The site of breakdown of the blood-aqueous barrier under the influence of vaso-dilator drugs

Pharmacokinetic aspects of retinal drug delivery

Drug delivery to the posterior eye segment is an important challenge in ophthalmology, because many diseases affect the retina and choroid leading to impaired vision or blindness. Currently, intravitreal injections are the method of choice to administer drugs to the retina, but this approach is applicable only in selected cases (e.g. anti-VEGF antibodies and soluble receptors). There are two basic approaches that can be adopted to improve retinal drug delivery: prolonged and/or retina targeted delivery of intravitreal drugs and use of other routes of drug administration, such as periocular, suprachoroidal, sub-retinal, systemic, or topical. Properties of the administration route, drug and delivery system determine the efficacy and safety of these approaches. Pharmacokinetic and pharmacodynamic factors determine the required dosing rates and doses that are needed for drug action. In addition, tolerability factors limit the use of many materials in ocular drug delivery. This review article provides a critical discussion of retinal drug delivery, particularly from the pharmacokinetic point of view. This article does not include an extensive review of drug delivery technologies, because they have already been reviewed several times recently. Instead, we aim to provide a systematic and quantitative view on the pharmacokinetic factors in drug delivery to the posterior eye segment. This review is based on the literature and unpublished data from the authors' laboratory.

Sildenafil accelerates anterior chamber refilling after paracentesis in sheep and rabbits

PURPOSE

Sildenafil increases ocular blood flow. Thus, the authors investigated if it also increases anterior chamber (AC) refilling after paracentesis.

METHODS

Corriedale sheep and albino rabbits were used as animal models. Intraocular pressure (IOP) was measured, paracentesis performed on one eye, and AC refilling followed by observation using oblique illumination. IOP measurements continued as the AC formed. After IOP stabilization, sildenafil (100 mg) was orally administered. Forty to 60 minutes later, AH was withdrawn from the contralateral eye. The point at which IOP recovered was used to determine refilling time. Paracentesis volumes were either 60, 120, or 300 μL in sheep, and 50 or 100 μL in rabbits.

RESULTS

IOP recovered in approximately 49, 56, and 50 minutes after the 60, 120, and 300 μL withdrawals in sheep. The refilling times of the contralateral eye after sildenafil ingestion were approximately 19, 26, and 37 minutes for the respective AH withdrawals. With rabbits, IOP recovered in approximately 13 minutes after the 50 and 100 μL AH withdrawals. After sildenafil, the IOP recovery times of the fellow eye were approximately 6 minutes. AH refilling rates were estimated by dividing the paracentesis volume by IOP recovery time. After sildenafil, such rates were larger than the AH formation rate attributed to secretion by the ciliary epithelium.

CONCLUSIONS

Sildenafil accelerates the rate of AC refilling and might have beneficial utility as an agent enhancing fluid entry into the AC of patients who experienced AH loss during eye surgery, as well as in some cases of ocular hypotony.

Nonhypotensive autonomic agents in veterinary ophthalmology

The parasympathetic and sympathetic divisions of the autonomic nervous system are involved in homeostatic control of a wide variety of ocular functions, including accommodation, pupillomotor control, lacrimation, eyelid position, and aqueous humor production. Familiarity with the functional anatomy of the autonomic nervous system is paramount to the understanding and application of the large number of autonomic drugs used in veterinary ophthalmology. The cholinergic and adrenergic agents discussed in this article are commonly employed to facilitate routine ophthalmic examination, in the diagnosis of autonomic dysfunction, and in the treatment of a variety of ocular diseases.

Laser flaremetric evaluation of experimentally induced blood-aqueous barrier disruption in cats

OBJECTIVES

To determine whether aqueous humor flare, measured by use of laser flaremetry, was proportional to aqueous humor protein concentration and to use laser flaremetry to evaluate disruption of the blood-aqueous barrier (BAB) in cats.

ANIMALS

30 healthy adult cats.

PROCEDURE

Laser flaremetry values for all eyes were compared with aqueous humor protein concentrations determined by use of a Coomassie blue microprotein assay. Laser flaremetry was then performed on both eyes before (0 hours) and 4, 8, and 26 hours after initiation of topical application of 2% pilocarpine (q 8 h) to 1 eye of 9 cats or paracentesis of the anterior chamber of 1 eye of 8 cats. Intraocular pressure and pupil size were also determined. Aqueous humor protein concentration was extrapolated from flare values by use of linear regression.

RESULTS

There was a linear relationship between flare values and aqueous humor protein concentrations. Topical application of 2% pilocarpine and paracentesis of the anterior chamber caused a breakdown of the BAB that was detected by use of laser flaremetry. The highest mean flare readings after application of pilocarpine or paracentesis were 24.4 and 132.8 pc/ms, respectively, which corresponded to aqueous humor protein concentrations of 85.5 and 434.9 mg/dl, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Paracentesis of the anterior chamber resulted in a more severe breakdown of the BAB in cats than topical application of 2% pilocarpine. Laser flaremetry may be a useful clinical method to detect increases in aqueous flare and, hence, disruptions of the BAB in cats.

Iris fluorescein angiography in clinical practice

Iris fluorescein angiography is a valuable technique for elucidating a wide range of eye pathologies, among them developmental anomalies, degenerative disease, retinal vascular occlusions, diabetic microangiopathy, uveitis of various etiologies, glaucoma, tumor, and surgical or accidental trauma. In fact, IFA is more sensitive than biomicroscopy in the evaluation of iris abnormalities. Moreover, it provides indirect but reliable information on the retinal circulation when this cannot be examined directly. Some of the most important clinical applications involve the early detection of iris new vessels, cataract management in diabetic patients, and monitoring of iris tumors. Equipment, procedures, and fluorescein angiography patterns in normal and disease states are described and extensively illustrated. More widespread use of this valuable technique is encouraged.

The expression of functionally-coupled B2-bradykinin receptors in human corneal epithelial cells and their pharmacological characterization with agonists and antagonists

1. Bradykinin (BK) and Lys-BK are peptides which are released at high nanomolar concentrations into the tear-film of ocular allergic patients. We hypothesized that these peptides may activate specific receptors on the ocular surface, especially the corneal epithelium (CE) and thus the CE cells may represent a potential target tissue for these kinins. 2. The purpose of the present studies, therefore, was to determine the presence of and the pharmacological characteristics of bradykinin receptors on normal cultured primary and SV40 virus-transformed human corneal epithelial (CEPI) cells by use of the accumulation of [3H]-inositol phosphates ([3H]-IPs) as a bioassay. 3. Bradykinin (BK) induced a maximal 1.95 +/- 0.24 fold (n = 17) and 2.51 +/- 0.29 fold (n = 26) stimulation of [3H]-IPs accumulation in normal, primary (P-CEPI) and SV40-immortalized (CEPI-17-CL4) cells, respectively. This contrasted with a maximal 3.2-4.5 fold and 2.0-2.9 fold stimulation by histamine (100 microM) and platelet activating factor (100 nM) in both cell-types, respectively. 4. The molar potencies of BK and some of its analogues in the CEPI-17-CL4 cells were as follows: BK (EC50 = 3.26 +/- 0.61 nM, n = 18), Lys-BK (EC50 = 0.95 +/- 0.16 nM, n = 5), Met-Lys-BK (EC50 = 2.3 +/- 0.42 nM, n = 5), Ile-Ser-BK (EC50 = 5.19 +/- 1.23 nM, n = 6), Ala3-Lys-BK (EC50 = 12.7 +/- 2.08 nM, n = 3), Tyr8-BK (EC50 = 19.3 +/- 0.77 nM, n = 3), Tyr5-BK (EC50 = 467 +/- 53 nM, n = 4) and des-Arg9-BK (EC50 = 14.1 +/- 2.7 microM, n = 4). The potencies of BK-related peptides in normal, P-CEPI cells were similar to those found in transformed cells, thus: BK, EC50 = 2.02 +/- 0.69 nM (n = 7), Tyr8-BK, EC50 = 14.6 +/- 2.7 nM (n = 3), Tyr5 = BK, EC50 = 310 +/- 70 nM (n = 4) and des-Arg9-BK, EC50 = 12.3 +/- 3.8 microM (n = 3). 5. The bradykinin-induced responses were competitively antagonized by the B2-receptor selective BK antagonists, Hoe-140 (D-Arg-[Hyp3, Thi5, D-Tic7, Oic8]BK; Icatibant; molar antagonist potency = 2.9 nM; pA2 = 8.54 +/- 0.06, n = 4; and slope = 1.04 +/- 0.08) and D-Arg0[Hyp3,Thi5,8, DPhe7]-BK (KB = 371 nM; pKB = 6.43 +/- 0.08, n = 4) in CEPI-17-CL4 cells. The antagonist potency of Hoe-140 against BK in normal, P-CEPI cells was 8.4 +/- 1.8 nM (pKi = 8.11 +/- 0.12, n = 4), this being similar to the potency observed in the immortalized cells. 6. This rank order of potency of agonist BK-related peptides, coupled with the antagonism of the BK-induced [3H]-IPs by the specific B2-receptor antagonists, strongly suggests that a B2-receptor subtype is involved in mediating functional phosphoinositide (PI) responses in the CEPI-17-CL4 and P-CEPI cells. 7. In conclusion, these data indicate that the P-CEPI and CEPI-17-CL4 cells express BK receptors of the B2-subtype coupled to the PI turnover signal transduction pathway. The CEPI-17-CL4 cells represent a good in vitro model of the human corneal epithelium in which to study further the role of BK receptors in its physiology and pathology, such as in allergic/inflammatory conditions, potential wound healing and other functions of the cornea.

The effect of epinephrine on ciliary process vasculature and IOP studied by intraocular microendoscopy in the albino rabbit

The effect of epinephrine on intraocular pressure (IOP) and the hemodynamics of the ciliary process vasculature in albino rabbits was studied by intraocular microendoscopy. Intraarterial application of epinephrine (15, 50, 250 ng/kg bw) lead to an immediate vasoconstriction and a reduction in blood flow velocity (BFV) in the iridial and major ciliary processes lasting from 30 to 120 sec. This anemic phase was followed by a hyperemic phase of about 60 to 240 sec. showing a vasodilation up to 150% of the initial diameters and an increase in BFV. The hyperemic phase can be prevented by pretreatment with indomethacin. Simultaneously measured IOP decreases in the anemic and increases in the hyperemic phase parallel with the changes in vascular diameter. After topical administration of epinephrine (25-50 micrograms/kg bw) a marked vasoconstriction followed by a vasodilatory phase was similarly found. However, the reactive changes of the ciliary process vasculature lasted considerably longer. The anemic phase lasted 15 minutes, the hyperemic phase 40 to 60 min. Again, this hyperemia can be prevented by indomethacin-pretreatment. In the iridial processes the anemic phase persisted till 70 minutes. No hyperemia and no substantial influence of indomethacin-pretreatment was found in this territory. In the anemic phase the IOP decreased in average from 20 mmHg to 15 mmHg. However, in contrast to the reactive changes of the IOP after intraarterial epinephrine application, the IOP did not increase again in the hyperemic phase, but decreased further to about 12 mmHg. After pretreatment with indomethacin the IOP remained at the level of 15 mmHg. The short-term IOP-changes after i.a. application of epinephrine, mirror the vasoconstrictory and vasodilatory reactions in the ciliary processes and might be due to volume changes in the eye (plethysmographic effect). However, the long lasting IOP reduction after topical epinephrine in the hyperemic phase can not be due to vascular reactions in the ciliary processes. There must be other factors responsible for the long lasting pressure reducing effect of epinephrine.

Histamine H1 receptor occupancy triggers inositol phosphates and intracellular calcium mobilization in human non-pigmented ciliary epithelial cells

Agonists and antagonists of histamine were used to characterize the stimulation of inositol phosphates formation and elevation of intracellular Ca2+ by histamine in cultured non-pigmented epithelial (NPE) cells from human ciliary body. Agonists specific for the H1 histamine receptor subtype were 20- to 200-fold more potent than the H2-specific agonists tested, and 5-16% as potent as histamine in inositol phosphates stimulation. An H1 antagonist was 10,000-fold more potent than an H2 antagonist in blocking histamine stimulation of inositol phosphates. H1 agonists also mimicked and H1 antagonists inhibited the elevation of intracellular Ca2+ by histamine. The first phase of the Ca2+ response to histamine was largely independent of extracellular Ca2+ while the second phase required extracellular Ca2+. Dose-response curves for histamine elevation of intracellular Ca2+ (EC50 = 10 microM, maximum at 100 microM) and inositol phosphates (EC50 = 2 microM, maximum at 100 microM) were similar. These data support the characterization of the NPE histamine receptor as an H1 receptor linked to elevation of inositol phosphates and intracellular Ca2+.

Review: mediation of the ocular response to injury

The structure of the anterior segment of the eye provide aqueous humour for metabolic traffic, regulation of intraocular pressure and the maintenance of a functional permeability barrier to separate internal compartments from general systemic influences. Irritative and injurous insults to the eye elicit an acute defensive miotic and vascular response which upsets the aqueous dynamics and provokes the influx of blood plasma proteins into the aqueous chambers. These events are initiated by antidromic activation of sensory elements within the anterior segment, releasing substance P and calcitonin gene-related peptide (CGRP) which, in lower mammals at least, stimulate respectively the miotic and vascular reactions. Considerable species differences can be found in the responsiveness of the eye to injury and in the effects of exogenous CGRP and substance P.

Recent experimental studies on the blood-aqueous barrier: the anatomical basis of the response to injury

In the mammalian eye the tissue layers which effectively separate the neural retina and the transparent refractive media from the circulating blood are generally known as the blood-retinal barrier (BRB) and blood aqueous barrier (BAB) respectively. These have been delineated by morphologically identifying the ultrastructural features which impede the passage of molecules from the blood to the working parts of the eye; physiologically, there is a certain amount of overlap in their function, as there is free diffusion of solutes between the aqueous humour, the vitreous humour and the interstitial tissue of the neural retina. Notwithstanding, it is convenient and appropriate to maintain the distinction as the more recent work has focused on the fine structural aspects of these barrier systems. The abundant literature which describes the work leading to the identification and characterisation of these barriers has been reviewed in depth with great authority by several eminent researchers, notably the late Giuseppina Raviola, and Jose Cunha-Vaz. Certain pathological conditions, trauma--including surgical manipulation, and chemical irritation of the eye can cause significant disruption of these barriers with important clinical consequences. In our laboratory interest has centred around vascular and epithelial changes which occur when the eye is injured and which can lead to a breakdown of the BAB. More recently attention has been drawn to functional and behavioural differences between species, particularly with regard to the relative stability of the barrier.

Enzymatic activities in the iris-ciliary body of the rabbit eye during experimentally induced acute ocular inflammation

Intravitreal injection of 5 micrograms of Shigella endotoxin, in the rabbit eye, induced an acute inflammatory response which was characterised by conjunctival hyperaemia, limbal and ciliary vascular injection, iritis, aqueous flare, miosis and reduction in intraocular pressure. Iris-ciliary body tissues, from normal and inflamed eyes, were fractionated into subcellular enriched fractions and the activities and distribution of selected enzymes were estimated. Alkaline phosphatase, a plasma membrane-bound enzyme, showed an increase in activity, whereas succinate dehydrogenase and Mn-Superoxide dismutase, both mitochondrial-bound enzymes, exhibited decreased activities. Lysosomal acid phosphatase displayed an increase in free activity and retention of latent activity inside the organelle. No alteration in free activity was shown by acid cathepsin. The cholinesterases did not exhibit any changes in activities nor did the cytosolic enzymes Cu/Zn-superoxide dismutase and lactate dehydrogenase. The decrease activity of the respiratory mitochondrial enzyme succinate dehydrogenase may contribute to the reduction in intraocular pressure, and the ability of the lysosomal organelles to retain their hydrolytic enzymes, ensures recovery of the cell from acute inflammatory attack.

Prostaglandin involvement in the responses of the rabbit eye to water-soluble marihuana-derived material

Both anticoagulants (heparin and streptokinase) and non-steroidal anti-inflammatory compounds (aspirin and indomethacin) were used against a water-soluble derivative of marihuana, MDM. While the anticoagulants had no effect on the ocular effects of MDM, both aspirin and indomethacin altered the time course and effected the MDM-induced reduction of intraocular pressure. The usual initial hypertensive effect of intravenous MDM was eliminated and the later intraocular pressure fall occurred earlier as well as being inhibited by about 35 to 50%. Assay for prostaglandins revealed that intravenous MDM (3.86 micrograms) caused a marked rise in PGE2 concentration of the aqueous humor and iris-ciliary body during the first hour or two after administration of MDM, but normal values occurred at 4, 6, and 8 hours when the intraocular pressure is reduced by up to 60%. Following intravitreal MDM (0.002 microgram), however, the PGE2 levels remained unchanged over 24 hours, despite the induction of a fall in intraocular pressure between 14 and 18 hours which lasts for many hours. Prostaglandin appears to be involved in the hypertensive phase of intraocular pressure change after intravenous MDM injection; and, while the fall in intraocular pressure may contain a component partially mediated by prostaglandins, there is no evidence that intravitreal MDM induces any effect on prostaglandin levels. The involvement of prostaglandins, therefore, in the mediation of MDM-induced ocular hypotensive effects is apparently small.

Calcitonin gene-related polypeptide as a mediator of the neurogenic ocular injury response

Calcitonin gene-related polypeptide (CGRP) has been localised immunochemically within the rat and guinea pig anterior uvea to nerve fibres of trigeminal origin. As with substance P (1-3) the level of CGRP in the iris-ciliary body is depleted after thermal damage to the Gasserian ganglion and elevated in chronically sympathectically denervated eyes. Unlike substance P, a potent pupillary constrictor (4,5), CGRP has no notable miotic action, but does, however, cause an elevation of the intraocular pressure (IOP) accompanied by disruption of the blood-aqueous barrier. It is proposed that the diverse actions of these two sensory neuropeptides conjointly mediate the antidromic ocular injury response.

Species differences in the responses of the eye to irritation and trauma: a hypothesis of divergence in ocular defense mechanisms, and the choice of experimental animals for eye research

Information published during the past century, especially the last decade, has identified pronounced species differences, not only in the morphological organization of ocular structures, but also in the functional responses of the eyes of different mammals to experimental and surgical procedures, as well as to drugs and autacoids. For the most part, these differences have been regarded as peculiarities or weakness rather than as fundamental evolutionary adaptations optimally suited to the environment and behavior of each species. This paper proposes a working hypothesis of evolutionary divergence in ocular defense mechanisms, based on some of the known morphological and functional differences among mammals, and discusses the implications of these differences with regard to the choice of appropriate animals for use as models in different areas of ophthalmic research.

Iris delay, a neglected factor in aqueous humour dynamics. A study in the cynomolgus monkey (Macaca fascicularis)

A method is described, intended for measuring the rate--over a short period of time--of aqueous humour outflow in non-anaesthetized primates. [125I] and [131I]-o-iodohippurates were infused s.c. at an interval into the conscious Cynomolgus monkey so as to give two time-displaced plasma curves. The isotope concentrations were measured in plasma by frequent sampling over the first 1-2 hr and in aqueous humour at the end of the experiment under brief anaesthesia. The coefficient of diffusion (kdiff) between plasma and aqueous, and the coefficient of loss (kout) out of the anterior chamber were calculated by making the usual assumptions about aqueous dynamics. Topical treatment of one eye with timolol consistently reduced kout, as expected, but many control values for kout were lower than expected or (impossibly) negative. Examination of the data reveals a likely source of error emanating from the commonly made assumption that diffusion of marker substance (in our case iodohippurate) from plasma to aqueous humour is proportional to the instantaneous concentration difference between plasma and aqueous. In fact, there may be a delay of several minutes during which the substance negotiates the stroma and anterior boundary layer of the iris. Evidence is presented that variation between and within individual monkeys may stem from the highly variable anatomy of the iris and from changes in sympathetic tone operating on the blood vessels and perhaps the anterior boundary layer.

Effect of serotonin, histamine and bradykinin on outflow facility following ciliary muscle retrodisplacement in the cynomolgus monkey

Cynomolgus monkeys with surgically untouched, aniridic, or ciliary muscle disinserted eyes underwent two-level constant pressure perfusion of the anterior chamber to determine total outflow facility before and after intracameral infusion of bradykinin triacetate (0.1, 1, 10 micrograms), histamine dihydrochloride (1, 10, 100 micrograms), or serotonin creatinine sulfate (1, 10, 100 micrograms). Bradykinin had no effect on facility in non-disinserted eyes, but significantly decreased facility in disinserted eyes at all three dosages. Histamine tended to decrease facility in non-disinserted and disinserted eyes at all three dosages, although the decrease was not consistently significant statistically. Serotonin tended to decrease facility in non-disinserted eyes at dosages of 10 and 100 micrograms, although the effects were not statistically significant; 1 microgram had no effect. In disinserted eyes, 1 microgram significantly decreased facility, while the higher dosages had no (10 micrograms) or less (100 micrograms) effect. No simple mathematical/physiological description of the drug-induced facility decreases could be formulated. The facility decreases could be due to drug effects directly on the outflow pathways or secondary to blood-aqueous barrier breakdown.

The effects of sensory denervation on the responses of the rabbit eye to prostaglandin E1, bradykinin and substance P

1 Six to eight days after diathermic destruction of the fifth cranial nerve in the rabbit, the ocular hypertensive and miotic responses to intracameral administration of capsaicin, bradykinin, and prostaglandin E1 were greatly reduced or completely abolished. The response to substance P was not abolished. 2 A response could still be obtained to chemical irritants 36 h after coagulation of the nerve and it is deduced that manifestation of the response is dependent upon functional sensory nerve terminals, and is independent of central connections. 3 It is suggested that prostaglandin E1 and bradykinin act directly upon the sensory nerve endings and that propagation of the response is augmented by axon reflex. 4 In view of the ability of substance P to induce miosis in the denervated eyes, it is presumed that its actions are not mediated via sensory nerves. 5 It is considered possible that the mediator(s) released from sensory nerve endings after chemical irritation or antidromic stimulation may act in the same way as substance P with regard to the miotic effect. 6 Synthetic substance P will only produce ocular hypertension in doses which induce a maximal miotic response. This may either be a question of access or a partial resemblance to the endogenous mediator.

Increased vascular permeability in the rabbit iris induced by prostaglandin E1. An electron microscopic study using lanthanum as a tracer in vivo

Prostaglandin E1 (PGE1) (25 micrograms) was applied topically to the eyes of albino rabbits, and lanthanum was used as an electron microscopic tracer to study possible vascular permeability changes in the iris of these eyes. Lanthanum was injected directly into the blood circulation 15 min after application of the PG, and the eyes were enucleated 30 to 60 min later. PGE1 was found to induce increased vascular permeability. Heavy deposits of the tracer were found in the walls of iridial venules, especially between the endothelium and its basement lamina. The highest concentration of lanthanum was found in close relation to the interendothelial clefts. Occasionally, gaps between neighbouring endothelial cells were observed. In control eyes, not treated with PG, no extravasation of the tracer was found. The study indicates that the iridial blood vessels contribute to the production of the plasmoid aqueous humour induced by PGE1 in the rabbit eye.

Cyclocryokoagulation. Sequelae of induced alterations and effect of a prostaglandin-inhibitor on the breakdown of the blood-aqueous barrier

Histopathological changes following cyclocryokoagulation in a group of rabbits pretreated with acetylsalicyclic acid (ASA) were studied and compared with similar changes in a control group. As observed clinically, macroscopical and histological differences were only noted in the early phase after the procedure. In this phase, all operated eyes revealed an immense vascular response with edema, exudation and extravasation of blood cells. The treated animals, however, showed some reduction of tissue edema than the animals not treated. At a later stage there is an necrosis of the stroma with reduction of small vessels and tissue cells. The reparative process, tending to restore both epithelial and stromal lesions, begins already in the 3rd. week after cyclocryokoagulation. Neither the epithelial nor the stromal alterations differed significantly in any of the operated eyes at this stage. This indicates that ASA mainly affects an obviously prostaglandin-mediated acute vascular response. The disruption of the epithelial barrier may be aggravated by the subsequent stromal alterations, but does not seem to be influenced by ASA.

Prostaglandin mediated inflammatory changes induced by alpha-adrenoceptor stimulation in the sympathectomised rabbit eye

Topical application of 50 microliter of 0.1% noradrenaline or 10% phenylepherine to either a surgically sympathectomised or a guanethidine treated rabbit eye induced a transient increase in intraocular pressure (IOP), between 90 and 150 min, followed by a fall to hypotensive levels. These changes were accompanied by the development of conjunctival and anterior uveal hyperemia and an aqueous flare. Fluorescein angiography and microscopic examination after intravenously injecting colloidal carbon showed that the major site of disruption of the blood-aqueous barrier occurs in the ciliary processes. Slightly elevated levels of protein and prostaglandin-like activity (PG) were detected in aqueous samples withdrawn 3 or 5 h after application of either adrenergic agent. Prior treatment with indomethacin or phentolamine prevented the ocular hypertensive phase as well as the inflammatory changes, suggesting that the inflammatory rebound is mediated by PG and that PG release under such circumstances is closely linked to alpha-adrenoceptor activation. By contrast, the rise in IOP occurring after each of several successive daily applications of an alpha-agonist to a normal rabbit eye was not accompanied by hyperemia or an aqueous flare and was not inhibited by indomethacin pretreatment.

Effect of prostaglandin E2 on the permeability of the iris vessels to horseradish peroxidase in the rabbit

Prostaglandin E2 (0.5 microgram in 10 microliter 10% ethanol) was introduced into the anterior chambers of rabbit eyes. Using the horseradish peroxidase method, it was shown under the electron microscope that the endothelial barrier of the iris vessels broke down. The peroxidase penetrated as far as the basis of the posterior epithelial cells, however, without entering their lateral intercellular spaces. The question of whether the effect of prostaglandins on the barrier was a direct effect or at least a partially indirect one, i.e., a haemodynamic action, is discussed.

Response of the human eye to laser irradiation of the iris

Irradiation of the iris of glaucoma patients with either a ruby or an argon laser caused no obvious disruption of the blood-aqueous barrier (as visualised by fluorescein angiography), in contrast to a sudden and pronounced effect in the rabbit eye. Only a low and variable increase in intraocular pressure occurred in the human subjects after the laser irradiation in comparison with a consistently high rise of ocular tension in the rabbit. This investigation indicates that the human eye is much less responsive to injury.

The effect of prostaglandin on iridial blood vessel permeability

Light and electron microscopy is used to examine the effect of exogenous PGE1 on the permeability and reactivity of rat iridial blood vessels. Results show that topical PGE1 causes an increase in the permeability of iridial vessles to carbon particles (200 A diameter). The technique of carbon labelling is used to quantitate increases in permeability caused by varying concentrations of PGE1 (0.001-1.0 mg/ml). Regression analysis shows that there is a linear relationship (P less than 0.02) between carbon labelling and PGE1 concentration over the range of concentrations tested. In other experiments rats were treated with the systemic histamine liberator Compound 48/80, or with topical applications of histamine diphosphate in order to examine the effects of exogenous and endogenous histamine upon iridial blood vessel permeability. These procedures produce only minimal labelling of iridial vessels. It therefore seems likely that PGE1 has a direct effect on iridial vessels and does not act indirectly by bringing about the liberation of endogenous histamine.

Electron microscopic studies on the blood-aqueous barrier of prostaglandin-treated rabbit eyes. I. Iridial and ciliary processes

Prostaglandins E1 and E2 were applied topically to rabbit eyes. Structures related to the blood-aqueous barrier in the iridial and ciliary processes, as well as the permeability of the ciliary epithelium to the protein tracer horseradish peroxidase, were studied with the electron microscope. Marked morphological changes, including dilations of the intercellular spaces and separation of the two epithelial cell layers, were found in the epithelium of the iridial processes. Only minor structural changes were found in the epithelium of the ciliary processes. Leakage of peroxidase through the intercellular spaces of the epithelium was demonstrated in the iridial processes and in the anterior parts of the ciliary processes. In the ciliary vessels of the same regions, opening of interendothelial gaps, platelet aggregations, microthrombi, and haemorrhages were found. In a previous in vitro study on the effects of prostaglandins on the movement of peroxidase in the ciliary epithelium, no structural changes of the epithelium were found, and the epithelial diffusion barrier to peroxidase was found to be intact. It is assumed that the breakdown of this barrier in vivo is secondary to vascular changes.

Electron microscopic studies on the blood-aqueous barrier of prostaglandin-treated rabbit eyes. II. Iris

The distribution of intravenously injected horseradish peroxidase in the iris of prostaglandin (E1 and E2)-treated rabbit eyes, has been studied with the electron microscope. Peroxidase was demonstrated in the stroma of all parts of the iris. The tracer was found throughout interendothelial clefts of iris vessels, indicating that these vessels had become permeable to peroxidase. The distribution of peroxidase in the iris epithelium indicates that the posteroir epithelial cells are girdled by zonulae occludentes.

In vitro studies on peroxidase movement in the epithelium of prostaglandin-treated rabbit ciliary bodies

The movement of horseradish peroxidase (HRP) in the epithelium of isolated rabbit iris/ciliary body preparations, has been studied with the electron microscope. HRP was applied at the stromal side of the epithelium, and was left for 60 and 120 min. The distribution pattern of HRP found in the epithelium of the iridial and ciliary processes is consistent with in vivo studies, i.e. the progression of HRP is blocked at the site of the zonula occludens of the superficial epithelium. The HRP distribution pattern found in the iris epithelium indicates that also the superficial epithelial cells of this epithelium are girdled by zonulae occludentes. Specimens treated with prostaglandins E1, E2, and F2alpha, showed no change in the epithelial distribution pattern of HRP, and the occluding zonules were found to be intact.

Provoked iris ischaemia in the rabbit. II. Histochemical localization of sodium fluorescein

The distribution of injected sodium fluorescein in the iris and ciliary body of albino rabbits was studied in normal animals and in animals subjected to different surgical procedures. The study was performed with two techniques: whole-mount preparations and the paraffin-section method on freeze-dried eyes. Fluorescein distribution showed marked changes after surgical procedures as a result of reduced blood perfusion. In normal iris vessels no penetration of fluorescein was observed. The permeability barrier was impaired if there were ischaemic conditions in the anterior segment. The results were compared with clinical and iris angiographic studies.