Topical proparacaine and episcleral venous pressure in the rabbit

Episcleral venous pressure response to brain stem stimulation: Effect of topical lidocaine

Episcleral venous pressure (EVP) is important for steady state intraocular pressure (IOP), as it has to be overcome by aqueous humor in order to leave the eye. Recent evidence suggests a neuronal tone being present, as topical anesthesia lowered EVP. The superior salivatory nucleus in the brainstem could be identified to elicit increases in EVP during electrical stimulation. In the present study the effect of topical anesthesia on the stimulation effect was investigated. 8 Spraque Dawley rats were anesthetized, artificially ventilated with CO2 monitoring and continuous blood pressure monitoring. Intraocular pressure was measured continuously through a cannula in the vitreous body. Episcleral venous pressure was measured by direct cannulation of an episcleral vein via a custom made glass pipette connected to a servonull micropressure system. Electrical stimulation of the superior salivatory nucleus (9 μA, 200 pulses of 1 ms duration) increased EVP from 8.51 ± 1.82 mmHg to 10.97 ± 1.93 mmHg (p = 0.004). After application of topical lidocaine EVP increased from 7.42 ± 1.59 mmHg to 9.77 ± 1.65 mmHg (p = 0.007). The EVP response to stimulation before and after lidocaine application was not statistically significantly different (2.45 ± 0.5 vs 2.35 ± 0.49 mmHg, p = 0.69), while the decrease in baseline EVP was (8.51 vs. 7.42 mmHg, p = 0.045). The present data suggest that distinct neuronal mechanisms controlling the episcleral circulation of rats exist. This is in keeping with previous reports of two distinct arterio-venous anastomoses, one in the limbal circulation and one in the conjunctival/episcleral circulation.

Vascular Aspects in Glaucoma: From Pathogenesis to Therapeutic Approaches

Glaucomatous optic neuropathies have been regarded as diseases caused by high intraocular pressure for a long time, despite the concept of vascular glaucoma dating back to von Graefe in 1854. Since then, a tremendous amount of knowledge about the ocular vasculature has been gained; cohort studies have established new vascular risk factors for glaucoma as well as identifying protective measures acting on blood vessels. The knowledge about the physiology and pathophysiology of the choroidal, retinal, as well as ciliary and episcleral circulation has also advanced. Only recently have novel drugs based on that knowledge been approved for clinical use, with more to follow. This review provides an overview of the current vascular concepts in glaucoma, ranging from novel pathogenesis insights to promising therapeutic approaches, covering the supply of the optic nerve head as well as the aqueous humor production and drainage system.

Episcleral Venous Pressure and the Ocular Hypotensive Effects of Topical and Intracameral Prostaglandin Analogs

There is a limit beyond which increasing either the concentration of a prostaglandin analog (PGA) or its dosing frequency fails to produce increases in ocular hypotensive efficacy with topical dosing. Intracameral PGA dosing with a bimatoprost implant, however, does not exhibit the same intraocular pressure (IOP)-lowering plateau at studied concentrations, and the maximum-achievable ocular hypotensive effects are not yet known. This suggests that the bimatoprost intracameral implant may activate another mechanism of action in addition to the mechanism(s) activated by topical application. Episcleral venous pressure (EVP) is a key determinant of IOP, and experimental manipulation of the episcleral vasculature can change both EVP and IOP. The recent observation that topical and intracameral PGA drug delivery routes produce different patterns of conjunctival hyperemia suggested that the differences in the IOP-lowering profiles may be caused by differing effects on the episcleral vasculature. Recent experiments in animals have shown that topical PGAs increase EVP, while the bimatoprost intracameral implant causes a smaller, transient increase in EVP, followed by a sustained decrease. The increase in EVP could be limiting the IOP-lowering efficacy of topical PGAs. In contrast, the decrease in EVP associated with the bimatoprost implant could explain its enhanced IOP-lowering effects. Further research on EVP as a target for IOP lowering is indicated to improve our understanding of this potentially important pathway for treating patients with glaucoma.

A model of the oscillatory mechanical forces in the conventional outflow pathway

Intraocular pressure is regulated by mechanosensitive cells within the conventional outflow pathway, the primary route of aqueous humour drainage from the eye. However, the characteristics of the forces acting on those cells are poorly understood. We develop a model that describes flow through the conventional outflow pathway, including the trabecular meshwork (TM) and Schlemm’s canal (SC). Accounting for the ocular pulse, we estimate the time-varying shear stress on SC endothelium and strain on the TM. We consider a range of outflow resistances spanning normotensive to hypertensive conditions. Over this range, the SC shear stress increases significantly and becomes highly oscillatory. TM strain also increases, but with negligible oscillations. Interestingly, TM strain responds more to changes in outflow resistance around physiological values, while SC shear stress responds more to elevated levels of resistance. A modest increase in TM stiffness, as observed in glaucoma, suppresses TM strain and practically eliminates the influence of outflow resistance on SC shear stress. As SC and TM cells respond to mechanical stimulation by secreting factors that modulate outflow resistance, our model provides insight regarding the potential role of SC shear and TM strain as mechanosensory cues for homeostatic regulation of outflow resistance and hence intraocular pressure.

Immunohistochemical Characterization of Neurotransmitters in the Episcleral Circulation in Rats

Purpose

Episcleral venous pressure (EVP) greatly influences steady-state IOP and recent evidence suggests a neuronal influence on EVP. Yet little is known about the innervation of the episcleral circulation and, more specifically, the neurotransmitters involved. We identify possible neurotransmitter candidates in the episcleral circulation of rats.

Methods

Eight immersion-fixated rat eyes taken from four animals were cut into serial sections, followed by standard immunohistochemistry. Antibodies against choline acetyltransferase, dopamine-β-hydroxylase, synaptophysine, PGP 9.5, VIP, neuronal nitric oxide synthase (nNOS), substance P, CGRP, and galanin were used. Additionally, colocalization experiments with smooth muscle actin and neurofilament (200 kDa) were performed.

Results

In all specimens, the episcleral vessels showed immunoreactivity for smooth muscle actin and were reached by neurofilament (200 kDa)-positive structures. Furthermore, these structures colocalized with immunoreactivity for PGP 9.5, synaptophysine, choline acetyl transferase (ChAT), dopamine-β-hydroxylase, VIP, CGRP, nNOS, substance P and galanin.

Conclusions

These findings indicate that there is neuronal input to the episcleral circulation. ChAT and VIP as well as dopamine-β-hydroxylase suggest parasympathetic and sympathetic innervation. Further studies are needed on whether the positively-stained structures are of functional significance for the regulation of the episcleral venous pressure and thereby IOP.

The Effects of Netarsudil Ophthalmic Solution on Aqueous Humor Dynamics in a Randomized Study in Humans

PURPOSE

Netarsudil, an inhibitor of Rho kinase and a norepinephrine transporter, has been shown to lower elevated intraocular pressure (IOP) in controlled studies of patients with open-angle glaucoma and ocular hypertension, and in healthy volunteers. The mechanism of this ocular hypotensive effect in humans is unknown.

METHODS

The objective of this study was to evaluate the effect of netarsudil 0.02% on aqueous humor dynamics (AHD) parameters. In this double-masked, vehicle-controlled, paired-eye comparison study, 11 healthy volunteers received topical netarsudil ophthalmic solution 0.02% or its vehicle once daily for 7 days (morning dosing). The primary endpoints were the change in AHD parameters, compared between active and vehicle-treated eyes.

RESULTS

In netarsudil-treated eyes, diurnal outflow facility increased from 0.27 ± 0.10 μL/min/mmHg to 0.33 ± 0.11 μL/min/mmHg (+22%; P = 0.02) after 7 days of treatment. In placebo-treated eyes, diurnal outflow facility did not significantly change (P = 0.94). The difference between netarsudil and placebo eyes in diurnal change of outflow facility was 0.08 μL/min/mmHg (P < 0.001). Diurnal episcleral venous pressure (EVP) in netarsudil-treated eyes decreased from 7.9 ± 1.2 mmHg to 7.2 ± 1.8 (-10%; P = 0.01). Diurnal EVP was not significantly different between netarsudil- and placebo-treated eyes. There was a trend toward decreasing aqueous humor flow rate (-15%; P = 0.08). No treatment changes were seen in uveoscleral outflow rate.

CONCLUSIONS

Once-daily dosing of netarsudil ophthalmic solution 0.02% lowered IOP through increasing trabecular outflow facility and reducing EVP. This suggests a combination of mechanisms that affect both the proximal and distal outflow pathways.

Additive Intraocular Pressure-Lowering Effects of Ripasudil with Glaucoma Therapeutic Agents in Rabbits and Monkeys

Ripasudil hydrochloride hydrate (K-115), a specific Rho-associated coiled-coil containing protein kinase (ROCK) inhibitor, is developed for the treatment of glaucoma and ocular hypertension. Topical administration of ripasudil decreases intraocular pressure (IOP) by increasing conventional outflow through the trabeculae to Schlemm's canal, which is different from existing agents that suppress aqueous humor production or promote uveoscleral outflow. In this study, we demonstrated that ripasudil significantly lowered IOP in combined regimens with other glaucoma therapeutic agents in rabbits and monkeys. Ripasudil showed additional effects on maximum IOP lowering or prolonged the duration of IOP-lowering effects with combined administration of timolol, nipradilol, brimonidine, brinzolamide, latanoprost, latanoprost/timolol fixed combination, and dorzolamide/timolol fixed combination. These results indicate that facilitation of conventional outflow by ripasudil provides additive IOP-lowering effect with other classes of antiglaucoma agents. Ripasudil is expected to have substantial utility in combined regimens with existing agents for glaucoma treatment.

Measurement of Outflow Facility Using iPerfusion

Elevated intraocular pressure (IOP) is the predominant risk factor for glaucoma, and reducing IOP is the only successful strategy to prevent further glaucomatous vision loss. IOP is determined by the balance between the rates of aqueous humour secretion and outflow, and a pathological reduction in the hydraulic conductance of outflow, known as outflow facility, is responsible for IOP elevation in glaucoma. Mouse models are often used to investigate the mechanisms controlling outflow facility, but the diminutive size of the mouse eye makes measurement of outflow technically challenging. In this study, we present a new approach to measure and analyse outflow facility using iPerfusion^™, which incorporates an actuated pressure reservoir, thermal flow sensor, differential pressure measurement and an automated computerised interface. In enucleated eyes from C57BL/6J mice, the flow-pressure relationship is highly non-linear and is well represented by an empirical power law model that describes the pressure dependence of outflow facility. At zero pressure, the measured flow is indistinguishable from zero, confirming the absence of any significant pressure independent flow in enucleated eyes. Comparison with the commonly used 2-parameter linear outflow model reveals that inappropriate application of a linear fit to a non-linear flow-pressure relationship introduces considerable errors in the estimation of outflow facility and leads to the false impression of pressure-independent outflow. Data from a population of enucleated eyes from C57BL/6J mice show that outflow facility is best described by a lognormal distribution, with 6-fold variability between individuals, but with relatively tight correlation of facility between fellow eyes. iPerfusion represents a platform technology to accurately and robustly characterise the flow-pressure relationship in enucleated mouse eyes for the purpose of glaucoma research and with minor modifications, may be applied in vivo to mice, as well as to eyes from other species or different biofluidic systems.

Effects of K-115 (Ripasudil), a novel ROCK inhibitor, on trabecular meshwork and Schlemm's canal endothelial cells

Ripasudil hydrochloride hydrate (K-115), a specific Rho-associated coiled-coil containing protein kinase (ROCK) inhibitor, was the first ophthalmic solution developed for the treatment of glaucoma and ocular hypertension in Japan. Topical administration of K-115 decreased intraocular pressure (IOP) and increased outflow facility in rabbits. This study evaluated the effect of K-115 on monkey trabecular meshwork (TM) cells and Schlemm’s canal endothelial (SCE) cells. K-115 induced retraction and rounding of cell bodies as well as disruption of actin bundles in TM cells. In SCE-cell monolayer permeability studies, K-115 significantly decreased transendothelial electrical resistance (TEER) and increased the transendothelial flux of FITC-dextran. Further, K-115 disrupted cellular localization of ZO-1 expression in SCE-cell monolayers. These results indicate that K-115 decreases IOP by increasing outflow facility in association with the modulation of TM cell behavior and SCE cell permeability in association with disruption of tight junction.

Effect of AR-13324 on episcleral venous pressure in Dutch belted rabbits

PURPOSE

AR-13324 is a potential new drug for the treatment of patients with glaucoma that has been shown to lower intraocular pressure (IOP) by increasing trabecular outflow facility and decreasing aqueous production. The present study tested the hypothesis that AR-13324 also lowers IOP by reducing episcleral venous pressure (EVP).

METHODS

In Dutch Belted (DB) rabbits (n=11), arterial pressure (AP), IOP, carotid blood flow (BFcar), heart rate (HR), and EVP were measured invasively. Animals were dosed with AR-13324 (0.04%, topical, n=6) once daily for 3 days. On day 3, the animals were anesthetized, and then, measurements were obtained before dosing with AR-13324 or vehicle (n=5) and for 3 h after dosing. The data (mean±standard error of the mean) were analyzed by repeated measures ANOVA with post hoc testing. Retrospective baseline data from prior similar studies in New Zealand White rabbits were also compiled.

RESULTS

Baseline values were as follows: AP, 101±3 mmHg; IOP; 33±3 mmHg; EVP, 16±1 mmHg; BFcar, 41±4 mL/min; and HR, 330±6 bpm. Three hours after AR-13324 dosing, IOP was reduced by 39%±7% (P<0.001) and EVP decreased by 35%±4% (P<0.05); after vehicle dosing, IOP was reduced by 24%±4% (P<0.05) and EVP increased by 25%±5% (P<0.05). AP, BFcar, and HR were unchanged.

CONCLUSIONS

AR-13324 produces statistically significant lowering of EVP in DB rabbits. In addition, the baseline values for AP, IOP, EVP, BFcar, and HR in the DB rabbit are higher than those previously reported in the New Zealand rabbit.

Autonomic control of the eye

The autonomic nervous system influences numerous ocular functions. It does this by way of parasympathetic innervation from postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia, and by way of sympathetic innervation from postganglionic fibers that originate from neurons in the superior cervical ganglion. Ciliary ganglion neurons project to the ciliary body and the sphincter pupillae muscle of the iris to control ocular accommodation and pupil constriction, respectively. Superior cervical ganglion neurons project to the dilator pupillae muscle of the iris to control pupil dilation. Ocular blood flow is controlled both via direct autonomic influences on the vasculature of the optic nerve, choroid, ciliary body, and iris, as well as via indirect influences on retinal blood flow. In mammals, this vasculature is innervated by vasodilatory fibers from the pterygopalatine ganglion, and by vasoconstrictive fibers from the superior cervical ganglion. Intraocular pressure is regulated primarily through the balance of aqueous humor formation and outflow. Autonomic regulation of ciliary body blood vessels and the ciliary epithelium is an important determinant of aqueous humor formation; autonomic regulation of the trabecular meshwork and episcleral blood vessels is an important determinant of aqueous humor outflow. These tissues are all innervated by fibers from the pterygopalatine and superior cervical ganglia. In addition to these classical autonomic pathways, trigeminal sensory fibers exert local, intrinsic influences on many of these regions of the eye, as well as on some neurons within the ciliary and pterygopalatine ganglia.

Episcleral venous pressure and IOP responses to central electrical stimulation in the rat

PURPOSE

Histological evidence suggests a role for the central nervous system in controlling episcleral venous pressure (EVP). Based on prior studies that identified candidate regions in the brain stem, the present study assessed the effect of electrical stimulation at the location of the superior salivatory nucleus (SSN) on EVP in rats.

METHODS

Male Sprague-Dawley rats (n = 11) were anesthetized using pentobarbital sodium (50 mg/kg intraperitoneally initially, supplemented intravenously [IV] as needed) and paralyzed with gallamine triethiodide (1 mg/kg, IV). The animals were artificially ventilated and the femoral artery and vein were cannulated for blood pressure measurement and drug administration. Carotid blood flow was measured with an ultrasound flow probe and heart rate with a cardiotachometer. IOP was measured through a cannula in the vitreous compartment and EVP was measured through a micropipette in episcleral veins using the servonull technique. After a craniotomy was performed, a unipolar stainless steel electrode was inserted into the brainstem at the coordinates of the SSN using a stereotactic instrument. Stimulations were performed at 20Hz, 9 μA, 1 ms pulse duration, and 200 pulses.

RESULTS

Stimulation at the SSN coordinates increased IOP from 10.6 ± 0.4 to 11.8 ± 0.6 mm Hg (P < 0.01) and EVP from 7.8 ± 1.3 to 10.7 ± 1.1 mm Hg (P < 0.01). Mean arterial pressure, carotid blood flow, and heart rate remained unaltered.

CONCLUSIONS

The present study indicates that the SSN may participate in regulating EVP.

In vitro and in vivo comparison of two suprachoroidal shunts

PURPOSE

To compare fibrosis, aqueous humor dynamics, and intraocular pressure (IOP) of two suprachoroidal shunts as part of a new class of glaucoma drainage devices.

METHODS

Following proliferation analysis in vitro, 20 rabbits were implanted with either a gold shunt (GS, GMSplus+, SOLX) or a polypropylene shunt (PS, Aquashunt, OPKO). Ten eyes received mitomycin C (MMC) and triamcinolone. Peak and trough IOP were monitored with a pneumatonometer and tono-pen for 15 weeks. Aqueous humor dynamics were evaluated fluorophotometrically and tonographically. Fibrosis was quantified.

RESULTS

In vitro proliferation was similar. In vivo, both shunts were devoid of foreign body reaction but exhibited fibrosis, and GS showed vascularization. There was no significant difference in aqueous or uveoscleral flow. Preoperative morning IOP was 23.7 ± 2 mm Hg, and evening IOP was 26.5 ± 2 mm Hg (P = 0.000). Morning IOP was decreased through 15 weeks and evening IOP through 8 weeks in all groups. The morning IOP decrease was most profound at 15 weeks in PS (41%) compared to GS (18%). Antifibrotics initially enhanced but eventually diminished shunt performance. At 15 weeks, thickness of scleral fibrosis was greater in GS (246 ± 47 μm) and PS (188 ± 47 μm, P = 0.285) compared with GS+MMC (109 ± 26 μm, P = 0.023 to GS) and PS+MMC (48 ± 30 μm, P = 0.028 to PS).

CONCLUSIONS

In a rabbit model, suprachoroidal polypropylene and gold shunts allow access to a new drainage pathway with different IOP profiles that can be modified with antifibrotics.

Effects of head down tilt on episcleral venous pressure in a rabbit model

In humans, changing from upright to supine elicits an approximately 10 mmHg increase in cephalic venous pressure caused by the hydrostatic column effect, but episcleral venous pressure (EVP) and intraocular pressure (IOP) rise by only a few mmHg. The dissociation of the small increases in IOP and EVP compared to the larger increase in cephalic venous pressure suggests a regulatory mechanism controlling EVP. The aim of the present study was to determine if the rabbit model is suitable to study the effects of postural changes on EVP despite its short hydrostatic column. In anesthetized rabbits (n = 43), we measured arterial pressure (AP), IOP, and orbital venous pressure (OVP) by direct cannulation; carotid blood flow (BFcar) by transit time ultrasound, heart rate (HR) by digital cardiotachometer, and EVP with a servonull micropressure system. The goal of the protocol was to obtain measurement of supine EVP for ≈10 min, followed by ≈10 min of EVP measurement with the rabbit in a head down tilt. The data were analyzed by paired t-tests and the results reported as the mean ± standard error of the mean. In a separate group of animals (n = 35), aqueous flow was measured by fluorophotometry. This protocol entailed measurement of aqueous flow in the supine position for ≈60 min, followed by ≈60 min of aqueous flow measurement with the rabbit in a head down tilt. From supine to head down tilt, AP and BFcar were unchanged, IOP increased by 2.3 ± 0.4 mmHg (p < 0.001), EVP increased by 2.4 ± 0.4 mmHg (p < 0.001), OVP increased by 2.5 ± 0.2 mmHg (p < 0.001) and HR decreased by 9 ± 3 bpm (p = 0.002). Head down tilt caused no significant change in aqueous flow. Although the hydrostatic column in the rabbit is shorter than humans, the rabbit model permits sufficiently sensitive measurements of the pressures and systemic parameters likely involved in the EVP responses to posture change. The present results indicate directionally similar EVP and IOP responses to tilt as occur in humans and, as in humans, the responses are smaller than would be expected from the change in the hydrostatic column height. Also, as in humans, the model reveals no change in aqueous flow during head down tilt. We conclude the rabbit model is appropriate for studying the mechanisms responsible for the relative immunity of EVP and IOP to posture change.

Effect of topical anesthesia on evaluation of corneal sensitivity and intraocular pressure in rats and dogs

OBJECTIVE

To determine the effect of 0.5% proparacaine in tonometry by evaluating corneal touch threshold (CTT) and intraocular pressure (IOP).

ANIMAL STUDIED

  Nine rats (18 eyes, Sprague-Dawley) and 10 dogs (20 eyes, Beagle)

PROCEDURES

The IOP and CTT were measured in each eye before and after topical anesthesia with 0.5% proparacaine. The IOP was evaluated using Tonopen for dogs and Tonolab for rats. The corneal sensitivity was evaluated by CTT through a Cochet-Bonnet aesthesiometer.

RESULTS

The mean IOP was not significantly changed in rats or dogs before and after topical anesthesia. However, after application of proparacaine, CTT was significantly increased in both animal groups compared with that before application of proparacaine.

CONCLUSION

  From this study, topical anesthesia was found to significantly lower the corneal sensitivity but have little effect on IOP measurements. In ophthalmologic examination, topical anesthesia can be used to reduce corneal sensation without an effect on IOP.

Measurement of episcleral venous pressure

Episcleral venous pressure (EVP) is an important determinant of intraocular pressure (IOP) and can be measured by using various techniques. It has been measured non-invasively by estimating the pressure required to compress an episcleral vein to a predetermined endpoint. However, the lack of objective endpoints makes EVP measurement in humans uncertain, and a wide range of mean EVP has been reported in the literature. We review the evidence for physiologic regulation of EVP and its role in glaucoma therapy, techniques that have been used to measure EVP and the need for objective measurements, and reported values for EVP. We also review recent progress toward developing an objective technique for EVP measurement.

A novel method for computerized measurement of episcleral venous pressure in humans

Episcleral venous pressure (EVP) is an important determinant of intraocular pressure (IOP) and can be estimated by the pressure required to compress an episcleral vein. However, the lack of objective measurement endpoints makes EVP measurements in humans uncertain. To address this issue, we developed a new method to measure EVP objectively and reproducibly, and demonstrated its utility on a group of normal subjects. Our system for pressure chamber based venomanometry included a computer-controlled motor drive to increase pressure automatically, a transducer to record pressure, and a high-definition video camera to record vein collapse. Pressure measurements were synchronized with the video stream to determine the pressure required to collapse the vein to a specific pre-determined degree. This system was used to measure EVP in 10 eyes from 5 young healthy volunteers. Episcleral veins were selected in each of 4 quadrants. EVP was calculated to be the pressure in the chamber that compressed the vein by 0% (by back-projection), 10% or 50% as determined by using image analysis of the video stream. For this group of subjects, mean EVP was 6.3 ± 2.8 mmHg (mean ± SD, n = 40 measurements), 7.0 ± 2.6 mmHg, and 9.6 ± 2.6 mmHg using the 0%, 10% and 50% reduction endpoints, respectively. Pressures and standard deviations determined from these endpoints were significantly different from each other (p < 0.001). Coefficients of variation between right and left eyes were 12.7%, 10.2%, and 6.8% using the 0%, 10% and 50% endpoints, respectively. Based on previous research and theoretical considerations, the 0% endpoint is assumed to provide the most accurate estimate of baseline EVP, and can only be estimated by analyzing the brightness profiles of the vessels in the video stream. Objective measurement of EVP is important for understanding normal aqueous humor dynamics and its changes in disease states and with therapies. EVP has typically been assumed to be constant because of the lack of a convenient means of its measurement. This new method provides a precise means to assess EVP based on specific endpoints of vessel collapse, and enables, for the first time, objective and non-invasive measurements of EVP changes.

Episcleral venous pressure responses to topical nitroprusside and N-Nitro-L-arginine methyl ester

PURPOSE

To determine the episcleral venous pressure (EVP) responses to nitroprusside (NP) and L-NAME.

METHODS

In anesthetized rabbits (n = 36), arterial pressure and IOP were measured by direct cannulation, and carotid blood flow and heart rate were measured with an ultrasound flowmeter and cardiotachometer. EVP was measured in two groups with a servonull system. Group 1 (n = 13) was given NP (50 microL, 10 mg/mL). Group 2 (n = 10) was given L-NAME (100 microL, 10 mg/mL) followed by NP (50 microL, 10 mg/mL). In group 3 (n = 13), fluorophotometric aqueous flow was measured before and after NP (100 microL, 10 mg/mL).

RESULTS

Systemic parameters were unaffected by treatment in all groups. In group 1, NP increased EVP from 9.1 +/- 0.6 to 11.6 +/- 0.8 mm Hg (P < 0.01) and IOP from 18.7 +/- 1.4 to 23.9 +/- 1.6 mm Hg (P < 0.01). In group 2, L-NAME lowered EVP from 11.5 +/- 1.2 to 8.8 +/- 1.0 mm Hg (P < 0.01) and subsequent NP increased EVP to 13.9 +/- 1.7 mm Hg (P < 0.01 versus L-NAME and baseline). L-NAME decreased IOP from 20.8 +/- 1.7 to 16.7 +/- 1.8 mm Hg (P < 0.01), and then it increased to 20.7 +/- 1.3 mm Hg after NP (P < 0.01 versus L-NAME and P > 0.05 versus baseline). In group 3, NP increased IOP from 16.6 +/- 0.7 to 20.0 +/- 0.9 mm Hg (P < 0.01) but did not alter aqueous flow (2.65 +/- 0.3 vs. 3.0 +/- 0.3 microL/min, P > 0.05).

CONCLUSIONS

Because a topical NO donor raises EVP and a topical NO synthase inhibitor lowers EVP, the authors conclude that EVP is modulated by NO.