Blood-aqueous barrier in pseudoexfoliation syndrome: evaluation by immunohistochemical staining of endogenous albumin

Background: Alterations of the integrity of the blood-aqueous barrier (BAB) are frequent findings in eyes with pseudoexfoliation syndrome (PSX). Methods: Immunohistochemical staining for the demonstration of albumin was used to analyze the BAB in 10 eyes with PSX without previous intraocular surgery and in 10 age-matched normal control eyes. Results: In eyes with PSX, small amounts of albumin were detected along the anterior surface of the iris in 7, in the anterior chamber in 1, along the ciliary epithelium in 4, and in the trabecular meshwork in 9 of 10 eyes. PSX material was also immunoreactive. In the 10 normal control eyes, albumin was detected anterior to the iris stroma in 1 eye, in the anterior chamber in 2 eyes, in the trabecular meshwork in 1 eye, but not internal to the ciliary epithelium. Conclusions: Our findings indicate that impairment of the BAB in PSX can be localized at the level of the iris and, less frequently or to a lesser extent, at the level of the ciliary body.     

Galactose-containing glycoconjugates of the iris,the aqueous outflow passages and the cornea in capsular glaucoma

Background: The aim of the study was to compare galactose-containing glycoconjugates of the iris, the aqueous outflow pas sages and the cornea with exfoliation material in capsular glaucoma. Methods: Six formalinfixed, paraffin-embedded human eyes with capsular glaucoma and six control eyes were studied by using a panel of 11 biotinylated lectins to galactose- and N-acetylgalactosamine-containing glycoconjugates. Results: The Gal ( 3) GaINAc-reactive lectins peanut agglutinin (PNA) and Bauhinia purpurea alba agglutinin (BPA) and the Gal (14)GlcNAc-reactive lectins Ricinus communis agglutinin (RCA-I) and Phaseolus vulgaris erythroagglutinin (PHA-E) gave the strongest label with exfoliation material. Lectin binding to the iris was variable. The binding of PNA, BPA, RCA-I, Erythrina cristagalli agglutinin (ECA), PHA-E and Glycine max agglutinin (SBA) to the subendothelial region of iris blood vessels closely resembled their binding to exfoliation material. RCA-I and PHA-E bound moderately to the aqueous outflow passages. The surface of the corneal epithelium showed positive reaction with most lectins studied, but the keratocytes reacted with RCA-I and PHA-E only. Neuraminidase pretreatment generally increased the reaction intensity. Conclusions: The findings suggest that the glycoconjugate composition of exfoliation material in the classical locations along the anterior and posterior chamber closely resembles that in the subendothelial region of iris blood vessels.The authors have no financial interest in any product or process mentioned herein     

Deamination of 3-hydroxykynurenine in bovine lenses: a possible mechanism of cataract formation in general

Background: 3-Hydroxykynurenine, a metabolite of tryptophan, acts as UV filter in the human lens. In this study, we looked for this substance and its metabolites in young and old bovine lenses, because of their possible role in the formation of cataract. Methods: The substances were detected by HPLC analysis. The fluorescent substance formed from 3-hydroxykynurenine was characterized by thin-layer chromatography followed by reaction with ninhydrin, UV and fluorescence spectrum analysis, and atom bombardment for molecular mass determination. The kynurenine aminotransferase activity was determined by the method of Tobes. Results: 3-Hydroxykynurenine was detected at concentrations of 0.07, 0.19, and 1.14 g/g of tissue in the bovine iris/ciliary body, retina, and transparent bovine lenses respectively. 3-Hydroxykynurenine was deaminated in old bovine eyes but not in calf eyes. In old eyes, kynurenine aminotransferase activity was 2.7, 3.5, and 9.6 mol/g of tissue per h in retina, iris/ciliary body, and lens respectively. Conclusion: The deamination of 3-hydroxykynurenine resulted in the formation of a fluorescent substance which was identified as oxidized xanthurenic acid. This substance, accumulating in the bovine lens and interacting with lens proteins, could induce cataract formation.     

Immunohistochemical localization of chondroitin sulfate proteoglycan and tenascin in the human eye compared with the HNK-1 epitope

Background: A previous study revealed the HNK-1 epitope in the human ciliary body beneath the ciliary epithelium. The molecules bearing this 3-sulphoglucuronic acid-containing oligosaccharide epitope in the eye remain unknown. As chondroitin sulphate proteoglycan (CSPG) and tenascin are potential candidates as bearers of the HNK-1 epitope, their distribution in the human eye was compared with that of the HNK-1 epitope. Methods: Fifty-five formalin-fixed, paraffin-embedded human eyes, including 20 normal eyes and 35 eyes with exfoliation syndrome or glaucoma, were studied immunohistochemically with monoclonal antibody (MAb) CS-56 to CSPG, MAb TN2 to tenascin, and MAbs HNK-I and VC1.1 to the HNK-1 epitope. Additionally, four frozen lens capsules with exfoliation material were studied by indirect immunofluoresence. Results: A population of dendritic cells in the inner connective tissue layer of the ciliary body and exfoliation material were immunoreactive with antibodies to the HNK-1 epitope, but no labelling for CSPG and tenascin was seen in them, including frozen sections. The inner surface of the nonpigmented ciliary epithelium was reactive for the HNK-1 epitope, and at the ora serrata also for CSPG. In some eyes with glaucoma, immunoreaction for CSPG and tenascin was seen beneath the epithelium and endothelium of the cornea. The nerve fibre layer of the retina was labelled for tenascin. In the sclera, all antibodies labelled the ground substance, and in some large blood vessels immunoreaction for CSPG and tenascin was seen subendothelially. Conclusion: Apart from the sclera, the distribution of CSPG and tenascin was different from that of the HNK-1 epitope, suggesting that this carbohydrate epitope may not be borne by these molecules in the human ciliary body.     

The ultrastructure of parapapillary chorioretinal atrophy in eyes with secondary angle closure glaucoma

Background: The present study was performed to investigate the ultrastructure of deep retinal layers and choroid corresponding to the parapapillary chorioretinal atrophy in eyes with secondary angle-closure glaucoma. Methods: The glaucomatous eyes included two eyes enucleated due to iris ring melanoma with high intraocular pressure and one eye with neovascular glaucoma enucleated due to ocular pain. The control eyes included one eye enucleated due to choroidal malignant melanoma with normal intraocular pressure and one eye enucleated during surgery for supramandibular carcinoma. These eyes were studied with light and electron microscopy. Results: In the region of parapapillary chorioretinal atrophy of glaucomatous eyes, the retinal pigment epithelial cells showed degenerative changes, such as loss of basal in foldings and microvilli, degenerated mitochondria, vacuolar degeneration and irregular distribution of melanin granules. The photoreceptors were decreased in number in this area of glaucomatous eyes. The lumen of the choriocapillary vessels adjacent to the optic nerve was collapsed. Conclusion: These results elucidate the fine structures of deep retina and choroid in the region of parapapillary chorioretinal atrophy of glaucomatous eyes, and suggest that the reduced choroidal perfusion might be the pathogenetic mechanism of glaucomatous parapapillary chorioretinal atrophy.     

The effect of argon laser trabeculoplasty on the blood-aqueous barrier and intraocular pressure in human glaucomatous eyes treated with diclofenac 0.1%

Background: We studied the effect of argon laser trabeculoplasty (ALT) on the bloodaqueous barrier (BAB) in 41 eyes of 41 patients with primary open-angle glaucoma, pseudoexfoliation glaucoma, or pigment dispersion glaucoma using the Fluorotron Master II. Methods: Fluorophotometry was performed the day before ALT and on the 3rd day after surgery at 30 and 60 min after intravenous injection of 7 mg/kg body weight sodium fluorescein 10%. Intraocular pressure (IOP) was measured using Goldmann applanation tonometry on the day before surgery and at 3rd days and 1 year (mean) after ALT. Patients were treated with argon laser by one surgeon (180°, 0.1 s, 50 m 0.6–1.0 W 56 laser burns). Eyes were randomly assigned to either diclofenac-sodium 0.1 % eye drops or vehicle. Eye drops were applied six times 1 h before ALT into the operated eyes and five times daily for 3 days postoperatively. Results: On the 3rd day after ALT there was significant disruption of the BAB in the placebo-treated eyes compared to the diclofenac 0.1%-treated eyes. In the placebo-treated eyes as well as in diclofenac-sodium 0.1 %-treated eyes there was a significant decrease of IOP postoperatively for up to 1 year. There was no significant difference concerning the IOP reduction after 1 year. Diclofenac-sodium 0.1 % eye drops significantly stabilized the BAB on the 3rd day after ALT, compared to placebo, in this model. Conclusion: Diclofenac-sodium 0.1 % significantly stabilized the disruption of the blood-aqueous barrier on the 3rd day after ALT. Concerning the IOP-lowering effect of ALT, the postoperative application of steroids should be avoided.     

Galactose-containing glycoconjugates of the ciliary body and lens in capsular glaucoma: a lectin histochemical study

Background: This study was carried out in order to obtain information on galactose-containing glycoconjugates in the lens and ciliary body of human eyes with capsular glaucoma and thus on the etiopathogenesis of exfoliation syndrome. Methods: Six formalin-fixed, paraffin-embedded human eyes with capsular glaucoma and six control eyes were studied using a panel of 11 biotinylated lectins with extended binding sites to galactose- and N-acetylgalactosamine-containing glycoconjugates. Both pepsin and neuraminidase pretreatments were performed. Results: Gal(ß13)Ga1NAc-reactive peanut (PNA) and Bauhinia purpurea alba (BPA) agglutinins and Gal(ß4)G1cNAc G1cNAc-reactive Ricinus communis (RCA-1) and Phaseolus vulgaris (PHA-E) agglutinins reacted strongly with exfoliation material, while binding of Gal(ß 4)G1cNAc-reactive Erythrina cristagalli and GaINAc-reactive soybean, Caragana arborescens, Vicia villosa, Helix pomatia (HPA) and Dolichos biorus agglutinins was generally weak. In the nonpigmented ciliary epithelium and on the zonular fibers and lamella, at least moderate reaction was detected with PNA, BPA, RCA-1 and PHA-E, and these tissues thus resembled exfoliation material in their reactivity. In contrast, the lens epithelium reacted weakly with PHA-E and HPA only, and the lens capsule was never labeled. No qualitative changes were seen after neuraminidase pretreatment. Conclusion: The findings suggest that galactose- and N-acetylgalactosamine-containing glycoconjugates in exfoliation material are at least partially produced by the nonpigmented ciliary epithelium, rather than by the lens epithelium. Moreover, PNA, BPA, RCA-1 and PHA-E seem to be the most suitable of the lectins tested for detection of exfoliation material in histological specimens.     

Microvascular change of the anterior eye segment after tenotomy in the rabbit

Background: The microvascular changes secondary to anterior segment ischemia following tenotomy of the extraocular muscles have not been studied in the rabbit. Methods: Using scanning electron microscopy of methylmethacrylate ocular microvascular luminal castings, the anterior eye segment vasculature after tenotomy was documented and compared to that after occlusion of the bilateral long posterior ciliary arteries and that in the eyes that were not subjected to any surgical intervention. Results: Five days and 1 week after the surgical intervention with tenotomy, microvascular change secondary to the anterior segment ischemia was not apparent, but 2 weeks after the tenotomy subtle evidence of ischemia such as new vessels in the iris was observed. Seven weeks after tenotomy, marked microvascular change was observed where corneal new vessels arose from the superior perilimbal arteries. In contrast, we found prominent microvascular changes 2 weeks after the occlusion of the long posterior ciliary arteries. Conclusions: Tenotomy of the rabbit eye causes microvascular change similar to that in occlusion of the long posterior ciliary arteries. This result suggests that the anterior ciliary artery of the rabbit contributes blood flow to the anterior eye segment and also has a stronger connection with the long posterior ciliary artery than previously reported.     

Endophthalmitis after Lasiodiplodia theobromae corneal abscess

Background:Lasiodiplodia theobromae is an exceptional cause of human keratomycosis. Patient: We treated a 53-year-old man with fungal keratitis, which had been treated with topical betamethasone and gentamicin for 1 month, and endophthalmitis due toLasiodiplodia theobromae. Despite intensive systemic, topical and intravitreal fungicidal treatment, enucleation had to be performed. Results: The vitreous aspirate cultures were negative as of the second amphotericin intravitreal injection. However, histology revealed that the fungus was present in the cornea, ciliary body, iris and retina. Conclusion: The use of topical steroids may worsen the outcome of the infection.     

Poly (triethylenglycol monomethacrylate) and poly(glycerol monomethacrylate) cross-linked gel as potential viscoelastics for intraoperative use

Background: The highly swelling poly(glycerol monomethacrylate) gel (polyGLYMA) and hydrophilic polymer poly(triethylenglycol monomethacrylate (polyTEGMA) were tested as potential viscoelastics for intraopertive use in anterior segment surgery. Methods: PolyGLYMA was implanted into the anterior chamber in 5 rabbits, and 40% polyTEGMA in 16 rabbits. The eyes were enucleated 1 week to 3 months after the operation. The corneal endothelium was examined with specular microscopy, and then the whole eye histopathologically. Results: In all eyes of the polyGLYMA group, the clinical findings were characterized by a marked ciliary injection and severe secondary glaucoma, and the histologic ones by a marked inflammatory infiltration and thickening of Descemet's membrane in the anterior chamber angle. Specular microscopy revealed a decrease in the endothelial cell density and polymorphism of the endothelial cells. In the polyTEGMA group, the anterior segment and the fundus were physiologic all the time, and specular microscopy and histologic findings showed no degenerative and/or inflammatory changes. Conclusions: PolyGLYMA proved unsuitable for intracameral application in rabbits. The new polymer polyTEGMA is characterized by high biologic tolerance after its implantation into the anterior chamber of rabbits. PolyTEGMA 40% might be considered as a potential viscoelastic material in humans.     

Integrins in human anterior chamber angle

Background: Integrins, which are composed of an and subunit, are capable of binding to a number of extracellular matrix proteins and, hence, affect cell adhesion and proliferation.Methods: The distribution of the integrin (₁, ₃-₅) and (_1–6 and _v) subunits in human anterior chamber angle was studied in eyes from subjects aged 9 months to 81 years using the indirect immunofluorescence technique.Results: Immunoreaction for the ₁ subunit was found throughout the trabecular meshwork (TM), in the cribriform layer, and in the endothelial lining of Schlemm's canal (SC). Labelling for the ₃ subunit was found in the TM and the cribriform layer only. In infant eyes the ₅ subunit was present in all three areas with the highest concentration in the cribriform layer, whereas no reaction was observed in adult eyes. The ₆ subunit was localized to the endothelium of SC only. Immunoreaction for the _v subunit was present in the TM and the cribriform layer of infants and young adults.Conclusion: The present results suggest the presence of several integrin heterodimers, acting as potential receptors for laminin, collagen, fibronectin, and vitronectin, in the anterior chamber angle.     

Fixation of whole eyes: the role of fixative osmolarity in the production of tissue artifact

Background: Whole eyes fixed in 4% buffered formaldehyde (10% neutral buffered formalin) demonstrate a variety of artifacts, including separation of the neurosensory retina from the retinal pigment epithelium. We postulate that the osmolarity of 4% buffered formaldehyde causes contraction of the internal compartments of the eye leading to several artifactual changes commonly observed in routine histologic sections. Methods: In part I of the study, enucleated animal eyes were examined histologically after immersion in different concentrations of formaldehyde. The variables of fixation and processing were kept constant except for the concentration and osmolarity of formaldehyde. In part II, enucleated animal eyes were used to empirically determine the optimal mixture of formaldehyde and glutaraldehyde for fixation based on subjective assessment of histologic sections. Results: In the first part of the study, the post-fixed volume of the anterior chamber and vitreous increased as the concentration (and osmolarity) of formaldehyde decreased. In part II of the study, fixation of whole eyes was optimal with a mixture of 1% buffered formaldehyde and 1.25% glutaraldehyde. The neurosensory retina was less likely to detach from the retinal pigment epithelium, and the anterior chamber retained a more normal shape with this fixative. Conclusions: Volume contraction of whole eyes fixed in 4% buffered formaldehyde is caused by the relatively high osmolarity of the fixative. Immersion fixation of whole eyes for 36 h (or longer) in 1% buffered formaldehyde/1.25% glutaraldehyde reduces tissue distortion without compromising cellular preservation.     

Surgical risk factors for severe postoperative proliferative vitreoretinopathy (PVR) in retinal detachment with grade B PVR

Background: Previous studies have shown that grade B proliferative vitreoretinopathy (PVR) is a considerable risk factor for the development of severe postoperative PVR. We conducted a prospective study to elucidate which surgical procedures used in retinal detachment management may stimulate the PVR process in such eyes. Materials and methods: The study included 156 eyes of 152 consecutive patients with rhegmatogenous retinal detachment complicated by grade B PVR referred before any failed surgery and operated on between 1983 and 1993. The parameters evaluated by multivariate statistical analysis included the cumulative circumferential extent of the retinal tears, the extent of the scleral buckle, gas injection, vitrectomy, the method used for retinopexy, and the time of surgical management during the period of the study. Results: The incidence of severe postoperative PVR was 25.8% in eyes managed with cryotreatment versus 2.2% in eyes managed with argon laser photocoagulation (P=0.001). The rate of severe postoperative PVR was not influenced by the other surgical variables. Conclusion: We conclude that cryotherapy may be a risk factor for the development of severe postoperative PVR in retinal detachments associated with grade B PVR.     

Pituitary adenylate cyclase-activating peptide-immunoreactive nerve fibers in the cat eye

Purpose: To study the occurrence and distribution of the neuropeptide pituitary adenylate cyclase-activating peptide (PACAP) in ocular and orbital structures of the cat. Methods: Immunocytochemistry to localize PACAP and double immunostaining to detect co-localization of PACAP with other neuropeptides. Results: Numerous PACAP-immunoreactive nerve fibers were observed in the lacrimal gland, choroid and retroocular arteries. There was a sparse supply of PACAP-containing nerve fibers in the iris, ciliary body and conjunctiva. Subpopulations of PACAP-containing nerve fibers stored vasoactive intestinal peptide (VIP) or calcitonin gene-related peptide (CGRP). Around 10% of the ganglion cells in the sphenopalatine ganglion harbored PACAP immunoreactivity. In the trigeminal ganglion around 5% of the neuronal cell bodies and in the ciliary ganglion only occasional ganglion cells contained PACAP immunoreactivity. PACAP immunoreactivity co-localized with VIP in the sphenopalatine ganglion and with CGRP in the trigeminal ganglion. Conclusion: PACAP-containing nerves in the eye and associated structures demonstrate a distribution pattern resembling that of VIP. Subpopulations of nerve fibers containing PACAP immunoreactivity store VIP or CGRP immunoreactivity. Neuronal PACAP in the eye and orbit may take part in regulation of smooth muscle tone, glandular secretion and sensory processing.     

Experimental microendoscopic photoablative laser goniotomy as a surgical model for the treatment of dysgenetic glaucoma

Purpose: The aim of this study was to investigate the feasibility of photoablative Er:YAG laser goniotomy under microendoscopic control in a surgical cloudy corneal model of primary infantile glaucoma. Methods: Pectinate ligaments of 12 freshly enucleated cadaver porcine eyes were treated by ab interno single-pulse (5 mJ, 200 s) Er:YAG laser (2.94 m) photoablation. Through a clear corneal incision near the limbus an ophthalmic microendoscope (18 and 20 gauge) was inserted into the anterior chamber. Internal structures were observed and photoablative laser goniotomy was conducted under video guidance. Following treatment all eyes were prepared for light and scanning electron microscopy. Results: Anterior chamber angle structures and tissue photoablation were clearly visualized on the videoscreen using ophthalmic microendoscopy. Energy settings of 5 mJ per pulse proved to be sufficient for reproducible photoablation of pectinate ligaments, accompanied by the root of the iris falling back and exposing trabecular meshwork. This was confirmed histopathologically. Scatter thermal damage was less than 30 m. Conclusion: This new therapeutic modality, which combines endoscopic visualization of the internal structures with photoablative laser goniotomy, can be effective in the management of dysgenetic glaucoma in the presence of a cloudy cornea. High reproducibility of contact laser photoablation enabled sufficient control of incision depth and was not accompanied by inadvertent tissue damage to adjacent intraocular structures.     

The effect of preservatives and antiglaucomatous medication on the histopathology of the conjunctiva

Background: Topical medication applied chronically for the treatment of glaucoma changes the cellular profile of the conjunctiva. We wanted to determine the role of preservatives, which are usually combined with the drugs, on this effect Methods: We applied metipranolol 0.3 % and pilocarpine 2 % with and without benzalkonium chloride 0.01 % and cetrimonium chloride 0.004 %, respectively. Twenty-four rabbits, divided into four groups, were treated for 3 months. The complete globes and the adherent bulbar conjunctiva were examined histopathologically and immunohistochemically Results: With special stains for collagen, a slight increase of the thickness of subepithelial collagen of the conjunctiva was present in both groups treated with medication and preservative compared with eyes treated with medication alone. This effect was also true for special antibodies for collagen type IV and -smooth muscle actin in the eyes treated with pilocarpine, but not in the eyes treated with metipranolol Conclusion: The results suggest that preservatives may have an additional adverse effect on the conjunctiva in addition to the effects of the medications alone.This paper was presented in part at the First Annual Meeting of the European Community Ophthalmic Research Association (ECORA), Bonn, Germany, 4 October 1993The authors have no financial or other interest in any of the substances used in this study     

Hamster Greene melanoma in the rabbit eye: immunosuppressive treatment to improve this tumor model

Background: The hamster Greene melanoma (HGM) implanted in the anterior chamber of the rabbit eye has been used to study experimental therapies for human uveal melanoma. However, the occurrence of spontaneous necrosis limits the value of this model for long-term evaluation of experimental treatments. In the present study we tested the hypothesis that an immune response is the cause of this necrosis and that prevention of the immune response can prolong the experimentation time Methods: HGM was implanted in the anterior chamber of control, presensitized and immunosuppressed rabbits. The effects of sensitization and immunosuppression were assessed by clinical and histological observation Results: Sensitization led to a significant slowing down of tumor growth, but not to a difference in necrosis. Immunosuppressive treatment with cyclosporin A improved the success rate of implantation and decreased the amount of necrosis in the tumor Conclusion: The immune response plays a role in the occurrence of necrosis. However, although immunosuppressive treatment with cyclosporin A decreased the amount of necrosis, significant necrosis still occurred, suggesting that other factors like angiogenesis play a part as well and still limit the usefulness of this model in the long-term evaluation of experimental therapies.     

Morphological differentiation of the conjunctival goblet cells in the chick (Gallus domesticus)

Background: These is no consensus in the literature regarding the differentiation of conjunctival goblet cells in vertebrates. Method: The conjunctival epithelium of the chick was studied before and after hatching in order to demonstrate the morphological evolution of the goblet cells. The entire conjunctiva was processed for light microscopy either on semithin sections stained with toluidine blue-pironine or on traditional sections stained with Alcian blue pH 2.5-PAS. Results: It was possible to demonstrate that goblet cells underwent remarkable changes in their secretory activity. At 12 h after hatching, isolated Alcian blue-positive cells were present in the fornix. At 24 h after hatching, cells positive for both Alcian blue and PAS were scattered among epithelial cells. Two days after hatching, cells which reacted positively only to PAS were also present. Conclusion: It is suggested that the differentiation of conjunctival goblet cells occurs first in the fornix, probably due to the particular vascular environment of this region, and then spreads all over the conjunctiva.     

An immunocytochemical study of isolated human retinal Müller tells in culture

Background: We report a modified method for the isolation and propagation of adult human Müller cells in culture. Methods: The retina of postmortem human eyes was mechanically dissociated and cultured. Using immunocytochemical techniques, these cells were stained with monoclonal antibodies specific for Müller cells, glial fibrillary acidic protein (GFAP), vimentin, glutamine synthetase (GS) and keratin. Transmission electron microscopy (TEM) was also performed. Results: The dissociated and cultured cells expressed vimentin and GS, but not GFAP. At least 85% of these cells stained with a Müller tell-specific monoclonal antibody. Using TEM, flat cells containing 13-nm intermediate filaments and glycogen were identified. Conclusion: Human retinal Müller cells tan be isolated and propagated in culture. Purified cell cultures are required for controlled studies of the normal physiology and pathologie responses of Müller cells.     

Effect of histamine and substance P on the rabbit and human iris sphincter muscle

Background and methods: In an attempt to clarify the functional action of histamine and substance P on atropine-resistant miosis, we isolated rabbit and human iris sphincter muscles and investigated their mechanical properties using the isometric tension recording method. Results: Substance P dose-dependently contracted the rabbit iris sphincter, but had no effect on the human iris sphincter. In the rabbit iris sphincter, histamine reduced the amplitude of twitch contraction evoked by field stimulation but had no effect on carbachol-induced contraction. Thioperamide, but not mepyramine or cimetidine, partially antagonized the histamine-induced reduction in the amplitude of twitch contractions. In the human iris sphincter, on the other hand, histamine dose-dependently provoked contraction and the amplitude of histamine-induced contraction was affected neither by atropine nor by indomethacin. Conclusions: These results provide evidence that histamine has strong contractile effect on the human iris sphincter muscle; the rabbit iris sphincter muscle, however, apparently lacks functional histamine receptors. In rabbits, exogenously applied histamine only activates H₃ receptors located on the cholinergic nerve terminal, hence the excitatory neuro-effector transmission is suppressed. Thus, histamine may have an important roles in atropine-resistant miosis in humans, but not in rabbits.