Aqueous Humor Formation Research Articles

Effects of endothelin A and B receptors on aqueous humor dynamics in the rabbit eye.

This study attempted to clarify the role of endothelin A and B (ETA and ETB) receptors in modulating aqueous humor dynamics in the rabbit eye. Intraocular pressure (IOP), aqueous flow, total outflow facility, and uveoscleral outflow were measured before, and 24 hours after, intravitreal injection (20 microliters) of a selective ETB agonist, sarafotoxin S6c (SRTX S6c, 10(-5) M), into one eye and its vehicle into the contralateral eye. The measurements were also performed before and after injection of a selective ETA antagonist, 97-139 (10(-2) M) + ET-1 (10(-5) M) into one eye, and vehicle + ET-1 (10(-5) M) into the contralateral eye. In the SRTX S6c-treated eye, total outflow facility increased significantly by 106% compared with that in the vehicle-treated eye 24 hours after injection; aqueous flow and uveoscleral outflow failed to change significantly. The ETA antagonist 97-139 inhibited the decrease in aqueous flow partially, but significantly, but failed to inhibit the increase in total outflow facility caused by ET-1. Therefore, both ETA and ETB play a significant role in modulating aqueous humor dynamics in the rabbit eye. ETA can modulate aqueous humor production, at least in part. ETB can modulate total outflow facility, and the possibility for ETB in modulating aqueous humor formation cannot be ruled out.

Effects of Topical K-Strophanthin on Aqueous Humor and Corneal Dynamics

Sodium-potassium-activated adenosine triphosphatase (Na/K ATPase) is present in the ciliary epithelium, where it serves a regulatory role in fluid transport and the production of aqueous humor. The present study investigates the ability of topical k-strophanthin, a water-soluble mixture of glycosides, to lower intraocular pressure. K-strophanthin was topically administered to one eye and vehicle to the fellow control eye of rabbits, cats, and human volunteers. Single drop 0.01 M k-strophanthin delivery lowered pressure in rabbits and cats to a maximal decrease of 5-6 mm Hg 3-4 h after drug administration. Higher drug concentrations irritated the eye, whereas lower concentrations had no effect. Pressure reduction in rabbits was associated with decreased aqueous humor formation, and no change in outflow facility and episcleral venous pressure. Drug delivery in humans had a minimal intraocular pressure effect. Slit-lamp and histologic examination of rabbit eyes after chronic drug administration was normal. However, single drop administration of 0.01 M drug was found to significantly increase corneal thickness, the aqueous to cornea fluorescein transfer coefficient, and the corneal endothelial permeability coefficient. Topical k-strophanthin lowers pressure via reduction of aqueous production and most probably by inhibition of the enzyme Na/K ATPase in the ciliary epithelium. The drug's effect on corneal function probably relates to a similar block of this enzyme in the epithelial layers of the cornea, which limits the usefulness of k-strophanthin and other previously studied Na/K ATPase inhibitors as an ocular hypotensive agent.

Molecular Cloning of the Human Volume-Sensitive Chloride Conductance Regulatory Protein, pI Cln, from Ocular Ciliary Epithelium

Chloride channels in the ocular ciliary epithelium are believed to play a key role in aqueous humor formation. We isolated a cDNA clone from a λUni-ZAP cDNA library of human nonpigmented ciliary epithelial (NPE) cells encoding the swelling-induced chloride channel/channel regulator PI Cln. The human clone contains an open reading frame of 237 amino acids (M r 26,293). The deduced human amino-acid sequence shows 90.2% and 92.7% identity with counterparts isolated from rat kidney and the canine kidney epithelial cell line MDCK. Human NPE cell lines exhibited significant levels of pI Cln transcripts. Complementary perforated-patch, whole-cell patch clamping demonstrated that swelling activates Cl − channels of the NPE cells, as suggested by ruptured-patch measurements. The results document the molecular isolation and identification of a human cDNA clone of a Cl − conductance regulator from ocular cells displaying volume-activated Cl −channels.

Mitomycin C suppresses aqueous human flow in cynomolgus monkeys.

To determine whether mitomycin C suppresses aqueous humor formation in cynomolgus monkeys. Three monkeys received subconjunctival injections (50 microL) in four quadrants bilaterally, one eye receiving mitomycin C (0.5 mg/mL) and the other receiving distilled water. Seven monkeys underwent 360 degrees conjunctival peritomy bilaterally and episcleral application of mitomycin C-soaked (0.5 mg/mL) cellulose sponges for 5 minutes in all four quadrants unilaterally. Aqueous humor flow was measured fluorophotometrically 1 and 3 days, and 1, 2, and 4 weeks after subconjunctival injection; and 3 days and 1, 2, 3, and 4 weeks after episcleral application. There was no change in aqueous flow in either eye and no difference between eyes following subconjunctival injection. Aqueous flow was reduced by 8% +/- 7% (mean +/- SEM), 20% +/- 3% (P < .01), 9% +/- 10%, and 0% +/- 4% compared with contralateral controls 1, 2, 3, and 4 weeks, respectively, after episcleral application of mitomycin C. Episcleral application of mitomycin C can produce at least a modest short-term reduction of aqueous humor flow in primates. Reduced aqueous flow might contribute to early postoperative hypotony following trabeculectomy with mitomycin C.

Effects of timolol, terbutaline and forskolin on IOP, aqueous humour formation and ciliary cyclic AMP levels in the bovine eye.

We have studied the effects of terbutaline, timolol, forskolin and 8-bromo cyclic AMP on aqueous humour formation, intraocular pressure and on ciliary epithelial cyclic AMP levels, either in presence or in absence of IBMX, using the bovine isolated arterially perfused eye, excised ciliary processes and cultured ciliary epithelium. Both terbutaline, a beta-adrenoceptor agonist, and timolol, a beta-adrenoceptor antagonist, caused significant reduction in aqueous humour formation and intraocular pressure but produced no effect on ciliary epithelial cyclic AMP content in the absence of IBMX. Even a three times higher dose of terbutaline was entirely ineffective in producing any effect on ciliary cyclic AMP in the perfused eye. On the other hand, terbutaline at the IOP-reducing dose, produced a significant increase in cyclic AMP when injected after 30 min perfusion with IBMX. Incubation of excised ciliary processes or cultured ciliary epithelial cells with terbutaline (10(-6) to 10(-4) M) produced concentration-dependent increases in cyclic AMP, in both tissues, even in the absence of IBMX. Forskolin, which stimulates cyclic AMP synthesis without interacting with cell surface receptors, was found to produce highly significant increases in ciliary cyclic AMP content both in presence and in absence of IBMX but had no effect on aqueous humour formation in the isolated eye. IBMX perfused at concentrations of 1 mM or 10 microM had no effect on basal levels of ciliary cyclic AMP but the 1 mM concentration produced a marked and significant reduction in IOP. Direct application of 8-bromo cyclic AMP, a cell permeable analogue, more resistant to hydrolysis by phosphodiesterases, had also no effect on aqueous humour formation in the perfused eye. It is concluded that in the bovine arterially perfused eye, the correlation between the aqueous humour formation rate and ciliary epithelial cyclic AMP content is unclear.

Effects of Cromakalim and Nicorandil on Intraocular Pressure After Topical Administration in Rabbit Eyes

In the study two potassium channel activators, cromakalim and nicorandil, were investigated in rabbits using three different levels of intraocular pressures (IOPs), including alpha-chymotrypsin-induced ocular hypertension, hypotension (IOP recovery method) and normal tension. Cromakalim significantly increased IOP during the initial period in normotensive and hypotensive rabbits, but in the later time, reduced IOP in the alpha-chymotrypsin-induced model. Nicorandil had a similar but less potent effect. In addition, cromakalim also affected ocular blood flows, which had a trend to reduce blood flow about 2-3 hr later, after topical administration of the eyedrop. Two possible mechanisms for increasing IOP effect of potassium channel activators were: (a) enhancing the formation of aqueous humor; (b) relaxing the trabecular meshwork to reduce the outflow of aqueous humor.

Regulation of cyclic AMP production in adult human ciliary processes

Cyclic AMP production in intact ciliary processes from elderly human donors is subject to stimulatory and inhibitory control by various agents. Stimulation of cAMP production is observed with forskolin, vasoactive intestinal peptide, or the beta-adrenergic agonist isoproterenol. Inhibition of forskolin-stimulated cAMP production is observed with endothelin-2 or PAC. The inhibitory effect of PAC is blocked by the specific alpha 2-adrenergic antagonist, yohimbine. Endothelin-2 has no effect on basal cAMP production. These data document the positive and negative regulation of cAMP responses in adult human ciliary processes and support the idea that cAMP is a key intermediate in the regulation of aqueous humor formation.

CDNA from human ocular ciliary epithelium homologous to βig‐h3 is preferentially expressed as an extracellular protein in the corneal epithelium

The non-pigmented ciliary epithelium is largely responsible for the formation of aqueous humor in the mammalian eye. To provide a basis for studies at the molecular level, a directional expression cDNA library was constructed in Uni-ZAP XR vector from poly A+ RNA of the human non-pigmented ciliary epithelial derived ODM-2 cell line. Fifty-three cDNA clones were isolated from the library and characterized by partial sequence analysis. Approximately 49% of the clones exhibited homology with known genes in the GenBank/EMBL databases. The putative identification of these clones may reflect the transcriptional activity of the ODM-2 cells in culture. One of the identified clones, ODM-42-I, was found to be specific and highly expressed in the corneal epithelium. This clone had an exact match with a recently discovered human gene, beta ig-h3 (Skonier et al., 1992, DNA Cell Biol., 11:511-522), which codes a surface recognition protein, inducible by transforming growth factor beta (TGF-beta), and containing a putative binding site (RDG) for integrins. The ODM-42-I cDNA clone displays a distinctive pattern of expression found in the human eye, expressed almost exclusively in the cornea. Further studies, using sera from a synthetic peptide to the carboxy-terminal region of ODM-42-I, reveal that the protein is heterogeneous in charge and is preferentially expressed on the extracellular surface of corneal epithelial cells, and might share immunologic properties with integrins beta 1.

Pharmacological advances in the treatment of glaucoma.

Glaucoma is a potentially blinding disease. The goal of glaucoma therapy is to reduce intraocular pressure to a predetermined target level. There are currently 5 classes of compounds used for the medical management of glaucoma. Four classes that appear promising for the long term management of glaucoma are in different phases of clinical investigation, and include the topically active carbonic anhydrase inhibitors, selective alpha 2-adrenergic agonists, prostaglandins and ethacrynic acid. The topically active carbonic anhydrase inhibitor dorzolamide (MK-507) is effective and well tolerated in clinical trials of up to 1 year's duration. Animal studies have demonstrated that this drug lowers intraocular pressure by reducing aqueous humour formation. The selective alpha 2-adrenergic agonists, brimonidine and apraclonidine, have been shown to be effective in reducing intraocular pressure in the short term. Long term effectiveness of these agents is under investigation. Prostaglandins (PG) of the PGF2-alpha isopropylester series caused marked reductions of intraocular pressure in laboratory and clinical trials. The newest prostaglandin analogue, latanoprost (PhXA41), effectively lowered intraocular pressure and was well tolerated in clinical trials of up to 4 weeks' duration. Prostaglandins reduce intraocular pressure by enhancing uveoscleral outflow. Ethacrynic acid enhanced traditional outflow facility and lowered intraocular pressure when applied topically or intracamerally in laboratory studies and clinical trials. Corneal adverse effects of ethacrynic acid have been noted. Reformulation of ethacrynic acid ointment may resolve this problem. These 4 classes of compounds will enhance our options for the medical management of glaucoma. They may be used instead of or in combination with some of the drugs currently in use, and may be better tolerated.

Role of cyclic AMP-induced Cl conductance in aqueous humour formation by the dog ciliary epithelium.

1. The effects of isoprenaline, a forskolin derivative NKH-477, and dibutyryl cyclic AMP (db cyclic AMP) on the membrane potential, conductance and cell volume of the dog non-pigmented ciliary epithelium (NPE) were investigated by intracellular potential recording, nystatin-perforated patch clamp technique and videomicroscopic cytometry. 2. The resting membrane potential of NPE was about -70 mV in physiological saline and was depolarized by isoprenaline in a dose-dependent manner with an ED50 of about 3 nM. This depolarization was competitively antagonized by the beta-adrenoceptor antagonist, timolol (pA2 = ca. 9) and almost completely blocked by the Cl transport blocker, DIDS. 3. In single dissociated NPE cells, 10 microM isoprenaline induced an inward current and caused a concomitant decrease in cell volume. The reversal potential measurement indicated that this inward current was carried mainly by Cl ion. DIDS (10 microM) abolished both the current and cell volume decrease. 4. NKH-477 (10 microM) or db cyclic AMP (1 mM) also induced an inward current together with a cell volume decrease, the properties of which were similar to those caused by isoprenaline. 5. These results suggest that beta-adrenoceptor stimulation in NPE leads to an increased rate of aqueous humour production by increasing Cl- efflux via an elevation of cyclic AMP and this effect is efficiently blocked by timolol.

Beta-human chorionic gonadotropin, progesterone, and aqueous dynamics during pregnancy.

To determine whether the altered hormonal milieu of pregnancy is associated with changes in the dynamics of aqueous humor formation and drainage. Nineteen women were studied during each trimester of a single pregnancy and post partum. Measures of aqueous dynamics included intraocular pressure, aqueous flow, facility of outflow, aqueous flare, and corneal thickness. Pregnancy was associated with relatively lower intraocular pressure, reduced aqueous flare, increased corneal thickness, and increased aqueous outflow facility. Aqueous flow was unchanged. The progesterone level increased during pregnancy and decreased during the postpartum period. The beta-human chorionic gonadotropin level was highest during the first trimester. The progesterone level, but not the beta-human chorionic gonadotropin level, was correlated with intraocular pressure, aqueous flare, and corneal thickness. The change in aqueous outflow facility that accompanied pregnancy could not be correlated directly to changes in beta-human chorionic gonadotropin or progesterone concentrations. Aqueous flow remains constant during and after pregnancy, but intraocular pressure decreases during pregnancy due to an increase in the outflow facility. The changes in aqueous dynamics are consistent with the hypothesis that excess progesterone during pregnancy blocks the ocular hypertensive effect of endogenous corticosteroids. However, we were unable to find a statistically significant correlation when a direct comparison between the observed changes in outflow facility and the observed changes in the progesterone level was made, perhaps because of intersubject variability of these changes. The changes in intraocular pressure and outflow facility could have been due to one of many other changes in pregnancy that were not measured.

Trabecular outflow facility and formation rate of aqueous humor during intravenous cocaine or lidocaine in rabbits.

This study examined the effect of cocaine and lidocaine on intraocular pressure (IOP), fluid formation, outflow facility, and compliance. Fourteen New Zealand White rabbits were anesthetized with halothane (0.8-1.0% inspired concentration) in nitrous oxide (2 L/min) and oxygen (1 L/min). Pancuronium was given intravenously (i.v.) to produce neuromuscular block, and the lungs were mechanically ventilated through a tracheal tube. In one group (n = 8) cocaine 0.5 mg/kg followed by 1.0 microgram.kg-1.min-1 i.v. was added to halothane/nitrous oxide/oxygen anesthesia. In the second group (n = 6) lidocaine 0.5 mg/kg followed by 1.0 microgram.kg-1.min-1 i.v. was added to halothane/nitrous oxide/oxygen anesthesia. In both groups a series of intraocular infusions was made via a 30-gauge needle in the anterior chamber and IOP, the rate of aqueous humor formation, trabecular outflow facility, and intraocular compliance were determined. In Group I, IOP was 15.0 +/- 6.3 mmHg prior to and 14.5 +/- 7.1 mm Hg after cocaine. The rate of anterior chamber aqueous formation was 3.79 +/- 1.25 microL/min and trabecular outflow facility was 0.141 +/- 0.090 microL.min-1.mm Hg-1. In Group 2, IOP was 14.4 +/- 4.8 mm Hg prior to lidocaine and IOP decreased (P < 0.05 compared to precocaine in Group 1 and prelidocaine in Group 2) to 11.9 +/- 6.8 mm Hg after lidocaine. The rate of anterior chamber aqueous formation was 2.59 +/- 0.86 microL/min and trabecular outflow facility decreased (P < 0.05 compared to Group 1) to 0.077 +/- 0.040 microL.min-1.mm Hg-1. Intraocular compliance during cocaine, 36 +/- 12 nL/mm Hg, was similar to that during lidocaine, 33 +/- 14 nL/mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion transport asymmetry and functional coupling in bovine pigmented and nonpigmented ciliary epithelial cells

The solute and water transport properties of the bovine ciliary epithelium were studied using isolated pigmented (PE) and nonpigmented (NPE) cells. It was shown that these cells were functionally coupled by demonstrating dye diffusion between paired PE and NPE cells after microinjection of lucifer yellow. Electronic cell sizing was used to measure cell volume changes of isolated PE and NPE cells in suspension after anisosmotic perturbations and after transport inhibition under isosmotic conditions. The PE cells showed the presence of a regulatory volume increase when subjected to osmotic shrinkage with NaCl, whereas the NPE cells did not demonstrate a regulatory volume increase under these conditions. In contrast, the NPE cells exhibited a regulatory volume decrease when subjected to osmotic swelling, whereas the PE cells did not recover from swelling. The regulatory volume decrease in NPE cells was inhibited by increased bath K or pretreatment with quinine (1 mM). The presence of a bumetanide-sensitive mechanism capable of moving measurable amounts of solute and water, probably Na-K-2Cl cotransport, was demonstrated in the PE cells but absent in the NPE cells. Bumetanide produced a dose-dependent shrinkage of PE cells at concentrations as low as 1 microM. Isosmotically reducing bath Cl, Na, or K concentration caused a rapid shrinkage of PE cells that was bumetanide inhibitable. The asymmetry of transport properties in PE and NPE cells supports a functional syncytium model of aqueous humor formation (39) across the two layers of the ciliary epithelium wherein ion uptake from the blood is carried out by the PE cells and ion extrusion by the NPE cells. Gap-junction coupling between the cells allows the ions taken up by the PE cells to move into the NPE cells. Extrusion of Na by the Na-K pump across the aqueous facing (basolateral) membranes of the NPE cells, most likely accompanied by Cl, determines the formation of the aqueous humor.

The potential for topical carbonic anhydrase inhibitors in glaucoma therapy

Carbonic anhydrase inhibitors have been investigated for topical administration in glaucoma therapy during the past few years. A group of substances termed thienothiopyran-2-sulfonamides seems to be the most promising. Additivity has been shown with β-blockers and is likely to occur with most other topical glaucoma medications. Topical carbonic anhydrase inhibitors lower intraocular pressure by reducing aqueous humor formation. Side effects include local irritation and allergies. Systemic side effects such as electrolyte imbalances could not be detected. Blood dyscrasias have not been reported. Dose-dependent blood dyscrasias are likely to be reduced because of the decreased amount of substance. Non-dose-dependent idiosyncratic blood dyscrasias may be expected in a small fraction of patients.

The Effect of Melatonin on Aqueous Humor Flow in Humans during the Day

Aqueous humor flow through the anterior chamber of the eye undergoes a circadian cycle. The rate of flow during the day is twice as high as the rate of flow at night. The pineal hormone, melatonin, also undergoes a circadian cycle. Melatonin levels are high at night, whereas aqueous humor flow is low. The authors studied the effect of oral melatonin on aqueous humor flow in humans. The effect of melatonin on aqueous humor flow was evaluated in 19 healthy human volunteers in a randomized, masked crossover study with a placebo control. The hormone or placebo was administered orally during the day when endogenous levels of melatonin are low. Aqueous flow was measured by fluorophotometry for 8 hours. The mean rate of flow during melatonin treatment was 2.71 +/- 0.64 microliters/minute (+/- standard deviation). The rate of flow during placebo treatment was 2.80 +/- 0.66 microliters/minute. There is no statistically significant difference between these two rates (P = 0.4). With a sample size of 19, the study has a power of 92% to detect at least a 15% difference in the rate of flow under the two conditions. Measurement of plasma concentration of melatonin in five subjects confirmed that concentrations after oral dosage reached peaks comparable with the normal endogenous nocturnal peaks. The authors conclude that melatonin concentrations during the day, comparable with plasma concentrations that occur spontaneously during sleep, do not suppress aqueous humor formation. The authors find no support for the idea that plasma melatonin, per se, can suppress aqueous formation or that the circadian rhythm of plasma melatonin is primarily responsible for the circadian rhythm of aqueous humor flow.

Effects of endothelin-1 on intraocular pressure and aqueous humor dynamics in the rabbit eye

The effects of endothelin-1 (ET-1) on intraocular pressure (IOP) and aqueous humor dynamics were studied in the rabbit eye. The intravitreal injection of 10(-5) M ET-1 (20 microliters) produced a biphasic IOP response consisting of an initial rise of 1 to 2 hours in duration, and a subsequent prolonged reduction lasting for more than 96 hours. Aqueous humor dynamics were determined 24 hours after the 10(-5) M ET-1 injection. Aqueous humor formation, measured fluorophotometrically, was decreased by 58%. Total outflow facility increased by 94%, according to measurement by two-level constant pressure perfusion. The change of uveoscleral outflow determined by fluorescein-dextran perfusion was not significant. The decrease in aqueous flow and the increase in total facility accounted for most of the IOP reduction after the ET-1 injection. Endothelin, which is endogenously present in the eye, may play a role in the regulation of intraocular pressure.

Evidence for a synergistic interaction between isoprenaline and timolol on intraocular pressure in conscious rabbits

1. Interaction between topical isoprenaline and timolol on intraocular pressure was studied in conscious rabbits through changes of the dose-response curves. 2. Both drugs caused a dose-dependent fall of intraocular pressure mainly by reducing the aqueous humour formation. 3. When combined, timolol increased the response to submaximal concentrations of isoprenaline as well as EC 50 (from 2.06 × 10 −3 M to 3.63 × 10 −3 M) and slope (from 1.21 to 1.89), but peak response to isoprenaline remained unchanged. 4. Discordance between experimental and theoretical additive curves as well as an interaction index smaller than 1 show that the interaction between both drugs is synergistic.

The bovine arterially-perfused eye: an in vitro method for the study of drug mechanisms on IOP, aqueous humour formation and uveal vasculature.

A method is reported in which the isolated bovine eye is perfused through a long posterior ciliary artery with buffered physiological saline, to provide simultaneous monitoring of drug effects on intraocular pressure (IOP), vascular resistance and the condition of the blood-aqueous barrier. With perfusion under constant pressure of 45 mm Hg, perfusate flows at 1.64 +/- 0.12 ml.min-1 (mean +/- SEM) and IOP is 7.26 +/- 0.16 mm Hg. Applying a constant flow rate of 2.25 ml.min-1, IOP averages 10.19 +/- 0.32 mm Hg and in both cases this can be maintained for around 2h. Increasing the perfusion flow rate from 1.5 to 3.5 ml.min-1 produces a 76% rise in perfusion pressure but IOP increases only insignificantly (< 10%). The inclusion in the perfusion fluid of dextran and albumin to maintain oncotic pressure similar to that of plasma makes no difference to the IOP achieved and does not affect the leakiness of the barrier. The preparation shows a net consumption of oxygen, supporting the hypothesis that the aqueous humour formed is secreted by active transport processes. Timolol (in bolus doses of 1-300 nmol) injected into the perfusing fluid is shown to induce a dose-dependent fall in IOP within 5 min, reaching a steady state within 40 min. Timolol, however, causes no significant change in vascular resistance, whether this is measured as perfusion flow rate under constant pressure or as perfusion pressure at constant flow rate, nor does it alter the permeability of the barrier. Other beta-blockers such as oxprenolol and betaxolol also induce dose-dependent decreases in IOP. By applying a fluorescein dilution technique, it is found that the aqueous formation rate (K(out) = 0.0046 min-1, or 12.9 microliters.min-1) is also reduced by timolol and, in a dose-dependent manner, by the new carbonic anhydrase inhibitor, MK-927. The bovine perfused eye offers a useful method for studying the mechanisms of action of drugs on IOP and aqueous humour formation, in isolation from the complicating influences of the CNS and the cardiovascular system and without the necessity to kill animals for experimental purposes.

Some aspects of aqueous humour drainage

It is now 50 years since the discovery of the aqueous veins by Karl Ascher. His finding had a great impact on ophthalmology since it showed that the aqueous humour is not a stagnant fluid; there had to be a continuous formation of aqueous humour by the ciliary processes, flow from the posterior chamber into the anterior chamber and outflow in the chamber angle. Since that time it has become clear that there is also some drainage of aqueous humour via uveoscleral routes. Furthermore the flow through the inner wall of Schlemm's canal has been shown to take place through about 20,000 pores each with diameter of around 0.1-3 microns. Recent measurements of the pressures in the outflow routes indicate that in normal monkey eyes the main outflow resistance is located close to the inner wall of the canal. Most of the aqueous humour leaving Schlemm's canal via the collector channels mixes with blood within the sclera. This is a consequence of the embryology of Schlemm's canal; it develops by the merging of blind extensions from intrascleral veins. Despite its specialisation the endothelium of Schlemm's canal retains the properties of a blood vessel. Platelets are likely to play in role in the integrity of Schlemm's canal as they do in blood vessels and may in fact control the size of the pores by occluding pores larger than 3 microns. It seems likely that in vitro perfusion of glaucomatous eyes with enzymes may be useful in the search for new methods of treatment for glaucoma.

Innervation of the ciliary process vasculature and epithelium by nerve fibers containing catecholamines and neuropeptide Y.

The ciliary processes (CPs) in the rat were investigated for the presence of neuropeptide Y (NPY)-like immunoreactivity and for histofluorescence indicating catecholamine (CAM)-ergic innervation in the rat, rabbit and cynomolgus monkey. Special attention was paid to those CP vessel segments which in previous studies showed distinct reactions on application of CAM or NPY, and on the respective sections of the CP epithelium. In the rat and cynomolgus monkey most CAM-ergic nerve fibers concentrated along the terminal arterioles and the epithelium of the anteriormost portion of the major CPs. In comparison, the rabbit displayed most intense CAM-ergic innervation along the terminal arterioles and the epithelium of both the iridial processes and the anterior portion of the major CPs. The NPY-ergic nerve fibers built up a dense subepithelial nervous plexus in the anterior portion of the rat CPs, diminishing towards the posterior CPs. Also, many NPY-ergic fibers were found along the terminal arterioles of the anterior CPs. The findings demonstrate that CAM- and NPY-ergic nerve fibers preferentially supply vasculature and epithelium of the anterior ciliary processes, suggesting a crucial function of these structures for the precise regulation of aqueous humor formation.