Effect of clonidine on aqueous humor flow in normal human eyes

Autonomic drugs in the treatment of canine and feline glaucoma--Part II: Medications that lower intraocular pressure by reducing aqueous humour production

One characteristic of the most common types of glaucoma is increased intraocular pressure (IOP), which has a damaging effect on optic nerve axons, leading to progressive loss of retinal ganglion cells. Therefore, ocular hypotensive drugs are the mainstay of pharmacological therapy for glaucoma. This review article, which is the second part of a two-part series, is dedicated to autonomic drugs which lower IOP by decreasing the aqueous humour production. These agents are subdivided into two groups: β-adrenergic antagonists and selective α2-adrenergic agonists. This paper summarizes the current state of knowledge on the mechanism of action of these drugs and their effect on IOP in dogs and cats. Moreover, it discusses their possible undesirable side effects of these medications and presents the current ideas about their role and position in the medical management of glaucoma in small animals.

Effects of common topical antiglaucoma medications on the ocular surface, eyelids and periorbital tissue

Glaucoma affects millions of people around the world. With the baby boom generation aging, the number of people affected by primary open-angle glaucoma in the US is expected to reach 3.3 million by 2020, and about half may not know they have the disease. The treatment of most forms of glaucoma includes the use of topical agents that enhance aqueous humour outflow, reduce aqueous production, or both. Topical intraocular pressure-lowering drugs must penetrate across the tissues of the eye to reach their therapeutic targets. Often, these tissues show the first signs and symptoms of drug toxicity and adverse effects. These include eyelid dermatitis, malpositions, lacrimal system scarring, ocular discomfort upon instillation, tear film instability, conjunctival inflammation, subconjunctival fibrosis, conjunctival epithelium changes, and corneal surface and endothelial impairment. For these reasons, ophthalmologists should evaluate the risks and benefits of ophthalmic medications before initiating therapy, identify the minimum dosages necessary to achieve a therapeutic benefit, and monitor patients for local and systemic adverse effects. Adverse events may be reduced by changing to a different class of topical medication, using corticosteroids, lubricating the eyes frequently, and reducing exposure to preservatives. This in turn can lead to higher levels of adherence to antiglaucoma therapy, improved outcomes and a reduction in the costs associated with long-term glaucoma complications. This article reviews the ocular adverse effects associated with the various classes of topical antiglaucoma drugs, with a particular focus on the ocular surface, eyelids and periorbital tissue.

Pharmacological evidence for heterogeneity of ocular $aL2adrenoceptors

Previous studies have shown that ocular alpha 2 adrenoceptors are located prejunctionally on sympathetic neurons and postjunctionally on cells in the iris/ciliary body. While the activation of alpha 2 adrenoceptors at each site has been postulated to alter aqueous humor dynamics, little is known about the pharmacological characteristics of these receptors or their role in the modulation of anterior segment function. The purpose of the current study was to determine the possible heterogeneity of ocular alpha 2 adrenoceptors using relatively selective alpha 2 adrenoceptor agonists and antagonists to examine ocular pre- and postjunctional alpha 2 adrenoceptors. Prejunctional alpha 2 effects were evaluated by means of the cat nictitating membrane (CNM) preparation. Postjunctional alpha 2 effects were evaluated by means of the cAMP assay in rabbit iris root/ciliary body. In the CNM, the administration of UK-14, 304 (UK) produced a dose-related inhibition of neuronally mediated contractions. Pretreatment with the alpha 2 antagonist rauwolscine caused a 1 to 2 log unit right shift in the dose-response curve of UK in the CNM. However, pretreatment with alpha 2 antagonist SKF 104078 had no demonstrable effect on UK-induced inhibition of neuronally mediated contractions of the CNM. In the rabbit iris root/ciliary body, UK produced a concentration-dependent inhibition of cAMP accumulation on isoproterenol- and VIP-induced cAMP production. Pretreatment of iris root/ciliary bodies with SKF 104078 or rauwolscine reversed the inhibitory effect of UK on isoproterenol- and VIP-induced accumulation of cAMP. These data provide the first evidence that the pre- and postjunctional alpha 2 adrenoceptors represent pharmacologically distinct subpopulations of receptors in the eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of alpha 2 receptors in ocular tissues

Alpha 2 adrenergic agonists are used for controlling intraocular pressure (IOP) in the treatment of glaucoma. They have also been shown to be neuroprotective to retinal cells in a variety of injury models. Despite this significance, the localization of the three known alpha 2 adrenergic receptors has not been unequivocally established. The aim of this study was to determine the location of the three alpha 2 adrenergic receptors in ocular tissues using immunohistochemical techniques. New antibodies were generated and their specificity was determined using Western blotting and preadsorption. In the anterior segment of the eye alpha 2A immunoreactivity was located in the nonpigmented ciliary epithelium, corneal, and conjunctival epithelia. Alpha 2B staining was not apparent in these tissues. Alpha 2C immunostaining was present in the membrane of pigmented ciliary epithelium and corneal and conjunctival epithelial cells. In the rat retina, all three receptor subtypes were present but were differentially localized. Alpha 2A was present in the somata of ganglion cell layer and inner nuclear layer somas, alpha 2B was located in the dendrites and axons of most of the neurons as well as glia, while alpha 2C was present in the somata and inner segment of the photoreceptors. In human and monkey retinas, similar pattern of labeling for alpha 2A and 2B receptors were observed, while alpha 2B was additionally present in the membranes of many cell somata in addition to dendrites and axons. Alpha 2C labeling was much weaker but exhibited similar pattern to that observed in the rat. These data provide additional information on the location of the alpha 2 receptors in the anterior portion of the eye and present new information on their specific location in the retina. This offers insights into possible targets for adrenergic agonists in a therapeutic context.

Management of glaucoma: focus on pharmacological therapy

Glaucoma represents a major cause of vision loss throughout the world. Primary open-angle glaucoma, the most common form of glaucoma, is a chronic, progressive disease often, though not always, accompanied by elevated intraocular pressure (IOP). In this disorder, retinal ganglion cell loss and excavation of the optic nerve head produce characteristic peripheral visual field deficits. Patients with normal-tension glaucoma present with typical visual field and optic nerve head changes, without a documented history of elevated IOP. A variety of secondary causes, such as pigment dispersion syndrome and ocular trauma, can result in glaucoma as well. Treatment of all forms of glaucoma consists of reducing IOP. With proper treatment, progression of this disease can often be delayed or prevented. Treatment options for glaucoma include medications, laser therapy and incisional surgery. Laser techniques for the reduction of IOP include argon laser trabeculoplasty and selective laser trabeculoplasty. Both techniques work by increasing outflow of aqueous humour through the trabecular meshwork. Surgical options for glaucoma treatment include trabeculectomy, glaucoma drainage tube implantation and ciliary body cyclodestruction. While each of these types of procedures is effective at lowering IOP, therapy usually begins with medications. Medications lower IOP either by reducing the production or by increasing the rate of outflow of aqueous humour within the eye. Currently, there are five major classes of drugs used for the treatment of glaucoma: (i) cholinergics (acetylcholine receptor agonists); (ii) adrenoceptor agonists; (iii) carbonic anhydrase inhibitors (CAIs); (iv) beta-adrenoceptor antagonists; and (v) prostaglandin analogues (PGAs). Treatment typically begins with the selection of an agent for IOP reduction. Although beta-adrenoceptor antagonists are still commonly used by many clinicians, the PGAs are playing an increasingly important role in the first-line therapy of glaucoma. Adjunctive agents, such as alpha-adrenoceptor agonists and CAIs are often effective at providing additional reduction in IOP for patients not controlled on monotherapy. As with any chronic disease, effective treatment depends on minimising the adverse effects of therapy and maximising patient compliance. The introduction of a variety of well tolerated and potent medications over the past few years now allows the clinician to choose a treatment regimen on an individual patient basis and thereby treat this disorder more effectively.

Goldmann's equation and clinical measures of aqueous dynamics

Goldmann's equation has served for over 50 years as an adequate description of aqueous humor dynamics for clinical applications. Recent advances in therapeutics for glaucoma have made it necessary to revise the equation and to reinterpret the meanings of its parameters.

5-HT(2) receptor-mediated phosphoinositide hydrolysis in bovine ciliary epithelium

The serotonin 2 (5-HT(2)) receptor antagonists, MCI-9042 (Anplag) and ketanserin, have been shown to lower intraocular pressure in rabbits (1) and humans (2). The mechanism of action of these drugs has not been determined, but it is hypothesized that 5-HT(2) receptors, and possibly alpha-adrenergic receptors, (3) may regulate in part aqueous humor production via an intracellular signal transduction pathway in the ciliary body. We therefore examined whether 5-HT(2) receptors were coupled to phosphoinositide hydrolysis in an organ culture system of isolated bovine ciliary epithelium. 5-HT stimulated [(3)H]inositol phosphates ([(3)H]InsPs) accumulation in a dose-dependent manner with a maximum increase approximately twice over the basal level. The mean EC(50) value was 1.1 microM, which was calculated from four dose-response curves. The 5-HT stimulated accumulation of [(3)H]InsPs was inhibited by spiperone (5-HT(2A/1A) and dopamine 2 (D(2)) antagonists), M-1 (a major metabolite of MCI-9042), ketanserin (5-HT(2A) antagonist), SB-206553 (5- HT(2B/2C) antagonist), and mesulergine (5-HT(2C) antagonist and D(2) agonist). It was not inhibited by chlorpromazine, which is a D(2) receptor antagonist. Accordingly, our study demonstrates that 5-HT(2) receptors are coupled to phospholipase C in bovine ciliary epithelium.

New glaucoma medications in the geriatric population: efficacy and safety

Glaucoma can be considered a disease of the aging eye. Most medications used to treat glaucoma are in topical eyedrop form and may cause numerous untoward systemic effects in older persons. In recent years, several new ocular hypotensive medications have become available. These medications are being used more commonly because there is a growing trend by ophthalmologists to aggressively lower intraocular pressure. Therefore, geriatricians require a comprehensive knowledge of medications used to treat glaucoma, in addition to an understanding of their mechanism of action profiles of untoward effects and possible interactions with other diseases or medications. Therefore, we performed a review of the medications recently introduced into clinical practice. We selected drugs approved by the U.S. Food and Drug Administration between 1996 and September 2001. The safety profiles of these agents and their untoward side effects were reviewed by class: topical carbonic anhydrase inhibitors (brinzolamide: ocular tolerance, taste perversion), beta-adrenoceptor antagonists (timolol: bradycardia and bronchospasm), alpha-adrenergic agonists (brimonidine: oral dryness, headache, and fatigue), and prostaglandin analogs (latanoprost, bimatoprost, travoprost, and unoprostone isopropyl: ocular hyperemia, iris color changes). The function of this review is to make geriatricians more aware of the efficacy and untoward effects of medications recently introduced into clinical practice. We recommend that geriatricians perform a medication review on all medications their patients use, including eye drops.

Effects of multiple dosing of epinephrine on aqueous humor dynamics in human eyes

Numerous studies have provided conflicting evidence to explain the ocular hypotensive mechanism of action of epinephrine. Although epinephrine has been shown consistently to increase outflow facility, its effects on aqueous flow and uveoscleral outflow are not as clear. The purpose of this study was to clarify the effects of multiple doses of topical epinephrine on aqueous humor dynamics in human eyes. This was done by evaluating the four main parameters that determine steady state intraocular pressure. These parameters were assessed at baseline and after a week of twice-daily treatment of epinephrine hydrochloride 2% to one eye. Twenty-six human volunteers were enrolled in the study. Intraocular pressure was measured by pneumatonometry, aqueous flow and trabecular outflow facility by fluorophotometry, episcleral venous pressure by venomanometry and uveoscleral outflow by mathematical calculation. In epinephrine-treated eyes compared to baseline, intraocular pressure and aqueous flow were reduced from 21.2 +/- 0.3 to 17.1 +/- 0.2 mmHg (19%, p = .01) and 3.3 +/- 0.2 to 2.9 +/- 0.2 microl/min (12%, p = .03), respectively. Trabecular outflow facility obtained by fluorophotometry was increased from 0.18 +/- 0.02 to 0.26 +/- 0.03 microl/min/mmHg (44%, p = .02). Topical epinephrine did not significantly affect uveoscleral outflow or episcleral venous pressure. In conclusion, multiple topical doses of epinephrine lowered intraocular pressure in human volunteers by reducing aqueous humor formation and increasing trabecular outflow facility. The increase in uveoscleral outflow suggested by other studies was not observed.

Effects of nitric oxide synthase inhibition on ciliary blood flow, aqueous production and intraocular pressure

A prior study found that inhibition of nitric oxide synthase with L-NAME causes a large, rapid decrease in IOP in anesthetized rabbits. In this follow-up study we sought to determine if this hypotensive effect was due to decreased aqueous production, possibly caused by ciliary vasoconstriction. Two protocols were performed in anesthetized rabbits. In the first protocol, mean arterial pressure (MAP) and IOP were measured by direct cannulation, and aqueous flow was measured by fluorophotometry, before and after L-NAME (5 mg kg(-1), i.v., n = 7). In the second protocol, ciliary blood flow was measured transclerally by laser Doppler flowmetry while MAP was varied mechanically over a wide range before and after L-NAME (5 mg kg(-1), i.v., n = 8). L-NAME caused a significant increase in MAP and decreases in IOP, ciliary blood flow and aqueous flow. L-NAME also caused a significant downward shift in the ciliary pressure-flow relation over the entire pressure range examined. The results indicate that L-NAME causes ciliary vasoconstriction and decreases aqueous production, suggesting that the L-NAME ocular hypotensive effect may be due in part to a blood flow-dependent decrease in aqueous production. However, assuming no uveoscleral outflow and constant episcleral venous pressure and outflow facility, the decrease in aqueous flow accounts for 66% of the drop in IOP, suggesting an additional effect of L-NAME on aqueous outflow.

Microdialysis and drug delivery to the eye

The eye presents unique challenges in both the development of tools for elucidating drug disposition as well as for the development of modes of drug delivery for treatment of ocular diseases. In this paper, we present a discussion of the anatomical and physiological characteristics and limitations present in the eye for microdialysis sampling of endogenous substrates and xenobiotics. To date, over twenty papers describing microdialysis approaches for assessment of ocular drug delivery and endogenous substrate characterization have been published. Although the majority of papers describe sampling of vitreous humor, recent efforts have been directed towards ocular anterior segment sampling using microdialysis. With this approach, an appreciable reduction in animal use has been realized. In addition, simultaneous examination of administered drug and endogenous substrates modulated by the drug is possible with this approach, facilitating construction of ocular pharmacokinetic/pharmacodynamic relationships through use of relevant surrogate markers.

Pharmacological therapy for glaucoma: a review

For some time the medical treatment of glaucoma has consisted of topical beta-blockers, adrenergic agents, miotics and oral carbonic anhydrase inhibitors (CAIs). However, the therapeutic arsenal available for the medical treatment of glaucoma has recently extended with new classes of ocular hypotensive agents i.e. prostaglandins, local CAIs and alpha2-adrenergic agents. Beta-blockers are still the mainstay in glaucoma treatment and are first line drugs. However, even if they are applied once daily, as with timolol in gel forming solution and levobunolol, the possible cardiopulmonary adverse effects of beta-blockers remain a cause for concern. When monotherapy with beta-blockers is ineffective in reducing intraocular pressure (IOP) or is hampered by adverse effects, a change of monotherapy to prostaglandins, local CAIs, alpha2-adrenergic agonists (brimonidine) or to dipivalyl epinephrine is advised. Prostaglandins, local CAIs and alpha2-adrenergic agonists, such as brimonidine, may in time become first line drugs because they reduce IOP effectively and until now systemic adverse effects have rarely been reported with these agents. The development of a pro-drug of either a local CAI or an alpha2-adrenergic agonist with a sustained and continuous effect on IOP level, which could be applied once a day is suggested. Because of these new developments, miotics, i.e. pilocarpine and carbachol, are recommended as second or third line drugs. The cholinesterase inhibitors are considered third line drugs as better agents with fewer local and systemic adverse effects have become available. Oral CAIs may be used temporarily in patients with elevated IOPs e.g. postsurgery or post-laser, or continuously in patients with glaucoma resistant to other treatment. Combining ocular hypotensive drugs is indicated when the target pressure for an individual patient cannot be reached with monotherapy. Combination therapy of beta-blockers is additive with prostaglandins, topical CAIs and miotics. Prostaglandins such as latanoprost can be combined with beta-blockers, adrenergic agents, local CAIs and miotics. Combinations with brimonidine or local CAIs need further investigation. Treatment of glaucoma with the new ocular hypotensive agents, either in monotherapy or combination therapy, may provide lower IOPs and delay or postpone the need for surgery.

Cell signaling in bovine ciliary epithelial organ culture

The ciliary epithelium secretes aqueous humor, an intraocular fluid whose production is regulated in part by transmembrane signaling pathways including those mediated by G protein-coupled receptors. Many drugs, such as beta-adrenergic receptor (AR) antagonists and alpha2-AR agonists, are used to lower intraocular pressure by presumably decreasing fluid transport across this epithelium. Hence, our purpose was to establish a ciliary epithelial organ culture system suitable for the study of cell signaling pathways. A trypsin-mediated dissection method was established to isolate bovine ciliary epithelial sheets. These sheets were cultured in a 5% CO2 incubator. The quality was assessed by light microscopy, by protein analysis, and by the evaluation of epinephrine-mediated phosphoinositide turnover. The cultured epithelial explants were viable as evidenced by minimal trypan blue staining. The explants were composed primarily of nonpigmented cells and some pigmented cells, but no other ciliary body tissues were present on histology. Membrane preparations showed proteins with a distribution from 31 to 116 kDa. Epinephrine caused a dose-dependent increase in [3H]inositol phosphates (InsPs) accumulation with a maximal increase of two- to three-fold over basal levels. This epinephrine-mediated increase was inhibited by prazosin. We established an organ culture system of isolated bovine ciliary epithelium suitable for the study of transmembrane signaling pathways.

Effect of brinzolamide and dorzolamide on aqueous humor flow in human eyes

PURPOSE

To measure the relative efficacy of brinzolamide hydrochloride 1% ophthalmic suspension, a new carbonic anhydrase inhibitor, compared with the currently used dorzolamide hydrochloride 2% ophthalmic solution as suppressors of aqueous humor flow in human eyes, and to study the difference of effect during the day and at night.

METHODS

A randomized, double-masked, placebo-controlled study of 25 normal human subjects was carried out at Mayo Clinic. The daytime rate of aqueous humor flow was measured every 2 hours from 8 AM to 4 PM by means of fluorophotometry. Likewise, the night-time rate of aqueous humor flow was measured every 2 hours from 12 AM to 6 AM. Intraocular pressure was measured at 4 PM and 6 AM.

RESULTS

Brinzolamide reduced aqueous flow by 0.47+/-0.20 microl per min (mean+/-SD) during the day, whereas dorzolamide reduced flow by 0.34+/-0.20 microl per min. Brinzolamide reduced aqueous flow by 0.16+/-0.12 microl per min during the night, whereas dorzolamide reduced flow by 0.10+/-0.13 microl per min. Brinzolamide reduced afternoon intraocular pressure by 1.5+/-1.1 mm Hg, and dorzolamide reduced afternoon intraocular pressure by 1.1+/-1.0 mm Hg. Brinzolamide reduced the morning awakening intraocular pressure by 0.3+/-1.6 mm Hg, and dorzolamide reduced it by 0.8+/-1.0 mm Hg.

CONCLUSIONS

Our data support the idea that brinzolamide is at least as efficacious as dorzolamide as a suppressor of aqueous humor flow in normal human eyes and that there is probably not a clinically significant difference between the two drugs in this efficacy. Clinicians who prescribe brinzolamide should expect similar ocular hypotensive responses from brinzolamide and dorzolamide.

Combined corticosteroid and catecholamine stimulation of aqueous humor flow

PURPOSE

Epinephrine is known to stimulate aqueous humor flow in humans. Corticosteroids are known to augment the effect of beta-adrenergic agonists on the ciliary body. This experiment was carried out to determine whether a corticosteroid can increase the stimulatory effect of epinephrine on aqueous humor flow.

METHODS

Twenty human volunteers were studied for 24 hours. Hydrocortisone was given orally and epinephrine was given intravenously during sleep while aqueous flow was monitored. Flow was compared with a second 24-hour study when oral and intravenous placebos were given. The sequence of administration was randomized. Subjects and investigators were masked. The flows also were compared with a previously published study in which the same dose of epinephrine was administered without steroid.

RESULTS

Epinephrine plus hydrocortisone compared with placebos increased aqueous flow 42% in subjects during sleep. The combination of epinephrine and hydrocortisone was a more potent stimulus to aqueous flow than epinephrine alone, which increased aqueous flow by only 27% (P = 0.045).

CONCLUSION

The two major hormones of the adrenal gland work in concert to increase the rate of aqueous humor flow in humans.

Long term effect of apraclonidine

AIMS

To evaluate the effect of the chronic use of apraclonidine 0.5% on the intraocular pressure (IOP) of patients with glaucoma; also, to study the side effect profile of this drug when used chronically.

METHODS

All patients who had uncontrolled IOP, who were either already on glaucoma medications, or who were intolerant of other glaucoma medications were enrolled. A total of 185 patients were started on apraclonidine 0.5% two to three times a day in one eye.

RESULTS

Follow up extended to 35 weeks. The mean difference in IOP between treated and control eyes was 2.1 (SD 5.0) mm Hg. A similar IOP lowering effect was obtained comparing IOP difference from baseline in the treated eye only.

CONCLUSION

By the end of the follow up period, 46% of patients were still on the medication. The drug was stopped in 23% of patients because of side effects and in 31% of patients because of failure to lower IOP significantly.

Evaluation of adverse reactions of aponidine hydrochloride ophthalmic solution

We prospectively evaluated the adverse reactions of apraclonidine in 20 normal volunteers by instilling a single drop of 1% apraclonidine in their right eyes. Examinations, including blood pressure, pulse rate, conjunctiva and cornea, intraocular pressure (IOP), pupil diameter, basal tear secretion and margin reflex distance of both upper and lower eyelids, were performed prior to entry and at 1, 3, 5 and 7 hours after instillation. The ocular hypotensive effects were statistically significant for apraclonidine-treated eyes throughout the study and also statistically significant for contralateral eyes from three hours after topical administration of 1% apraclonidine. Decreases in systolic blood pressure were statistically, but not clinically, significant. No significant changes in diastolic blood pressure, pulse rate and basal tear secretion were noted. Conjunctival blanching and mydriasis were commonly found. Upper lid retraction was frequently noted. While the elevations of the upper lid margin in most subjects were not more than 2 mm and did not cause noticeable change in appearance, one subject suffered from mechanical entropion and marked corneal abrasion 3 hours after instillation of the medication. This may well be a particularly notable finding in Asian people.

Dexmedetomidine as intramuscular premedication for day-case cataract surgery. A comparative study of dexmedetomidine, midazolam and placebo

The effects of dexmedetomidine 1.0 microgram.kg-1, midazolam 20 micrograms.kg-1 and saline placebo were assessed in a double-blind, randomised study in 90 patients undergoing day-case cataract surgery under regional anaesthesia. The trial drug was injected into the deltoid muscle 45 min before the peri-ocular block. Dexmedetomidine 1.0 microgram.kg-1 decreased intra-ocular pressure before, during and after surgery. The maximum reduction in mean (SD) intra-ocular pressure occurred in the dexmedetomine group just before discharge from hospital (17.7 (2.8) mmHg to 11.5 (2.9) mmHg) (p < 0.001 compared with midazolam and placebo). In contrast, midazolam did not differ from saline placebo. Dexmedetomidine and midazolam produced a similar sedative effect of short duration. Dexmedetomidine induced a moderate decrease in blood pressure (p < 0.001 compared with placebo) and a slight but statistically significant decrease in heart rate throughout the study period (p < 0.001 compared with placebo). Dexmedetomidine 1.0 microgram.kg-1 intramuscularly, effectively reduced intra-ocular pressure and produced short-acting sedation with marginal cardiovascular effects; it may be a useful premedicant drug for elderly patients undergoing day-case cataract surgery under regional anaesthesia.

Ocular alpha 2-receptor subclasses and antiglaucoma efficacy

An overview of the ocular hypotensive actions of some alpha 2-agonists with imidazoline structures is presented. These agents inhibit isoproterenol-stimulated adenylate cyclase (AC) activity in ciliary process membrane through a Na+ and GTP-dependent mechanism. Receptor binding studies with the alpha 2-agonist radioligand [125I] p-iodoclonidine ([125I]PIC) in rabbit ciliary body membranes indicate that the alpha 2-receptor subtype is alpha 2A. Gpp(NH)p and NaCl dose-dependently decreased the number of [125I]PIC binding sites by shifting the receptor-G protein complexes from the high affinity state to the low affinity state for agonist binding. This is consistent with the observations that inhibition of AC was Na+ and GTP dependent. The NaCl and Gpp(NH)p effects on binding appeared to be through different mechanisms. The alpha 2-receptor in ciliary process thus appears to be an alpha 2A-receptor that is negatively coupled to the AC-cAMP generating system.

Regulation of adenylyl cyclase in rabbit iris ciliary body

The enzyme adenylyl cyclase has been shown to be important in the regulation of intraocular pressure. We therefore studied the activity of adenylyl cyclase (AC) activity in the rabbit iris/ciliary body (I/CB) after pre-treatment with the beta-adrenergic agonist isoproterenol (ISO) which activates cAMP dependent protein kinase A, and phorbol 12,13 dibutyrate (PDB) which activates protein kinase C. When I/CB was pre-treated with ISO (10 microM) or PDB (1 microM), attenuated AC activity (approximately 35%) resulted when the activity of the enzyme was assessed by rechallenge with isoproterenol. However, when AC activity was assessed by rechallenge with forskolin or prostaglandin, enhanced activity resulted. In an effort to identify the mechanism of this apparent heterologous regulation of AC, studies were performed that showed no significant changes in the density of beta-adrenergic receptors or the affinity of the receptors for the ligand (125I)-Iodopindolol occurred in ISO or PDB treated tissue. Similarly, in membranes prepared from ISO or PDB treated tissue, no significant changes in the functional activity of the guanine nucleotide binding proteins Gi or Gs could be ascertained as assessed by somatostatin inhibition of forskolin-stimulated AC (to assess Gi function), or in an adenylyl cyclase complementation assay (to assess Gs function). However, AC activity stimulated by Mn2+ and purified Gs was enhanced (approximately 2X) following isoproterenol or phorbol ester pre-treatment, suggesting that an alteration at the level of the catalytic subunit of AC resulted from ISO or PDB pretreatment. Therefore, the assessment of net changes in receptor coupled AC activity induced by phorbol esters or isoproterenol appears to be dependent on the drug used to rechallenge the AC system and cAMP production is dependent on the sum of diverse effects on multiple components of the AC pathway.

Apraclonidine and clonidine: a comparison of efficacy and side effects in normal and ocular hypertensive volunteers

We performed a prospective, randomized double blind study comparing the cardiovascular and intraocular pressure (IOP) effects of unilateral therapy with clonidine 0.125% and apraclonidine hydrochloride 1.0% in 15 normal and 15 ocular hypertensive volunteers. Baseline values were obtained prior to instillation. One drop of test medication (clonidine, apraclonidine or placebo) was instilled unilaterally, and the post-instillation measurements were taken at 1, 2, 4, 6 and 8 hours. Apraclonidine 1% produced a maximum 31.4% +/- 6.9% (4.83 +/- 1.17 mmHg) decrease in mean IOP in ocular normotensive volunteers and 33.9% +/- 6.9% (10.10 +/- 2.45 mmHg) in ocular hypertensive patients (p < 0.001). These values were 22.1% +/- 6.9% (2.90 +/- 1.94 mmHg) and 22.7% +/- 6.9 (6.80 +/- 2.31 mmHg), respectively in clonidine group (p < 0.001). In apraclonidine group, there were no changes in contralateral IOP, blood pressure or pulse rate. Clonidine produced a significant decrease in contralateral IOP, but this reduction was not statistically significantly different than that of placebo. In clonidine group, there was no change in pulse rate, but a significant decrease in blood pressure. Eyelid retraction, conjunctival blanching and mydriasis were noted in eyes treated with apraclonidine. However there were no statistically and clinically significant changes in pupil size or interpalpebral fissure width with clonidine. This study suggests that apraclonidine appears to be safer and more effective ocular hypotensive agent than clonidine in treatment of glaucoma.

New developments in the drug treatment of glaucoma

This article reviews standard treatment modalities for patients with glaucoma and describes 3 classes of drugs which are undergoing development: apraclonidine (aplonidine, ALO 2145), an alpha 2-adrenergic agonist which has been released for clinical use; topical carbonic anhydrase inhibitors, a modification of the systemic carbonic anhydrase inhibitors currently in use; and prostaglandins (PGs), a new class of drugs with topical ocular hypotensive activity. Standard treatment modalities include parasympathomimetic agents such as pilocarpine, carbachol, and phospholine iodide, which lower intraocular pressure (IOP) by increasing aqueous outflow through the trabecular meshwork. A newer form of pilocarpine as a gel produces a longer action. Adrenergic agonist medications, such as epinephrine (adrenaline) and its prodrug dipivefrine (dipivalyl epinephrine), function by increasing uveoscleral outflow and trabecular outflow facility. A decrease in aqueous formation by the ciliary processes is thought to be the mechanism of action of beta-adrenoceptor antagonists, but the physiological basis for this action has not been clearly demonstrated. A newer beta-blocker, betaxolol, has relatively selective beta 1-blocking activity. Carbonic anhydrase inhibitors are nonbacteriostatic sulphonamide derivatives which decrease aqueous formation by the ciliary body. Almost 50% of patients taking these medications are unable to tolerate them because of their adverse effects, and there is thus much interest in the development of a topical carbonic anhydrase inhibitor with the potential for fewer adverse effects. MK 507 is the most recent and most potent compound in the series of topically active carbonic anhydrase inhibitors. Apraclonidine hydrochloride is a derivative of clonidine hydrochloride, an alpha 2-adrenergic agonist. Clonidine has previously been shown to lower IOP significantly, but has the potential to produce marked lowering of both systolic and diastolic blood pressures. Its major ocular effect appears to be a decrease in aqueous production. The structural modification to apraclonidine decreases corneal absorption and the drug's ability to cross the blood-brain barrier, minimising the risk of centrally mediated cardiovascular side effects. Apraclonidine may also influence secondary avenues of aqueous outflow, such as uveoscleral outflow, and may also affect conjunctival and episcleral vascular flow. It produces a mean decrease in IOP of 25% for as long as 12 hours. Adverse effects include blanching of the conjunctiva, minimal mydriasis and eyelid retraction. This drug has been approved in the US for use in prevention of elevated IOP after argon laser trabeculoplasty and iridotomy, and has potential uses in preventing an IOP rise after YAG laser posterior capsulotomy and cataract surgery in patients already on other antiglaucomatous medications.(ABSTRACT TRUNCATED AT 400 WORDS)

Corneal and conjunctival/scleral penetration of p-aminoclonidine, AGN 190342, and clonidine in rabbit eyes

The ocular penetration pathways of three alpha 2-adrenergic agents (p-aminoclonidine, AGN 190342, and clonidine) were investigated in rabbits both in vitro and in vivo. The corneal permeabilities of the compounds correlated positively with their octanol/water distribution coefficients. The ocular drug absorption via corneal and conjunctival/scleral penetration routes was evaluated separately after drug perfusion in vivo. In most cases, the corneal route was the major pathway for the intraocular drug absorption. However, the conjunctival/scleral penetration pathway was the predominant pathway for the delivery of p-aminoclonidine, the least lipophilic compound among the three drugs, to the ciliary body. The drug concentration in the iris was contributed mainly by the corneal route and correlated well with drug lipophilicity.

Neuropeptide Y and somatostatin inhibit stimulated cyclic AMP production in rabbit ciliary processes

The interaction of adrenergic and peptide receptors linked to adenylate cyclase and the inhibition by bioactive peptides of stimulated cyclic AMP production has been investigated in intact, excised rabbit ciliary processes. Cyclic AMP production stimulated by isoproterenol, vasoactive intestinal peptide, or forskolin was inhibited by the biologically active peptides neuropeptide Y, somatostatin, and the synthetic somatostatin analogue SMS 201-995. IC50s determined from dose-response curves of inhibition are consistent with the known abilities of these ligands to modulate cyclic AMP and physiological responses in other tissues. Inhibition by neuropeptide Y or SMS 201-995 was unaffected by the specific alpha 2-adrenergic antagonist yohimbine, which shows that peptide inhibition is not occurring via peptide binding to the inhibitory alpha 2-adrenergic receptor. These results suggest that endogenous peptides may participate in modulation of cyclic AMP production and subsequent physiological events influenced by cyclic AMP levels in rabbit ciliary processes by inhibiting stimulated cyclic AMP synthesis.

A limited comparison of apraclonidine's dose response in subjects with normal or increased intraocular pressure

We performed a multicentered, placebo-controlled, randomized, crossover study comparing the efficacy of 0.5% and 1.0% apraclonidine hydrochloride in 15 normal volunteers and 17 subjects with increased intraocular pressure. Apraclonidine 1% produced a maximum 30.4% +/- 14.0% (4.7 +/- 2.4 mm Hg) decrease in mean intraocular pressure in normal eyes and a 31.3% +/- 16.5% (7.6 +/- 4.2 mm Hg) decrease in eyes with increased pressure. Apraclonidine 0.5% produced a maximum 25.8% +/- 9.7% (4.0 +/- 1.7 mm Hg) decrease in mean intraocular pressure in normal eyes and a 27.4% +/- 16.0% (6.8 +/- 4.5 mm Hg) decrease in eyes with increased pressure. There was no statistically significant difference in mean percent intraocular pressure lowering effect between the 0.5% and 1.0% apraclonidine concentrations. Most subjects treated with apraclonidine had a greater than or equal to 20% reduction in intraocular pressure from baseline. Twelve hours after instillation of apraclonidine, nine of the normal volunteers had an intraocular pressure of 10 mm Hg or less. Apraclonidine produced the same percent intraocular pressure decrease regardless of the initial level of intraocular pressure.

Alpha 2-adrenergic and VIP receptors in rabbit ciliary processes interact

The interaction between alpha 2-adrenergic and VIP receptors has been studied by examining inhibition of VIP-stimulated cyclic AMP production by adrenergic agonists in intact, excised rabbit ciliary processes. Epinephrine, norepinephrine, isoproterenol, dopamine, and the specific alpha 2-adrenergic agonists clonidine and p-aminoclonidine exhibit dose-dependent inhibition of VIP-stimulated cyclic AMP production. I50s, clonidine (0.05 microM) = p-aminoclonidine (0.05 microM) congruent to epinephrine (0.1 microM) less than norepinephrine (2.0 microM) less than isoproterenol (15 microM) = dopamine (15 microM), are consistent with the characteristic binding affinities of these adrenergic agonists for alpha 2-adrenergic receptors. Inhibition of VIP-stimulated cyclic AMP production by clonidine, epinephrine, isoproterenol, and dopamine is blocked by yohimbine but not by prazosin. These data establish the alpha 2-adrenergic specificity of the inhibitory effects observed. We have previously shown that beta 2-adrenergic receptor-mediated stimulation of cyclic AMP production in rabbit ciliary processes is also inhibited by postjunctional alpha 2-adrenergic receptors. These studies support the idea that the catecholamines may regulate aqueous humor formation by inhibiting stimulation of cyclic AMP production via postjunctional alpha 2-adrenergic receptors in ciliary processes.

Aqueous flow is reduced by the alpha-adrenergic agonist, apraclonidine hydrochloride (ALO 2145)

Apraclonidine hydrochloride (ALO 2145), a clonidine derivative that does not cross the blood-brain barrier, was applied topically to one eye of each of 20 normal human subjects. The rate of aqueous humor flow and the permeability of the blood-aqueous barrier were measured by fluorophotometry. Four hours after administration, the flow rate in the apraclonidine-treated eyes was 35% lower than that measured in the control eyes. Three hours after instillation, the intraocular pressure (IOP) was 34% lower in the apraclonidine-treated eyes when compared with control eyes. Both these differences were statistically significant. The drug had little, if any, effect on blood-aqueous permeability.

ALO 2145 reduces the intraocular pressure elevation after anterior segment laser surgery

The effect of an alpha adrenergic agonist, ALO 2145 (para-amino-clonidine [PAC]), was examined in a double-masked, multicenter study on the postoperative intraocular pressure (IOP) elevation after laser surgery in 165 patients (83 trabeculoplasty, 36 iridotomy, and 46 capsulotomy). One drop of 1.0% ALO 2145 or vehicle was instilled 1 hour before and immediately after laser surgery. The mean IOP increase in the ALO 2145-treated group was lower (P less than 0.05) than in the placebo group at each of the first three postoperative hours. Overall, 18% of placebo-treated eyes experienced IOP increases greater than or equal to 10 mmHg as compared with 4% of ALO 2145-treated eyes (P less than 0.003). Ocular and systemic side effects were minimal and did not differ between the treatment groups. ALO 2145 safely and effectively reduced the incidence and magnitude of potentially harmful IOP elevations after anterior segment laser surgery.

Ocular effects of a relatively selective alpha 2 agonist (UK-14, 304-18) in cats, rabbits and monkeys

UK-14, 304-18 (UK), a relatively selective alpha 2-agonist, was examined for its effects on intraocular pressure (IOP) and pupil diameter (PD) in rabbits, cats and monkeys and on noradrenergic function in the cat nictitating membrane (CNM) preparation. Topical, unilateral administration of UK (0.0005-0.5 mg) produced dose-dependent decreases in IOP and pupil size in normal, unanesthetized rabbits, cats and monkeys. The ocular hypotensive effect of UK in the ipsilateral eye was delayed relative to the contralateral eye in all three species; UK produced an initial transient ocular hypertension in rabbits which was abolished by surgical transection of three major extraocular muscles. Mean arterial blood pressure in rabbits was not affected by 0.005 mg UK topically. The ocular hypotensive and miotic effects of UK were attenuated in superior cervical ganglionectomized (SX) cats and rabbits. Intra-arterially administered UK (0.33, 1.0, 3.3 and 10 micrograms) produced dose-related systemic hypotension and inhibition of contractions of the CNM elicited by electrically stimulating the pre- and postganglionic sympathetic trunks in the urethane/chloralose anesthetized cat. This inhibition was reversed and prevented by 300 micrograms rauwolscine but not by 300 micrograms domperidone. UK also enhanced the contractile response of the CNM to injected norepinephrine (10 micrograms). UK suppressed ocular hypertension induced by water loading and IOP recovery rate following hypertonic saline infusion in rabbits suggesting that aqueous flow was inhibited. These results indicate that UK lowers IOP, in part, by suppressing sympathetic neuronal function which causes a reduction in aqueous flow.

Measurement of aqueous humor flow with scanning ocular fluorophotometers

A one-dimensional scanning fluorophotometer (Fluorotron Master) and a two-dimensional scanning fluorophotometer (McLaren and Brubaker) were used to measure rate of aqueous humor flow in normal volunteers. The precision of three protocols of dye application and flow determination was investigated. In all three protocols rate of flow was determined by observing the rate of disappearance of topically-applied fluorescein during an 8-hour period. The three protocols studied were: 1) Two drops of 10% fluorescein washed away five minutes later immediately before beginning measurements of fluorescence. 2) Application of 0.25% fluorescein every 5 minutes for 30 minutes, 6 hours before beginning measurements. 3) Iontophoresis for 7 seconds and irrigation of eye immediately before beginning measurements. Rate of flow was calculated by the methods of Jones and Maurice. The results were compared to measurements made with a slit-lamp fluorophotometer using the technique described by Coakes and Brubaker. The most satisfactory method of measuring flow when either scanning fluorophotometer was employed was to instill fluorescein many hours before measurements begin (protocol 2).