-dependent inactivation of CYP3A activity. No thiol adducts were formed directly from 2-hydroxycarbamazepine. However, glutathione- and N-acetylcysteine-conjugates were formed with 2-OHIS or CBZ-IQ as substrates. Thus, CYP3A4-dependent secondary oxidation of 2-hydroxycarbamazepine represents a potential carbamazepine bioactivation pathway leading to the formation of thiol-reactive metabolites, intermediates that may play a role in the etiology of idiosyncratic toxicity attributed to carbamazepine.     

Induction of histamine release from human skin mast cells by bradykinin analogs

Kinins are potent proinflammatory peptides that induce histamine release from rodent mast cells. We examined the ability of bradykinin, lysylbradykinin and a series of kinin analogs to cause histamine release from human basophils, human lung mast cells and human skin mast cells. At concentrations ranging from 0.1 microM to 1 mM, bradykinin failed to cause histamine release from any of the human histamine-containing cells studied. Lysylbradykinin was also without effect on basophils and lung mast cells, but was a weak secretagogue for human skin mast cells, inducing 5.5 +/- 3% (mean +/- SD) of total cellular histamine release at a concentration of 10(-5) M. Similarly, when sixteen recently developed bradykinin antagonists were examined, these compounds had no effect on basophils or lung mast cells but all sixteen induced dose-dependent histamine release from skin mast cells. The release process was temperature dependent and, at a concentration of 10(-5) M, the antagonists induced 8-27% histamine release. Although preincubation of cells with 10(-3) M bradykinin or des(Arg9) bradykinin significantly inhibited antagonist-induced histamine release, the requirement for such high concentrations of these peptides to cause inhibition suggested that histamine release is not mediated by either B1 or B2 kinin receptors. To understand further the mechanism of histamine release, we examined a series of bradykinin analogs with single amino acid substitutions in the bradykinin sequence. Replacement of proline7 in the bradykinin sequence with D-phenylalanine is the essential change used to convert kinin analogs into antagonists, and 10(-5) M [DPhe7]-bradykinin induced 8-10% histamine release. Other analogs, devoid of antagonist activity, however, such as [DPhe6]-bradykinin and [LPhe7]-bradykinin were able to induce equivalent levels of histamine release. The ability to induce histamine release appears to be related, at least in part, to aromaticity, since [DTrp6]-bradykinin and [DTrp7]-bradykinin induced greater amounts of histamine release than equivalent [DPhe]-analogs, causing approximately 20% histamine release at 10(-5) M. By contrast, [DAla7]-bradykinin was an ineffective stimulus. In summary, a single amino acid substitution can convert bradykinin into a secretagogue for human skin mast cells. The ability of kinin analogs to induce histamine release from skin mast cells, but not lung mast cells or basophils, emphasizes the heterogeneity of human histamine-containing cells.