Mast cells in rat gastrointestinal mucosa. 3. Reactivity towards compound 48/80

Heterogeneity of Human Mast Cells With Respect to MRGPRX2 Receptor Expression and Function

Mast cells and their mediators play a role in the control of homeostasis and in the pathogenesis of several disorders. The concept of rodent mast cell heterogeneity, initially established in the mid-1960s has been extended in humans. Human mast cells isolated and purified from different anatomic sites can be activated via aggregation of cell surface high affinity IgE receptors (FcεRI) by antigens, superantigens, anti-IgE, and anti-FcεRI. MAS-related G protein-coupled receptor-X2 (MRGPRX2) is expressed at high level in human skin mast cells (MCs) (HSMCs), synovial MCs (HSyMCs), but not in lung MCs (HLMCs). MRGPX2 can be activated by neuropeptide substance P, several opioids, cationic drugs, and 48/80. Substance P (5 × 10⁻⁷ M – 5 × 10⁻⁶ M) induced histamine and tryptase release from HSMCs and to a lesser extent from HSyMCs, but not from HLMCs and human cardiac MCs (HHMCs). Morphine (10⁻⁵ M – 3 × 10⁻⁴ M) selectively induced histamine and tryptase release from HSMCs, but not from HLMCs and HHMCs. SP and morphine were incomplete secretagogues because they did not induce the de novo synthesis of arachidonic acid metabolites from human mast cells. In the same experiments anti-IgE (3 μg/ml) induced the release of histamine and tryptase and the de novo synthesis of prostaglandin D₂ (PGD₂) from HLMCs, HHMCs, HSyMCs, and HSMCs. By contrast, anti-IgE induced the production of leukotriene C₄ (LTC₄) from HLMCs, HHMCs, HSyMCs, but not from HSMCs. These results are compatible with the heterogeneous expression and function of MRGPRX2 receptor on primary human mast cells isolated from different anatomic sites.

Disodium cromoglycate reverses colonic visceral hypersensitivity and influences colonic ion transport in a stress-sensitive rat strain

The interface between psychiatry and stress-related gastrointestinal disorders (GI), such as irritable bowel syndrome (IBS), is well established, with anxiety and depression the most frequently occurring comorbid conditions. Moreover, stress-sensitive Wistar Kyoto (WKY) rats, which display anxiety- and depressive-like behaviors, exhibit GI disturbances akin to those observed in stress-related GI disorders. Additionally, there is mounting preclinical and clinical evidence implicating mast cells as significant contributors to the development of abdominal visceral pain in IBS. In this study we examined the effects of the rat connective tissue mast cell (CTMC) stabiliser, disodium cromoglycate (DSCG) on visceral hypersensitivity and colonic ion transport, and examined both colonic and peritoneal mast cells from stress-sensitive WKY rats. DSCG significantly decreased abdominal pain behaviors induced by colorectal distension in WKY animals independent of a reduction in colonic rat mast cell mediator release. We further demonstrated that mast cell-stimulated colonic ion transport was sensitive to inhibition by the mast cell stabiliser DSCG, an effect only observed in stress-sensitive rats. Moreover, CTMC-like mast cells were significantly increased in the colonic submucosa of WKY animals, and we observed a significant increase in the proportion of intermediate, or immature, peritoneal mast cells relative to control animals. Collectively our data further support a role for mast cells in the pathogenesis of stress-related GI disorders.

Electroacupuncture Attenuates 5'-Guanidinonaltrindole-Evoked Scratching and Spinal c-Fos Expression in the Mouse

The present study was undertaken to investigate the influence of electroacupuncture (EA) on compulsive scratching in mice and c-Fos expression elicited by subcutaneous (s.c.) administration of a known puritogen, 5'-guanidinonaltrindole (GNTI) to the neck. Application of EA to Hegu (LI4) and Quchi (LI11) acupoints at 2 Hz, but not 100 Hz, attenuated GNTI-evoked scratching. In mice pretreated with the µ opioid receptor antagonist naloxone, EA 2 Hz did not attenuate GNTI-evoked scratching, whereas EA at 2 Hz did attenuate GNTI-evoked scratching in mice pretreated with the κ opioid receptor antagonist nor-binaltorphimine. Moreover, intradermal (i.d.) administration of the selective µ opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate (DAMGO) attenuated GNTI-evoked scratching behavior, while s.c. administration of DAMGO was ineffective. GNTI provoked c-Fos expression on the lateral side of the superficial layer of the dorsal horn of the cervical spinal cord. Application of 2 Hz EA to LI4 and LI11 decreased the number of c-Fos positive nuclei induced by GNTI. It may be concluded that application of 2 Hz EA to LI4 and LI11 attenuates scratching behavior induced by GNTI in mice and that the peripheral µ opioid system is involved, at least in part, in the anti-pruritic effects of EA.

Adipose tissue as a dedicated reservoir of functional mast cell progenitors

White adipose tissue (WAT) is a heterogeneous tissue, found in various locations throughout the body, containing mature adipocytes and the stroma-vascular fraction (SVF). The SVF includes a large proportion of immune hematopoietic cells, among which, mast cells that contribute to diet-induced obesity. In this study, we asked whether mast cells present in mice adipose tissue could derive from hematopoietic stem/progenitor cells (HSPC) identified in the tissue. We therefore performed both in vitro and in vivo experiments dedicated to monitoring the progeny of WAT-derived HSPC. The entire study was conducted in parallel with bone marrow-derived cells, considered the gold standard for hematopoietic-lineage studies. Here, we demonstrate that adipose-derived HSPC contain a precursor-cell population committed to the mast cell lineage, and able to efficiently home to peripheral organs such as intestine and skin, where it acquires properties of functional tissue mast cells. Additionally, WAT contains a significant mast cell progenitor population, suggesting that the entire mast cell lineage process take place in WAT. Considering the quantitative importance of WAT in the adult organism and the increasing roles recently assigned to mast cells in physiopathology, WAT may represent an important source of mast cells in physiological and pathological situations.

Amphetamine modulates cellular recruitment and airway reactivity in a rat model of allergic lung inflammation

Asthma is characterized by pulmonary cellular infiltration, vascular exudation and airway hyperresponsiveness. Several drugs that modify central nervous system (CNS) activity can modulate the course of asthma. Amphetamine (AMPH) is a highly abused drug that presents potent stimulating effects on the CNS and has been shown to induce behavioral, biochemical and immunological effects. The purpose of this study was to investigate the effects of AMPH on pulmonary cellular influx, vascular permeability and airway reactivity. AMPH effects on adhesion molecule expression, IL-10 and IL-4 release and mast cell degranulation were also studied. Male Wistar rats were sensitized with ovalbumin (OVA) plus alum via subcutaneous injection. One week later, the rats received another injection of OVA-alum (booster). Two weeks after this booster, the rats were subjected to AMPH treatment 12 h prior to the OVA airway challenge. In rats treated with AMPH, the OVA challenge reduced cell recruitment into the lung, the vascular permeability and the cellular expression of ICAM-1 and Mac-1. Additionally, elevated levels of IL-10 and IL-4 were found in samples of lung explants from allergic rats. AMPH treatment, in comparison, increased IL-10 levels but reduced those of IL-4 in the lung explants. Moreover, the tracheal responsiveness to methacholine (MCh), as well as to an in vitro OVA challenge, was reduced by AMPH treatment, and levels of PCA titers were not modified by the drug. Our findings suggest that single AMPH treatment down-regulates several parameters of lung inflammation, such as cellular migration, vascular permeability and tracheal responsiveness. These results also indicate that AMPH actions on allergic lung inflammation include endothelium-leukocyte interaction mechanisms, cytokine release and mast cell degranulation.

Nalfurafine prevents 5'-guanidinonaltrindole- and compound 48/80-induced spinal c-fos expression and attenuates 5'-guanidinonaltrindole-elicited scratching behavior in mice

The aims of the present study were to establish if nalfurafine, a kappa opioid agonist, inhibits compulsive scratching in mice elicited by the s.c. administration (behind the neck) of 5'-guanidinonaltrindole (GNTI), a kappa opioid antagonist; to assess if nalfurafine prevents c-fos expression provoked by GNTI or compound 48/80, two chemically diverse pruritogens; and to distinguish on the basis of neuroanatomy, those neurons in the brainstem activated by either GNTI-induced itch or formalin-induced pain (both compounds given s.c. to the right cheek). Pretreatment of mice with nalfurafine (0.001-0.03 mg/kg s.c.) attenuated GNTI (0.3 mg/kg)-evoked scratching dose-dependently. A standard antiscratch dose of nalfurafine (0.02 mg/kg) had no marked effect on the spontaneous locomotion of mice. Tolerance did not develop to the antiscratch activity of nalfurafine. Both GNTI and compound 48/80 provoked c-fos expression on the lateral side of the superficial layer of the dorsal horn of the cervical spinal cord and pretreating mice with nalfurafine inhibited c-fos expression induced by both pruritogens. In contrast to formalin, GNTI did not induce c-fos expression in the trigeminal nucleus suggesting that pain and itch sensations are projected differently along the sensory trigeminal pathway. Our data indicate that the kappa opioid system is involved, at least in part, in the pathogenesis of itch; and that nalfurafine attenuates excessive scratching and prevents scratch-induced neuronal activity at the spinal level. On the basis of our results, nalfurafine holds promise as a potentially useful antipruritic in human conditions involving itch.

Topography and morphometry of intestinal mast cells in children with Hirschsprung's disease

Mast cells (MC) are source of many biological active compounds like cytokines, arachidonic acid derivates, proteoglicanes, prostaglandins, proteases, free oxygen radials, NGF, PAF and many more. The role of MC in pathogenesis of Hirschsprung's disease (HD) is not clear. Substances produced by MC may exert an important effect on embryology, growth, differentiation and regeneration of intestinal nervous system. Additionally, MC products modulate inflammation processes thus influencing on the clinical course of HD. Present study was established to evaluate the morphologic MC examination as a support of making diagnosis in HD. The MC topography and morphometry were evaluated in specimens collected from aganglionic colon of patients with diagnosed HD. The results were compared with corresponding data from normally innervated colon of patients suffering from constipation, and normal colon of children not presenting defecation problems. MC were visualized using indirect immunohistochemical method LSAB with mouse antibody against human tryptase. The MC visualized in submucosa and muscular layer in Hirschsprung's disease were significantly larger in comparison with control group (p<0.05). Also the number of MC/mm2 in mucosa and lamina propria in HD was significantly elevated (p<0.05). However, the MC density in submucosa was also higher but it was not high statistically significant. In muscular layer and in serosa density of MC/mm2 was not statistically significant. In the intestinal wall MC in aganglionic segment in Hirschsprung's disease are significantly activated comparing with normally innervated colon segments taken from the patients from other groups. This may confirm the role of MC both in pathogenesis of HD and in the reparation processes of bowel nervous system.

Pulmonary neutrophil recruitment and bronchial reactivity in formaldehyde-exposed rats are modulated by mast cells and differentially by neuropeptides and nitric oxide

We have used a pharmacological approach to study the mechanisms underlying the rat lung injury and the airway reactivity changes induced by inhalation of formaldehyde (FA) (1% formalin solution, 90 min once a day, 4 days). The reactivity of isolated tracheae and intrapulmonary bronchi were assessed in dose-response curves to methacholine (MCh). Local and systemic inflammatory phenomena were evaluated in terms of leukocyte countings in bronchoalveolar lavage (BAL) fluid, blood, bone marrow lavage and spleen. Whereas the tracheal reactivity to MCh did not change, a significant bronchial hyporesponsiveness (BHR) was found after FA inhalation as compared with naive rats. Also, FA exposure significantly increased the total cell numbers in BAL, in peripheral blood and in the spleen, but did not modify the counts in bone marrow. Capsaicin hindered the increase of leukocyte number recovered in BAL fluid after FA exposure. Both compound 48/80 and indomethacin were able to prevent the lung neutrophil influx after FA, but indomethacin had no effect on that of mononuclear cells. Following FA inhalation, the treatment with sodium cromoglycate (SCG), but not with the nitric oxide (NO) synthase inhibitor L-NAME, significantly reduced the total cell number in BAL. Compound 48/80, L-NAME and SCG significantly prevented BHR to MCh after FA inhalation, whereas capsaicin was inactive in this regard. On the other hand, indomethacin exacerbated BHR. These data suggest that after FA inhalation, the resulting lung leukocyte influx and BHR may involve nitric oxide, airway sensory fibers and mast cell-derived mediators. The effect of NO seemed to be largely restricted to the bronchial tonus, whereas neuropeptides appeared to be linked to the inflammatory response, therefore indicating that the mechanisms responsible for the changes of airway responsiveness caused by FA may be separate from those underlying its inflammatory lung effects.

Microcirculatory dysfunction during intestinal ischemia-reperfusion

Oxido-reductive stress is a crucial factor of the tissue response during ischemia-reoxygenation injuries. Reperfusion affects primarily the microvasculature in a manner consistent with an acute inflammatory reaction. In this respect, the salient data suggest an important connection between endothelial cell-derived humoral mediators and the perivascular mast cell system. Increased endothelin-1 and decreased nitric oxide formation, mast cell degranulation and leukocyte accumulation coexist in gastrointestinal ischemia-reperfusion syndromes too. Constitutively produced nitric oxide inhibits, while increasingly formed endothelin-1 significantly enhances the degranulation of the intestinal mast cells. The endothelin-A receptor-dependent mast cell degranulation per se plays a secondary role in reperfusion-induced structural injury, but contributes significantly to leukocyte recruitment into the reperfused intestinal mucosa. It is conceivable therefore, that the nitric oxide--endothelin-1--mast cell cycle is involved in the mechanism of ischemia-reperfusion-induced endothelial cell-leukocyte interactions, where mast cells act to amplify the process of leukocyte sequestration. The alteration in the balance between endothelial cell-derived proadhesive vasoconstrictor and antiadhesive vasodilator factors exerts a significant influence on the mucosal integrity, and the antagonism of endothelin-A receptor activation in this setting tips the equilibrium toward tissue salvage.

Acute otitis media develops in the rat after intranasal challenge of Streptococcus pneumoniae

OBJECTIVES/HYPOTHESIS

The rat is a frequently used animal model for middle ear research. To date, acute otitis media (AOM) has been evoked after instillation of bacteria directly into the middle ear cavity or after traumatizing the tympanic membrane. The purpose of the study was to examine whether, with an intact tympanic membrane and middle ear cavity, intranasally deposited bacteria cause AOM and how tympanic membrane stimulation influences this procedure.

STUDY DESIGN

In vivo, murine model.

METHODS

In a rat model, Streptococcus pneumoniae, type 3, was intranasally inoculated for 5 consecutive days. The tympanic membrane was treated with saline or with compound 48/80 or was left untreated. The development of AOM was evaluated by otomicroscopy, light microscopy, and middle ear culture.

RESULTS

Ninety percent of the ears developed AOM. However, when the tympanic membranes were treated with saline or compound 48/80, only 40% and 57%, respectively, developed AOM. In all, 23 of 40 ears developed AOM and 20 ears showed growth of bacteria.

CONCLUSION

Repeated intranasal deposition of S. pneumoniae, type 3, causes AOM in the rat. The development of AOM can be influenced by tympanic membrane stimulation.

Involvement of unique mechanisms in the induction of scratching behavior in BALB/c mice by compound 48/80

Compound 48/80 induced scratching behavior in BALB/c mice, and the role of mast cell mediators in this behavior was examined. Mouse scratching behavior was detected and evaluated using a new apparatus, MicroAct. Compound 48/80 increased the incidence of scratching behavior and scratching time in a dose-dependent manner, accompanied by a potent activation of mast cells and a potent increase in vascular permeability. Dibucaine and mu-opioid receptor antagonists inhibited the scratching behavior. Although histamine H(1) receptor antagonists potently inhibited the vascular permeability increase, they did not affect the scratching behavior. Methysergide inhibited the scratching behavior slightly without affecting the vascular permeability increase, whereas cyproheptadine inhibited both. A cyclooxygenase inhibitor, a 5-lipoxygenase-activating protein inhibitor and a PAF receptor antagonist did not affect the scratching behavior. High doses of serotonin induced scratching behavior less frequently than did compound 48/80. Furthermore, mast cell-deficient WBB6F1-W/W(v) mice exhibited frequent scratching behavior after injection of compound 48/80. These results clearly indicate that compound 48/80 can induce scratching behavior in mice independent of mast cell mediators.

Mast cells and angiogenesis

Angiogenesis is tightly regulated by pro- and anti-angiogenic factors. Secreting mast cells are able to induce and enhance angiogenesis via multiple in part interacting pathways. They include mast cell-derived (i) potent pro-angiogenic factors such as VEGF, bFGF, TGF-beta, TNF-alpha and IL-8, (ii) proteinases and heparin, that release heparin-binding pro-angiogenic factors lodged on cell surfaces and in the extracellular matrix (ECM), (iii) histamine, VEGF, and certain lipid-derived mediators that induce microvascular hyperpermeability having pro-angiogenic effects, (iv) chemotactic recruitment of monocytes/macrophages and lymphocytes that are able to contribute with angiogenesis-modulating molecules, (v) activation of platelets that release pro-angiogenic factors, (vi) activation of neighboring stationary non-mast cells, which secrete pro-angiogenic factors, ECM-degrading proteinases and stem cell factor which attracts, mitogenically stimulates and activates mast cells, (vii) auto- and paracrine stimulation of mast cells by stem cell factor, (viii) recruitment of mast cells by pro-angiogenic factors such as VEGF, bFGF and TGF-beta. As a result of ECM-degradation and changes in the microenvironment following initial mast cell secretion, the mast cell populations may change significantly in number, phenotype and function. In tumor models, mast cells have been shown to play a decisive role in inducing the angiogenic switch which precedes malignant transformation. There is, moreover, strong evidence that mast cells significantly influence angiogenesis and thus growth and progression in human cancers.

Pharmacological modulation of allergic inflammation in the rat airways and association with mast cell heterogeneity

Administration of ovalbumin by aerosol to sensitised rats produced a rapid (15 min) protein exudation in different airway tissues, as determined by Evans blue staining. This was associated with marked mast cell degranulation determined by histological examination, with there being no difference between mucosal and connective tissue mast cells. A 5-day administration regimen with compound 48/80 selectively depleted connective tissue mast cell (positive to berberine staining) without modifying ovalbumin-induced plasma protein extravasation. Treatment of rats with dexamethasone (1 mg/kg, -12 h) or nor-dihydroguaiaretic acid (30 mg/kg i.p., -30 min) significantly reduced ovalbumin-induced protein extravasation and preserved mucosal mast cell morphology. Indomethacin (4 mg/kg i.v., -30 min) exerted no effect on either parameter. In conclusion, we propose the mucosal mast cell as a target cell responsible at least partly for the inhibitory actions of known anti-inflammatory drugs. We suggest an involvement of endogenous leukotriene(s), but not prostanoid(s), in mucosal mast cell activation/degranulation.

Degranulation of mast cells provokes a massive inflammatory reaction in the tympanic membrane

OBJECTIVE

The pars flaccida is extremely rich in mast cells. On stimulation the mast cells release preformed and de novo synthesized inflammatory substances. The purpose of this study was to examine how these mast cell substances provoke inflammatory changes in the tympanic membrane.

STUDY DESIGN

In vivo, murine model.

METHODS

In a rat model, the mast cell secretagogue compound 48/80 was applied locally to the tympanic membrane on 4 consecutive days and the ensuing inflammatory changes were evaluated by otological, light, and electron microscopy 3, 6, 9, 12, 18, 24, 36, and 48 hours and 4, 6, and 8 days later.

RESULTS

Degranulation of the mast cells occurred within 3 hours of applying compound 48/80. Release of the mast cell substances coincided with an inflammatory event characterized by a two-stage reaction: an edema stage, peaking 6 hours after application, followed by a massive invasion of inflammatory cells, peaking at 24 and 48 hours. Pars flaccida and pars tensa were both involved, pars flaccida showing the earliest changes. Pars tensa exhibited the same biphasic reaction as pars flaccida, but approximately 6 hours later.

CONCLUSIONS

The mast cells of the pars flaccida have the capacity to elicit an intense inflammation of the tympanic membrane. The biphasic reaction pattern resembles that observed in experimental otitis media, suggesting involvement of the mast cells in this inflammatory condition of the middle ear.

Reversible Expression of Tryptases and Chymases in the Jejunal Mast Cells of Mice Infected with Trichinella spiralis

It is has been established that mouse mast cells (MCs) can reversibly alter their expression of serglycin proteoglycans and the homologous granule chymases that have been designated mouse MC protease (mMCP)-1, mMCP-2, and mMCP-5 in vivo. Nevertheless, it remained to be determined whether these immune cells could modify their expression of other chymases and the granule tryptases mMCP-6 and mMCP-7. As assessed immunohistochemically, we now show that MCs reversibly change their expression of the recently described chymase mMCP-9 and both tryptases as these cells traverse the jejunum during the amplification and regression stages of the reactive MC hyperplasia. In noninfected mice, most jejunal MCs reside in the submucosa and express mMCP-6 and mMCP-7, but not mMCP-9 or the chymase mMCP-2. During the inductive phase of the helminth-induced inflammation, when the jejunal MCs move from the submucosa to the tips of the villus, the MCs briefly express mMCP-9, cease expressing mMCP-6 and mMCP-7, and then express mMCP-2. During the recovery phase of the inflammation, jejunal MCs cease expressing mMCP-2 and then express varied combinations of mMCP-6, mMCP-7, and mMCP-9 as they move from the tips of the villus back toward the submucosa. In other model systems, mMCP-6 elicits neutrophil extravasation, and mMCP-7 regulates fibrin deposition and fibrinogen-mediated signaling events. Thus, the ability of a jejunal MC to reversibly alter its tryptase expression during an inflammatory event has important functional implications.

The mast cell: origin, morphology, distribution, and function

The mast cell remains an enigmatic cell more than 100 years after its discovery by Paul Ehrlich at the turn of the century. It is a cell that is found widely distributed in the body particularly associated with connective tissues. It can be recognised by its content of metachromatic granules when appropriately fixed and stained with metachromatic dyes such as toulidine blue. The metachromatic granules of the mast cell remain an important differentiating characteristic from other cells although it is by no means absolute. In the early days of its discovery it was thought to originate from primitive mesenchyme, thymocyte or lymphocyte. More recent evidence suggests that it may have originated from the monocyte. Current evidence points to an origin from haemopoietic tissue in the bone marrow, the progenitors differentiate from primitive cells under the influence of cytokines (IL3), migrate to other body sites and then undergo differentiation and maturation under the influence of growth and other factors. The mast cell has many functions exerted through its ability to produce a host of biologically active substances the most notable being heparin, serotonin, dopamine, tryptase and chymase. These substances may be released in response to immunological and neural stimuli. Mast cells are found to be functionally heterogeneous, possibly site specific and have the ability to adapt to their environment, producing secretions commensurate with the needs of any situation. The mast cell is involved in immunological, neoplastic, inflammatory and other conditions. Much about its function has been unravelled but there remains more to be uncovered.

Number, fixation properties, dye-binding and protease expression of duodenal mast cells: comparisons between healthy subjects and patients with gastritis or Crohn's disease

There is an accumulation of evidence to suggest that mast cells may play a key role in gastrointestinal inflammation. We have investigated the numbers and heterogeneity in staining properties of mast cells in biopsies of the duodenum of normal subjects (n = 10), and of normal duodenum from patients with Crohn's disease of the ileum and/or colon (n = 7) or with Helicobacter-associated gastritis of the antrum/corpus (n = 6). In normal donors, two subsets of mast cells, one located in the duodenal mucosa and the other in the submucosa, were clearly distinguished by their morphology and dye-binding properties. Whereas submucosal mast cells stained metachromatically with Toluidine Blue after neutral formalin fixation and emitted a yellow fluorescence after staining with Berberine sulphate, those in the mucosa were invisible using these stains. In patients with gastritis or Crohn's disease, there were marked changes in the numbers of mucosal mast cells compared with control subjects even though the duodenal biopsies were from apparently uninvolved tissue. Gastritis was associated with increased mucosal mast cell numbers (controls: 187 +/- 23 cells mm-2; gastritis: 413 +/- 139 cells mm-2; p = 0.0004), but mean mucosal mast cell counts in the uninvolved duodenum of Crohn's patients were actually decreased (34 +/- 30 cells mm-2, p = 0.0147). The clear differentiation between mucosal and submucosal mast cells on the basis of metachromasia with Toluidine Blue was not seen in biopsies from the patients with gastritis or Crohn's disease. Previous studies which have suggested that there are no distinct mucosal and submucosal mast cell subsets in the human intestine may, therefore, have been affected by the use of tissue from diseased subjects. Heterogeneity in the expression of mast cell tryptase and chymase was seen by immunohistochemistry using specific antibodies, but the relative numbers of mast cell subsets were critically dependent on the methods used. Using a sensitive staining procedure, the majority of mucosal mast cells stained positively for chymase as well as for tryptase, an observation confirmed by immunoelectron microscopy and immunoabsorption studies. Our findings suggest that early stages in intestinal inflammation may be reflected in changes in mast cell numbers and in their staining properties, and call for a reappraisal of mast cell heterogeneity in the human intestinal tract.

Histochemistry and morphology of porcine mast cells

Mast cells have been described extensively in rodents and humans but not in pigs, and the objective of this study was to characterize porcine mast cells by histochemistry and electron microscopy. Carnoy's fluid proved to be a good fixative but fixation with neutral buffered formalin blocked staining of most mast cells. Alcian Blue stained more mast cells than did Toluidine Blue (pH 0.5), although Alcian Blue also stained goblet cells. In pigs, unlike rodents, the Alcian Blue method did not distinguish between mast cells in the intestinal mucosa and those in the connective tissue of the intestinal submucosa, tongue and skin. Mast cells were significantly larger in adult pigs than in piglets; in adult pigs and piglets, mast cells in the intestinal mucosa were significantly larger than those in submucosal connective tissue, and they were more varied in shape in piglets and adults. Granules in mast cells in the intestinal mucosa stained less intensely than those in mast cells in connective tissue of tongue, skin and intestinal submucosa. Mast cells in the connective tissue of the tongue, skin and intestinal submucosa fluoresced strongly when stained with berberine sulphate or with a mixture of berberine sulphate and Acridine Orange, but mast cells in the intestinal mucosa did not. All mast cells reacted positively in an enzyme-histochemical method previously used to detect human tryptase but not in a method previously used to detect human chymase. Mast cells in the medulla of thymus stained similarly to mast cells in the intestinal mucosa. Ultrastructural differences between mast cells were not detected.

Mast cells in the anterior uvea of the rabbit. Intraocular effects of compound 48/80 in the rabbit

In the present study, the presence and localization of mast cells and the intraocular effects of compound 48/80 have been studied in detail in the rabbit eye using histochemical and physiological methods. In histochemical studies mast cells were localized in the anterior uvea, especially in the ciliary and iridial processes. Intracamerally injected, compound 48/80 caused an increase in the intraocular pressure, disruption of the blood-aqueous barrier and an increase in the cAMP content in the aqueous humour. Miosis was observed only after higher doses of compound 48/80 (greater than 100 micrograms) and even then only one-half of the eyes responded. The intraocular effects, excluding miosis, of compound 48/80 resembled an on/off-type of response, where 20 micrograms caused only minor changes, if any, and 50 micrograms gave a maximal response. The ocular hypertensive reaction developed a tachyphylaxis so that the second and third consecutive dose of compound 48/80 (100 micrograms) produced no significant change in IOP. The results indicate that mast cells, which are present in the anterior uvea in an extent not known previously, might be involved in certain inflammatory reactions in the rabbit eye. The inconsistent and slight miosis after the intracameral application of compound 48/80 indicates that the mechanism is different from that caused by sensory nerve stimulation. The rapid development of tachyphylaxis after consecutive application of compound 48/80 suggests that mast cells are easily depleted which might be useful for further studies to evaluate the functional role of mast cells in different pathophysiological conditions in the rabbit eye.

Biological properties of human skin mast cells

Mast cells and basophils, although sharing many constitutive properties, are quite distinct in their development, functions and biological properties. Mast cell granules are composed of a macromolecular matrix of proteoglycan and neutral protease of which heparin and tryptase, respectively, are predominant. The distribution of the other major neutral protease, chymase, allows human mast cell subpopulations to be subdivided immunocytochemically. All human mast cells respond to IgE-dependent stimulation with the secretion of the preformed mediator, histamine, and the newly generated lipid-derived eicosanoids PGD2 and LTC4. Although amounts of these products vary between mast cells dispersed from different tissues, it is uncertain whether this reflects true heterogeneity. Mast cells of the human skin, but not those of other tissues, are sensitive to stimulation by substance P, compound 48/80 and other basic non-immunological stimuli. The mechanism of mediator secretion induced by these agents is distinct from that induced by IgE-dependent stimulation. However, the morphological characteristics of degranulation are similar, suggesting that the distinct biochemical pathways merge into a common pathway before effecting degranulation.

Droperidol-fentanyl inhibits mast cell histamine release in rat synovium, but not skin, after immunologic activation: evidence for mast cell heterogeneity

The characteristics of mast cell heterogeneity include differences in mast cell staining, cellular structure, content of proteoglycans and vasoactive amines, and in vitro responses to secretagogues. We have found that the anesthetic agent, droperidol-fentanyl, significantly decreased rat synovial vascular permeability after selective synovial mast cell degranulation. In contrast, there was no effect on skin mast cell-induced vascular permeability. To address the possibility that this difference in response represents mast cell heterogeneity and uniqueness of synovial mast cells, we investigated the effects of three different anesthetic agents on synovial mast cells. In these studies we examined whether (1) the drugs affect synovial mast cells and (2) whether the drugs affect synovial mast cells differently from mast cells in skin. Our data indicate that, compared to inhaled methoxyflurane and ethyl ether, subcutaneous droperidol-fentanyl had a significant inhibitory effect on synovial, but not skin, mast cell-mediated vascular permeability, which was associated with a significant inhibition of synovial mast cell histamine release. Thus, the permeability differences were not due only to effects on histamine receptors. Our data suggest the possibility that there are significant functional differences between synovial and skin mast cells, a phenomenon that may have important therapeutic implications for treatment of mast cell-mediated synovial inflammation.

Lectin histochemistry of the mast cell: heterogeneity of rodent and human mast cell populations

Mast cell granules contain a variety of N-linked saccharides. Heterogeneity of the expression of these saccharides in mast cells was studied in rodent and human tissues which were so selected as to contain all the mast cell subsets previously identified using other criteria. Dermal and intestinal mucosal mast cells were stained with lectins using an avidin-biotin system. It was found that dermal and subepidermal mast cells in the rat and mouse, and mucosal and dermal human mast cells showed very similar lectin binding properties to each other, with a fine variation in the intensity of staining with certain lectins. Rat mucosal mast cells, however, showed a distinctive pattern of lectin binding which was not seen in mast cells from any other tissue studied. The chemical basis of the differences seen were deduced and the possible significance of these structural variations is discussed.

A mast cell secretagogue, compound 48/80, prevents the accumulation of hyaluronan in lung tissue injured by ionizing irradiation

Irradiation with a single dose of 30 Grey on the basal regions of the lungs of Sprague-Dawley rats induced a peribronchial and alveolar inflammation. Infiltration of mast cells in the edematous alveolar interstitial tissue and also in the peribronchial tissue were characteristic features of the lesion. The appearance of mast cells was already seen 4 wk after irradiation and by weeks 6 to 8 there was a heavy infiltration. The staining properties suggested that they were connective tissue-type mast cells. The infiltration of mast cells was paralleled by an accumulation of hyaluronan (hyaluronic acid) in the alveolar interstitial tissue 6 and 8 wk after irradiation. The recovery of hyaluronan (HA) during bronchoalveolar lavage (BAL) of the lungs also increased at this time. Treatment with a mast cell secretagogue, compound 48/80, induced a distinct reduction of granulated mast cells in the alveolar tissue. Regular treatment with compound 48/80 from the time of irradiation considerably reduced the HA recovery during BAL and the HA accumulation in the interstitial tissue but did not affect the interstitial infiltration of mononuclear cells and polymorphonuclear leukocytes. By contrast, an accumulation of HA in the alveolar interstitial space was induced when compound 48/80 was given not until mast cell infiltration of the lung had started. The effects of compound 48/80 indicate that the connective tissue response after lung irradiation is dependent on whether or not mast cell degranulation is induced before or after the mast cell infiltration of the alveolar tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Eosinophils and mast cell homogeneity of the guinea pig eyelid skin, conjunctiva, and ileum

Mast cell heterogeneity has been described on the basis of differential staining reactions, light microscopic morphology, anatomic location, degranulation after polyamines, biochemical contents, growth requirements, and reactions to lymphokines. We have demonstrated typical "connective-tissue mast cells" by using anatomic criteria, histological staining reactions, electron microscopy, and reaction to compound 48/80 in the guinea pig conjunctiva, eyelid skin, and ileum. A second, much larger population of cells in the ileal mucosa and the conjunctiva, and rarely in the eyelid skin stained reddish-blue with acid toluidine blue in tissue fixed in ethanol-acetate-lead subacetate (BLA) and with alkaline Giemsa in formaldehyde-fixed tissue, did not stain with ethanolic or acid toluidine blue in formaldehyde-fixed tissue or with alkaline Giemsa in BLA-fixed tissue, and did not degranulate after 48/80 treatment. These are features of the rat intestinal "mucosal mast cells"; however, ultrastructural and light microscopic studies with the orcein Giemsa stain demonstrated these cells in the guinea pig to be eosinophils. Tissue culture, biochemical, and immunological studies indicate the existence of a second type of mast cell (bone-marrow-derived mast cell), ultrastructurally almost indistinguishable from the connective tissue mast cell. Our studies demonstrate only one mast cell type in the guinea pig and support the contention that other forms of mast cells are immature forms or variants of the connective-tissue mast cell.

Histochemical comparison of mast cells obtained from the airways of mongrel dogs and Basenji-Greyhound dogs by bronchoalveolar lavage

An abnormally large number of mast cells in the airway lumen may be an important factor in the pathogenesis of bronchial hyperreactivity. However, it is unclear just how many mast cells are present in the lumen of normal or hyperreactive airways, in part because of differences in the histochemical techniques that have been used to identify mast cells and questions about the heterogeneity of mast cells. The present study was done (1) to compare the effectiveness of six techniques in the identification of mast cells obtained from dogs by bronchoalveolar lavage (BAL), (2) to compare the mast cells in the airways of normal mongrel dogs with those from a breed of dog (Basenji-Greyhound) known to have bronchial hyperreactivity, and (3) to determine whether the two-type histochemical classification used for rodent mast cells (formaldehyde-resistant or typical and formaldehyde-sensitive or atypical) applies meaningfully to the mast cells in BAL fluid from dogs. Cells obtained by BAL were fixed with Mota's basic lead acetate or formaldehyde. Mast cells were identified by metachromatic staining with toluidine blue or methylene blue, staining of highly sulfated proteoglycans with Alcian blue or berberine sulfate, and a histochemical reaction for chloroacetate esterase (mast cell chymase). After Mota's fixation, the methods were relatively similar in their effectiveness in determining the number of mast cells in lavage fluid from the mongrel dogs, in that all of the values fell within a narrow range: 0.53 to 0.96% of the total number of cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Trypanosoma cruzi: antibody-dependent killing of bloodstream trypomastigotes by mouse bone marrow-derived mast cells and by mastocytoma cells

Two different populations of mast cells, that is, mastocytoma cells (P815) that were maintained either in vitro or in vivo, and mast cells obtained by differentiation of bone marrow precursor cells (MMC) in conditioned medium, were used as effector cells in antibody-dependent cytotoxic reactions (ADCC) against bloodstream trypomastigotes (BT) of Trypanosoma cruzi. The assay consisted of incubating effector cells with parasites that had been previously sensitized with immune mouse sera, immune IgG isotypes, or with medium. After the incubation period, the number of live BT was assessed. It was found that (a) cytotoxicity is antibody dependent; (b) the main isotypes involved are IgG1, IgG2a, and IgG2b; (c) both types of mast cells (mastocytoma and MMC cells) are equally efficient in killing BT; (d) mastocytoma cells degranulated by pretreatment with compound 48/80 are still able to effect ADCC; (e) on optical microscope examination, large numbers of parasites were often seen attached to the cells, but only when anti-T. cruzi antibodies were present; and (f) on electron microscope examination, no integral or ruptured parasites were seen inside the cells. We conclude that both T dependent and T independent mast cells are capable of mediating ADCC by a mechanism that is probably not dependent on granule extrusion.

The development of neurogenic plasma extravasation in the rat dura mater does not depend upon the degranulation of mast cells

Mast cells were visualized in stretch preparations of the rat dura mater and were found mostly in relation to small and large blood vessels. The overall number of dural mast cells was unaffected by electrical trigeminal or chemical deafferentation. As in other tissues, mast cell degranulation increased at sites of local injury (electrode penetration) or after systemic treatment with compound 48/80. However, mast cells did not degranulate following electrical trigeminal stimulation, or after injection of drugs (capsaicin or substance P) which promote plasma extravasation in the dura. Furthermore, pretreatment with a mast cell stabilizer (sodium dicromoglycate) or with large doses of H1 and H2 histamine receptor blockers (mepyramine and cimetidine), did not block electrically- or chemically-induced neurogenic plasma extravasation (NPE). Daily pretreatment with 48/80 however completely attenuated or abolished NPE. Taken together these data suggest that as assessed by the extrusion of metachromatic granules, mast cells are not essential to the development of neurogenic inflammation within the rat dura mater. However, these findings cannot exclude the possibility that mast cells may amplify or modulate this process.

Development of preclinical models for testing antiasthma drugs

The antiasthmatic agent, sodium cromoglycate, owes its discovery to a series of antigen challenge tests carried out during the 1960s by an asthmatic, Roger Altounyan, on himself. Until recently, research efforts to identify new antiasthma drugs have relied heavily on screening methods which involved passively-sensitised mast cells. In theory these tests, such as rat passive cutaneous anaphylaxis, appeared relevant and showed sodium cromoglycate to have a stabilising effect on the mast cell membrane. In practice no new drugs were discovered, since this type of activity in animal models was not predictive of antiasthmatic potential. A more relevant research programme has subsequently evolved, which attempts to approach more closely the conditions prevailing in the asthmatic lung. The use of a model of immune lung inflammation in macaque monkeys in conjunction with a model of bronchial hyper-reactivity in the dog has been successful in producing the new compound, nedocromil sodium, which is proving to be an effective addition to the drugs available for the treatment of inflammatory diseases of the airways.

The mast cells of the human uterus

Staining of mast cells in the human uterus has been studied using four fixatives and five staining methods to determine whether there are subpopulations of mucosal (endometrial) and connective tissue (myometrial) mast cells, and to discover how they can best be demonstrated. Following formalin fixation none of the staining methods showed maximum staining of mast cells in either endometrium or myometrium. The best demonstration of uterine mast cells is by fixation with either isotonic formol acetic acid or Mota's basic lead acetate followed by staining with the long toluidine blue technique. Although the degree of MC understaining following formalin fixation was greater for the endometrium than for the myometrium this is inadequate evidence to designate two cell populations. The findings suggest that the mast cells of the human uterus are all one population but show heterogeneity of histological properties possibly related to their functional state.

Phenotypic expression of proteoglycan in mast cells of the human nasal mucosa

The phenotypic expression of the proteoglycan of human mast cells in the nasal mucosa and normal skin was analysed using histochemical techniques. Nasal mucosa was obtained from normal subjects, from patients with seasonal allergic rhinitis before and during the pollen season and from patients with nasal polyps. In the latter groups, specimens were taken from both polyp tissue and adjacent nasal mucosa. Formaldehyde treatment blocked the cationic dye binding in 75-84% of the mast cells located in the nasal mucosa, as compared to the optimum fixation with IFAA (iso-osmotic formaldehyde-acetic acid). A significantly lower degree of blocking of dye binding was obtained in the human skin where 45% of the mast cells were susceptible to formaldehyde treatment (P less than 0.01). The mast cells of the polyp tissue also showed a relatively low degree of blocking (54%), which was significantly lower than the blocking of mast cells of the nasal mucosa taken from the same individuals (P less than 0.05). Staining of serial tissue sections in Alcian Blue containing graded concentrations of MgCl2 was used to determine the critical electrolyte concentration (CEC) of the dye binding, defined as the salt concentration at which the staining of 50% of the mast cells is extinguished. The CEC of the skin mast cells was 0.64M MgCl2 which is significantly higher than that of the mast cells of the nasal mucosa of normal subjects [0.49M (P less than 0.05)], allergic subjects [0.52M (P less than 0.01)], patients with polyp disease [0.52M (P less than 0.01)] and the polyp tissue proper [0.57M (P less than 0.05)].(ABSTRACT TRUNCATED AT 250 WORDS)

Density heterogeneity of human lung mast cells

Suspensions of enzymatically dispersed human lung parenchymal mast cells were fractionated according to density by flotation through discontinuous Percoll gradients and examined for their responsiveness to release stimulants and pharmacologic agonists. Mast cells localized to all six density fractions (I-VI) examined: densities varied from specific gravities of 1.053 gm/ml to 1.123 gm/ml. Most (67%) lung mast cells localized to fractions III and IV, corresponding to specific gravities of 1.077 to 1.088 gm/ml, respectively. Histamine content increased with density from 2.7 +/- 0.3 pg per cell in fraction 1 to 4.8 +/- 0.7 pg per cell in fraction VI (mean +/- SEM; n = 19). Fraction III was least responsive to high concentrations of anti-IgE than to any other fractions and, along with fraction IV, the most responsive to ionophore A23187. All fractions released the arachidonate mediators prostaglandin D2 and leukotriene C4 in response to anti-IgE. In four of eight lungs tested, formyl methionine peptide (10(-6) to 10(-4) mol/L) weakly elicited histamine release (3% to 6%) in fractions I and II cells. Compound 48/80 (0.1 to 10 micrograms/ml; n = 3) failed to induce histamine release in any fractions. The cyclic adenosine monophosphate-active drugs, isoproterenol (10(-4) mol/L), dibutyryl cyclic adenosine monophosphate (3 mmol/L), and isobutylmethylxanthine (3 X 10(-4) mol/L) inhibited anti-IgE-induced histamine release from all fractions equivalently. Dimaprit (3 X 10(-5) mol/L) and cromolyn sodium (10(-5) -3 x 10(-3) mol/L) failed to significantly inhibit any fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Mast cell numbers in appendices with threadworm infestation

A study has been made of mast cells (MC) in surgically resected appendices using a long toluidine blue (LTB) staining method. The numbers of MC in measured areas of both mucosa and submucosa/muscularis were counted and comparisons made between 22 appendices containing threadworms and 22 which were histologically normal. There was considerable variation in MC numbers from case to case and the patients aged under 15 as a group had a higher mean number of mucosal MC than the older patients. The reasons for the high individual variation could not be identified from the histological sections, and no correlation was found between MC numbers and the presence of threadworms.

Comparison of human lung and intestinal mast cells

Since much of the data on heterogeneity in the rodent systems are based on differential effects of drugs, we compared responses of human lung mast cell and intestinal mucosal mast cell populations to agents that have revealed mast cell heterogeneity in rat models, as well as to other agents that are effective in both rat peritoneal and intestinal mast cell preparations. Disodium cromoglycate and theophylline inhibit histamine release (HR) from rat peritoneal, but not intestinal, mast cells. We found disodium cromoglycate (10(-4) to 10(-8) mol/L) to be ineffective in both human lung and human intestinal mast cell preparations, whereas theophylline (10(-3) to 10(-6) mol/L) inhibited both lung and intestinal mast cell HR. Quercetin (10(-4) to 10(-8) mol/L) and doxantrazole (3 X 10(-4) to 10(-7) mol/L), which inhibit both rat mast cell types, also inhibit HR in both human mast cell preparations. We also examined the actions of indomethacin and exogenous arachidonic acid and found that neither the lung nor the intestinal mast cells demonstrated the enhancement of HR stimulated by these agents in the basophil. In summary, we have found no pharmacologic differences between the human lung and intestinal mucosal mast cell such as exist in the rodent models. This underscores the limitations of applying the data from animal mast cells to the human mast cells, and at the same time demonstrates that human basophils and mast cells are different.

Histamine storage and formation in canine gastric mucosa--effect of pentagastrin stimulation

Histamine storage and formation in the dog gastric mucosa were studied during basal conditions and after pentagastrin stimulation. Histamine formation (histidine decarboxylase activity), histamine content as well as the density of mast cells of the oxyntic gland mucosa were evenly distributed. Histamine content of the mucosa was significantly correlated to the density of mucosal mast cells. In the basal secretory state, histamine formation and histamine content of the oxyntic gland mucosa were of the same magnitude as in the antral mucosa. Pentagastrin stimulation induced a small but significant decrease in histamine content of the oxyntic gland mucosa and a subsequent acceleration in the rate of amine formation. Neither histamine content nor histidine decarboxylase activity of the antral mucosa was affected by pentagastrin infusion.

Two biochemically distinct populations of histaminocytes separated by isokinetic sedimentation of dispersed rat gastric cells

Two populations of histaminocytes, with different sedimentation rates (SR), were separated by a computer developed isokinetic gradient using dispersed rat gastric mucosal cells. Histamine content, histidine decarboxylase (HDC) activity and incorporation of radiolabelled histidine metabolites were used to assess the migration of specific cells throughout the gradients. One histaminocyte population, with cells of lower SR, contained high HDC activity and undetectable levels of histamine, whereas the other population, with cells of higher SR, contained lower HDC activity and high concentration of histamine. Both types of histaminocytes incorporated 3H-histidine metabolites. Electron microscopy showed that the fractions containing histaminocytes with lower SR had 3.5 times more endocrine ECL cells than the original population of dispersed fundic cells and lacked A and D cells, whereas the fractions with histaminocytes of higher SR were associated with a 2.7 times higher concentration of A and D cells and with a 7.7 times higher ratio of a variety of partial cells with a distinct mitochondrial morphology. These results are consistent with prior novel information regarding the separation of two populations of rat histaminocytes using different sedimentation techniques.

Homology of the rat basophilic leukemia cell and the rat mucosal mast cell

Secretory granules of the rat basophilic leukemia (RBL-1) cell, a chemically generated tumor cell line maintained in tissue culture, were shown to stain with alcian blue but not with safranin counterstain and to have sparse, small, electron-dense granules. A Mr 25,000 protein was the major [3H]diisopropyl fluorophosphate-binding protein in extracts of RBL-1 cells. Double-immunodiffusion analysis of extracts revealed immunoreactivity for rat mast cell protease (RMCP)-II, a Mr 25,000 neutral protease present in the secretory granules of rat mucosal mast cells and cultured rat bone marrow-derived mast cells, but no immunoreactivity for RMCP-I, the predominant neutral protease of rat connective tissue mast cells. By radial immunodiffusion, there was 66.8 ng of RMCP-II per 10(6) cells. Whereas rat connective tissue mast cells stain with alcian blue and safranin and contain heparin proteoglycan, rat mucosal and rat bone marrow-derived mast cells stain with alcian blue only and contain a non-heparin proteoglycan and lesser amounts of histamine. Proliferation of rat mucosal mast cells in vivo and rat bone marrow-derived mast cells in vitro requires T-cell factors, whereas no comparable requirement has been observed for connective tissue mast cells. The transformed RBL-1 tumor cells, whose growth is independent of factors other than those present in standard tissue culture medium, has previously been shown to contain predominantly chondroitin sulfate di-B proteoglycans and low amounts of histamine. The similar histology and secretory granule biochemistry of the rat mucosal mast cells, rat culture-derived mast cell, and RBL-1 cell suggest that they comprise a single mast cell subclass distinct from the rat connective tissue mast cell.

Ultrastructural study of the mast cells of the human duodenal mucosa

The ultrastructure of the process of degranulation of mast cells of human duodenal mucosa was examined. In normal controls little degranulation was seen, but in persons with false food allergy (pseudo-allergy) considerable degranulation of mast cells was detected. This is consistent with the hypothesis that some persons have an abnormal fragility of duodenal mast cells in the presence of histamine-releasing substances. Incubation of duodenal biopsy material with various histamine-releasing agents (compound 48/80, Concanavalin A, the calcium ionophore A 23187, and anti-IgE) confirmed the susceptibility of duodenal mast cells for antigen non-specific release of histamine, or that mediated by IgE. In a group of patients with immediate-type, anaphylactic, food allergy, mast cells in the absence of antigen are in a normal state, but degranulation occurs on exposure in vitro or in vivo to specific antigen. The susceptibility to degranulation continues in persons cured of their food allergy. This suggests that a clinical cure is not due to a change of susceptibility of duodenal mast cells to release histamine, but is possibly associated with formation of blocking antibodies, and/or a modification in reactivity of basophils and mast cells of other organs.

Mast cell heterogeneity: evidence and implications

Mast cells and basophils play a fundamental role in the pathogenesis of allergic disease, although their physiologic role is largely unknown. A large body of evidence now indicates that the properties of mast cells are dependent on the tissue and species from which they are derived. Such mast cell heterogeneity encompasses differences in morphology, development, cytochemistry, and function. The evidence for such heterogeneity, and some of its clinical implications, is discussed.

Separation and characteristics of two histaminocytes from rat gastric mucosa

To determine the properties of rat gastric cells involved in histamine metabolism (histaminocytes), fundic mucosa was enzymatically dispersed prior to separation by sedimentation methods. The distribution of histamine content, histidine decarboxylase (HDC) activity and incorporation of radioactive histidine metabolites were used to determine the characteristics of various populations of gastric cells. All activities measured, as well as most of the dispersed gastric cells, occurred in a narrow range of density between 1.083 and 1.091 g/ml. Velocity sedimentation showed that two populations of histaminocytes can be distinguished. One population has a higher sedimentation rate, suggesting a larger size, contains histamine, HDC activity and incorporates radioactive metabolites. Another population, in fractions with lower sedimentation rates, contains little histamine, has a higher HDC activity than the previous population and also incorporates radiolabelled histidine metabolites. For the first time, two populations of viable histaminocytes have been separated that differ in their biochemical properties.