Continuous, simultaneous, and instant display of aqueous humor dynamics with a micro-spectrophotometer and a sensitive drop counter

Patterns of X and Y optic nerve fibre terminations in the dorsal lateral geniculate nucleus of the cat

The distributions of X and Y optic nerve fibre terminals in the A and A1 laminae of the dorsal lateral geniculate nucleus (LGNd) of the cat have been determined by a method that eliminates the Y fibres. A pressure-blocking technique was used in a sterile operation to produce anterograde degeneration in the Y fibres with minimal effect on the X fibres. Subsequently the Fink/Heimer technique was used to stain for degenerating fibres. This showed a strong peak of degeneration in the ventral regions of the laminae. Tritiated leucine was injected into one eye either of a normal cat or of one in which the optic nerve had been pressure-blocked at least one week previously. Subsequent examination of the LGNd by autoradiography showed a more uniform distribution of label in the laminae deprived of Y input (i.e. the pattern of distribution of X fibres). Subtraction of this distribution from that produced in a normal cat (i.e. X + Y input) gave the Y distribution. As in the degeneration studies, this revealed a peak of label in the most ventral part of each lamina but also showed a smaller peak in the most dorsal regions.

Estimation of lung liquid production in fetal sheep with blue dye dextran and radioiodinated serum albumin

Lung liquid production and reabsorption rates and lung volumes were measured in 99 fetal sheep (119-148 days of gestation) by indicator-dilution methods with the simultaneous use of blue dye dextran (BDD) and radioiodinated serum albumin (RISA). There were no significant differences between rates of lung liquid production or reabsorption by the two methods (n = 71 pairs; paired t-test; Wilcoxon test; ANOVA); this was equally true for rates in milliliters per hour or milliliters per kilogram body weight per hour and was independent of age. Volumes measured by both methods showed a close linear relationship (r = 0.97; for slope P < 0.0001; n = 99), whether expressed as milliliters or milliliters per kilogram body weight. Either method could give the higher volume. Values differed by only approximately 4%, independent of age or parameter (ml or ml/kg body wt; volumes regressed to original volume, or as measured in untreated control hours). However, this small difference was significant by paired t-test or Wilcoxon test when all data were combined irrespective of age; it was not significant after allowance for gestational age (two-way ANOVA). Both indicators showed the same increase in lung volume toward birth and the same fall when related to body weight (slopes significant P = 0.0003-0.0004; r = 0.93). Two-way ANOVA showed that the declines were significant (P = 0.003). The data suggest that 1) there was no significant difference in production or reabsorption rates measured by BDD or RISA, 2) differences in volumes measured by the two indicators were only significant if gestational age was ignored and were too small to have physiological importance, and 3) although BDD and RISA each may have methodological weaknesses, for purposes of measuring lung liquid volumes both are sufficiently accurate and reproducible to obtain meaningful physiological results.

Timolol may inhibit aqueous humor secretion by cAMP-independent action on ciliary epithelial cells

The beta-adrenergic antagonist timolol reduces ciliary epithelial secretion in glaucomatous patients. Whether inhibition is mediated by reducing cAMP is unknown. Elemental composition of rabbit ciliary epithelium was studied by electron probe X-ray microanalysis. Volume of cultured bovine pigmented ciliary epithelial (PE) cells was measured by electronic cell sizing; Ca(2+) activity and pH were monitored with fura 2 and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, respectively. Timolol (10 microM) produced similar K and Cl losses from ciliary epithelia in HCO/CO(2) solution but had no effect in HCO/CO(2)-free solution or in HCO/CO(2) solution containing the carbonic anhydrase inhibitor acetazolamide. Inhibition of Na(+)/H(+) exchange by dimethylamiloride in HCO/CO(2) solution reduced Cl and K comparably to timolol. cAMP did not reverse timolol's effects. Timolol (100 nM, 10 microM) and levobunolol (10 microM) produced cAMP-independent inhibition of the regulatory volume increase (RVI) in PE cells and increased intracellular Ca(2+) and pH. Increasing Ca(2+) with ionomycin also blocked the RVI. The results document a previously unrecognized cAMP-independent transport effect of timolol. Inhibition of Cl(-)/HCO exchange may mediate timolol's inhibition of aqueous humor formation.

Safety and ocular hypotensive efficacy of a single dose of metoclopramide or droperidol in healthy subjects

The purpose of this study was to evaluate the IOP lowering effect of two topical dopamine antagonists, metoclopramide or droperidol, in healthy subjects. Forty healthy volunteers were randomly assigned to receive, in double-masked fashion, topical administration of a single drop of 0.5% metoclopramide or 0.25% droperidol, with the fellow eye receiving isotonic saline as placebo. IOP was measured before and 1, 3, 5, and 7 hours after instillation of drugs. Ocular irritation and conjunctival congestion were also recorded at the time of each measurement. In the metoclopramide group, the maximal mean percentage change in IOP was -14.4% in tested eyes as compared with -10.8% in placebo-treated eyes 3 hours after instillation. All the changes were not significantly different between the metoclopramide-treated and the placebo-treated eyes at all time points. In the droperidol group, the maximal mean percentage change in IOP was -19.6% in tested eyes as compared with -17.7% in placebo-treated eyes at 3 hours after instillation. There was also no significant difference between the droperidol-treated and the placebo-treated eyes. None of the volunteers reported ocular irritation or conjunctival congestion after instillation of the drugs. In conclusion, while topical droperidol or metoclopramide tended to lower IOP in healthy subjects, the decrease in IOP did not differ significantly from that in placebo-treated eyes. Both drugs appear to be safe. Further studies with larger numbers of subjects at higher doses in glaucomatous subjects are needed before definite conclusions on drug efficacy can be drawn.

Lung-liquid production in vitro by lungs from fetal guinea pigs: effects of amiloride on responses to aldosterone

Lungs from near-term fetal guinea pigs (62 ± 2 days of gestation) were supported in vitro for 3 h; lung-liquid production was monitored by a dye-dilution method based on Blue Dextran 2000. Untreated preparations produced fluid at 1.26 ± 0.14 mL∙kg−1 body mass∙h−1, with no significant change over the ensuing hours (ANOVA, regression analysis; n = 16). Experimental preparations received aldosterone at plasma concentrations reported to be present at birth. Aldosterone produced rapid, significant reductions in fluid production, and occasionally reabsorptions, which persisted beyond treatment. Reductions during treatment were as follows: 10−8 M aldosterone, 90.8 ± 4.9% (P < 0.001; n = 4); 2 × 10−9 M aldosterone, 64.1 ± 16.6% (P < 0.05–0.001; n = 6), and 7 × 10−10 M aldosterone, 48.6 ± 11.7% (P < 0.005–0.001; n = 6). The linear log dose response curve (r = 0.99) showed a theoretical threshold at 3.4 × 10−11 M aldosterone. Responses to 7 × 10−10 M aldosterone were abolished by 10−6 M amiloride. At the highest concentration of aldosterone (10−8 M), 10−6 M amiloride significantly reduced responses, and the changes were no longer significant by ANOVA. At both high and low aldosterone concentrations, responses with amiloride were significantly lower than those without amiloride (ANOVA, P < 0.03–0.04). Amiloride controls and untreated preparations showed no significant changes in fluid production. It is concluded that aldosterone at plasma concentrations present at birth can cause reductions in lung-liquid production or reabsorption through effects on amiloride-sensitive Na+ channels, and that the responses are remarkably rapid.

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.

Chirality and drug targeting: pros and cons

The theoretical basis of stereoselectivity in drug targeting in terms of eudismic ratio and quantitative structure-activity relationship has been presented. Specific examples of the advantages of using the correct stereoisomers (eutomers) rather than the distomers or racemic mixtures have been discussed. With the recent development of new methods of chiral synthesis and chiral analysis, it is not longer justifiable to continue to use racemic mixtures as therapeutic agents, unless it can be proven to be safe to do so. In the area of using peptides, proteins, glycoproteins and polysaccharides as therapeutic agents, mother nature has been very selective in choosing the optically active starting materials (Bradley, 1994), it is a necessity to design and test the proper optical isomer when a mimetic or antagonist compound is contemplated as a therapeutic agent.

Treatment of open angle glaucoma and ischemic retinopathy with dopamine antagonists

Ocular blood flow in the retina, choroid and optic nerve head should be measured in addition to the intraocular pressure (IOP) in order to better diagnose glaucoma disease and to better evaluate the efficacy of antiglaucoma drugs. It was found that beta-adrenergic blockers, the most widely used antiglaucoma drugs, reduced ocular blood flow in animals and worsened glaucoma disease in some patients, even though the IOP was reduced to the normal range. Therefore, their use for glaucoma treatment requires careful re-evaluation. On the other hand, most dopamine antagonists were found to reduce the IOP and increase the blood flow to the retina, choroid, iris and ciliary muscle. Therefore, these agents could be used for glaucoma treatment. Further, dopamine antagonists were found to improve retinal function after ischemia and, thus, also could be used for the treatment of ischemic retinopathy.

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 EC50 (from 2.06 x 10(-3) M to 3.63 x 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.

Effects of D-timolol and L-timolol eye drops on intraocular pressure and aqueous flow. A dose-response study in normal eyes

The effect on intraocular pressure of 0.5, 1.0, 2.0 or 4.0% D-timolol and 0.25 or 0.5% L-timolol was determined in healthy volunteers after a single topical application. A dose-response relationship for D-timolol could be established. The effect on a diurnal pressure curve and on aqueous humour production was about twice as large with 0.25% L-timolol as with 4.0% D-timolol.

Development of D-timolol for the treatment of glaucoma and ocular hypertension

It has been found that D-timolol is equipotent or slightly less potent than L-timolol to lower the intraocular pressure (IOP) in normotensive rabbits, water loaded ocular hypertensive rabbits, alpha-chymotrypsin induced glaucoma rabbits, hypertonic saline infused IOP recovery model of rabbits, normotensive human volunteers, glaucoma patients and ocular hypertensive human individuals. Although L-timolol has been used widely for the treatment of glaucoma and ocular hypertension, it produces numerous side effects including cardiovascular disturbances, asthmatic attack, psychological depression, etc. Since D-timolol has much weaker affinity toward beta-adrenergic receptors, it was found to have 1/80-1/300 the beta-adrenergic blocking potency of L-timolol to block beta-adrenergic receptors in guinea pig tracheal preparations and 1/90 of L-timolol to block beta-adrenergic receptors in guinea pig atrial preparations. As a result, D-timolol showed no subjective nor objective side effects on pupil size, conjunctiva, cornea, blood pressure and pulse rate. Further, D-timolol was reported to increase retinal and choroid blood flow in rabbits without affecting overall ocular blood flow. On the contrary, L-timolol was found to significantly reduce the overall ocular blood flow and retinal and choroid blood flows in rabbits, although it might slightly increase the retinal blood flow in normotensive individuals. D-Timolol was well absorbed across the cornea as L-timolol and produced the duration of action as long as L-timolol. These results indicate that D-timolol could be a better agent than L-timolol for the treatment of glaucoma and ocular hypertension.

Effects of D-timolol and L-timolol on ocular blood flow and intraocular pressure

Effects of D-timolol and L-timolol on IOP were compared with two rabbit models. When the drug solution was injected into vortex vein, 1% D-timolol produced ocular hypotension just like 0.5% L-timolol except D-timolol was less potent than L-timolol to lower the IOP. On the other hand, when 0.5% of D-timolol and L-timolol were instilled into the rabbit eye on IOP recovery model both agents showed equipotency to delayed the IOP recovery. Effects of D-timolol and L-timolol on ocular blood flow were also studied with two rabbit models. D-Timolol at 0.5% did not affect the ocular pulsatile blood flow measured with Langham's pneumatonometer whereas 0.5% L-timolol significantly suppressed it. D-Timolol (0.5%) was found to increase retinal and choroidal blood flows measured with laser Doppler method whereas L-timolol suppressed it. These results indicate that D-timolol though less potent than L-timolol to lower IOP, is superior over L-timolol to improve the blood flow in retina and choroid.

Effects of D-timolol on intraocular pressure (IOP), beta blocking activity, and the dynamic changes of drug concentrations in aqueous humor

L-timolol has been used successfully in the treatment of glaucoma. Because of its powerful beta blocking activity, however, more and more side effects have been reported. D-timolol, an optic isomer of L-timolol with much less beta blocking activity, was noted to have similar ocular hypotensive effects through inhibition of aqueous production as L-timolol. Thus, it is expected that D-timolol may reduce the side effects considerably if it is used in the clinics. In this study, we observed the ocular hypotensive effect of 0.5% D-timolol in normotensive and water-loaded rabbits as well as in patients with glaucoma or ocular hypertension. A significant reduction of intraocular pressure (IOP) which lasted at least six hours was observed. No side effects, either locally or systemically, were found. The concentration of D-timolol was traced in rabbits' aqueous humor after topical administration. The drug quickly appeared in the aqueous 30 minutes after instillation of 100 microliters 1% D-timolol. The peak concentration was 1916.5 ng/ml. It declined to a minimum in four hours, although the hypotensive effect could last at least six hours. For estimating the beta blocking potency of D-timolol, radioligand binding assay was employed to calculate the Ki value from isolated cell membrane of rabbits' lung. It showed that the beta blocking potency of D-timolol was much less than that obtained from L-timolol. It is concluded that D-timolol may be a valuable antiglaucoma agent comparable to L-timolol in lowering the IOP. The side effects induced by beta blocking agents we are using currently might be considerably reduced with D-timolol.

Timolol inhibits adenylate cyclase activity in the iris-ciliary body and trabecular meshwork of the eye and blocks activation of the enzyme by salbutamol

The enzymatic activity of adenylate cyclase in homogenates and membrane-rich fractions prepared from rabbit iris-ciliary bodies and bovine trabecular meshwork was found to be inhibited by timolol. Treatment of iris-ciliary body homogenates with Triton X-305 resulted in abolition of the inhibitory effect of the drug on the activity of the enzyme. The stimulatory effect of salbutamol on the enzyme was also susceptible to blockade by timolol. It is suggested that the hypotensive action of timolol on intraocular pressure results from structural and functional changes induced on the plasma membranes of the iris-ciliary body and trabecular meshwork by the thiadiazole group of the molecule, and, also, from the occupation of the adrenergic receptors of the iris-ciliary body by the tert-butylamino-2-hydroxypropoxy part of the compound.

Is topical haloperidol a useful glaucoma treatment?

A randomised, double blind, single dose study of topical haloperidol, a dopamine receptor blocking drug, was performed on 20 healthy volunteers. After its administration a modest reduction in intraocular pressure was recorded over the six-hour study period, but the difference was not significant at the p less than 0.05 level. Although dopamine blocking agents are effective in reducing intraocular pressure in experimental animals, topical haloperidol appears unlikely to be clinically useful in the treatment of glaucoma.

A synopsis of recent developments in antiglaucoma drugs

Open-angle glaucoma is treated primarily with drugs, some of which have been used clinically for years. These drugs include: 1) cholinergic agonists that increase aqueous humor outflow, 2) adrenergic agonists and antagonist that affect both aqueous humor formation and outflow, and 3) carbonic anhydrase inhibitors that decrease aqueous humor formation. Several new classes of drugs are being tested for efficacy and mechanism of action. They include: 1) the D-isomer of timolol that reduces aqueous humor formation without producing adrenergic blockade, 2) dopaminergic agonists and antagonists, including bromocriptine and butyrophenones that reduce intraocular pressure, and 3) cannabinoids that reduce aqueous humor formation and increase outflow. In addition, several other types of drugs, such as prostaglandins, diuretics, Na+,K+-ATPase inhibitors, and adenyl cyclase stimulators are just now beginning to be studied.

Cellular mechanisms in the actions of antiglaucoma drugs

There are several classes of drugs currently in use for the therapeutic management of the glaucomas. Although the ocular hypotensive effects of these agents have been well characterized and described, little is known of their site of action and cellular mechanism. This review attempts to describe those cellular mechanisms that may be linked to the actions of several classes of antiglaucoma drugs. Special emphasis was placed on drug actions and 1) the adenylate cyclase system; 2) receptor-coupled phosphoinositide turnover; 3) prostaglandins and 4) ion transport processes. Models are presented depicting proposed cellular sites of the interaction of the antiglaucoma drugs with these cellular processes.

Ocular effects of adrenergic stereoisomers in the rabbit

Because of the need to develop ocular hypotensive agents with low incidence of side effects, the dextrorotatory enantiomers of adrenergic agents have prompted some attention. In the present study, the relative potency and efficacy of various adrenergic stereoisomers with respect to their ocular hypotensive and mydriatic effects have been determined after topical administration to conscious rabbits. The relative order of potency was found to be, iris: 1-epinephrine much greater than d-epinephrine congruent to 1-norepinephrine congruent to dl-alpha-methyl-norepinephrine greater than dl-alpha-methyl-epinephrine greater than d-norepinephrine; intraocular pressure (IOP): l-epinephrine greater than dl-alpha-methyl-epinephrine greater than d-epinephrine greater than dl-alpha-methyl-norepinephrine greater than d-norepinephrine congruent to 1-norepinephrine. The pupil and the initial ocular hypertensive responses clearly demonstrated the phenomenon of stereoselectivity of adrenergic stimulants, in that the 1-form was relatively more potent than the d-form. However, such order of potency does not seem to hold for IOP. The apparent discrepancy and reversal of order of potency for IOP responses may be related to the hypothesis that the net ocular hypotensive effect of adrenergic agents is the summation of the initial hypertension and the delayed hypotension produced by these agents. It is anticipated that the results of the present study may improve our understanding of ocular pharmacology of adrenergic stereoisomers. The ability to separate the dose-response profiles of pupillary and IOP effects of these agents may have potential therapeutic significance and thus warrants further investigation in other species.

Existence and role of endogenous ocular melatonin

The proposed role of melatonin as an endogenously synthesized modulator of intraocular pressure in the eye was investigated. Melatonin levels were determined by radioimmunoassay in the iris, ciliary body and retina-choroid of pinealectomized or sham-operated chickens by day and by night. Pinealectomy had no effect on melatonin levels in the ciliary body or retina of chicken eyes; a diurnal rhythm continued to be observed in these tissues, with values higher by night than by day. Chloroform-extracted melatonin levels in the rabbit ciliary body showed a diurnal rhythm but melatonin levels in rabbit retina did not. Intracameral infusion of melatonin into cat eyes caused aqueous humor synthesis to decrease but caused a greater decrease in aqueous humor outflow facility, leading to a significant increase in intraocular pressure. The results suggest that melatonergic mechanisms in the eye could be responsible for the diurnal rhythm in IOP, and the synthesis and diurnal rhythm of this melatonin are independent of the pineal gland.

R-enantiomer of timolol: a potential selective ocular antihypertensive agent

Various studies were conducted to evaluate the effects of timolol, an S-enantiomer, relative to its R-enantiomer upon intraocular pressure and related ocular systems in the rabbit. The R-enantiomer was about one-third as potent as timolol in displacing 3H-dihydroalprenolol binding to iris-ciliary body tissue, reducing aqueous humor formation, and lowering intraocular pressure of alpha-chymotrypsin hypertensive eyes. In contrast, the R-enantiomer was 50 to 90 times less potent than timolol in antagonizing the effects of isoproterenol on pulmonary and atrial beta-adrenergic receptors. The data indicate that the R-enantiomer may lower intraocular pressure in man at concentrations less likely than timolol to block extraocular beta-adrenergic receptors. Finally, to account for the differential effect of the R-enantiomer upon ocular as opposed to extraocular beta-adrenergic receptors, it is tentatively suggested that this agent may also act upon a population of ocular beta-adrenergic receptors showing relatively poor stereoselectively.

Laboratory and clinical studies on the mechanism of action of timolol

Timolol is now widely used in the treatment of glaucoma but its mechanism of action is unclear. The drug decreases aqueous humor formation by the ciliary processes and is a potent beta-adrenergic antagonist. However, the physiological basis for such a drug to decrease intraocular pressure has not been clearly demonstrated.

A major pathway for the regulation of intraocular pressure

There has been a suspicion on the part of many clinicians and research scientists that intraocular pressure can be regulated by neural and/or humoral influences upon the rate of aqueous humor formation. It has been difficult, if not impossible, to separate specific influences of the central nervous system upon intraocular pressure from vascular induced or other secondary alterations. The past two decades have witnesses a great deal of study of the role of the adrenergic nervous system upon the regulation of intraocular pressure. From the investigations it is possible to formulate an integrated concept that can place years of work and speculation on a firm molecular foundation. The secretory tissue of the eye, the ciliary processes, contain an enzyme receptor complex, comprised by receptor complex, comprised by receptor bound membrane proteins, the catalytic moiety of the enzyme, a guanyl nucleotide regulatory protein (or N protein) and other features. The enzyme can be activated by well known neurohumoral or humoral agents that consist of catecholamines, glycoprotein hormones produced by the hypothalamic pituitary axis, and other related compounds, including placental gonadotropin. These compounds cause the ciliary epithelia to produce cyclic AMP at an accelerated rate. Cyclic AMP, as a second messenger, causes, either directly or indirectly, a decrease in the rate of aqueous humor formation that may be modulated by cofactors. Clinical syndromes fit the experimental data so that an integrated explanation can be given for the reduced intraocular pressure witnessed under certain central nervous system and adrenergic influences. The molecular biology of this concept provides important leads for future investigations that bear directly both upon the regulation of intraocular pressure and upon glaucoma.

Effects of alpha 1 and alpha 2 activation of adrenergic receptors on aqueous humor dynamics

Effects of phenylephrine (alpha 1-adrenergic agonist), prazosin (alpha 1-adrenergic antagonist), clonidine (alpha 2-adrenergic agonist), and yohimbine (alpha 2-adrenergic antagonist) on aqueous humor (AH) dynamics were studied with a cat eye model. Phenylephrine (130 microgram/ml) inhibited AH outflow (67% at 90 min. period) more than AH formation (26% at the same period) indicating the intraocular pressure (IOP) might be raised by the administration of phenylephrine. Prazosin (0.1 microgram/ml) produced effects opposite to those of phenylephrine (55% reduction of AH formation and 25% reduction of AH outflow at 3 hr. period) suggesting the alpha 1-adrenergic receptor is responsible for increases rather than decreases of IOP. Both clonidine (10 microgram/ml) and yohimbine (0.1-1.0 microgram/ml) inhibited AH formation (60% inhibition) more than AH outflow (no inhibition for clonidine and 40% inhibition for yohimbine) to lower IOP. The conventional theory of receptor antagonism does not seem to function at alpha 2-receptor sites.

Ocular effects of beta-adrenergic agents. XII Jules Stein Lecture

The ocular effects of beta-adrenergic agonists and antagonists are reviewed. The permeability of the rabbit corneal epithelium was estimated for various instilled drugs, and difference in pharmacokinetics of timolol between the albino and pigmented rabbit demonstrated. The effects of the drugs on the cornea and the lens epithelium, and also the distribution of the adrenergic receptors in the intraocular muscles were discussed. The beta-adrenergic receptors are present in the ciliary processes of the rabbit as shown through studies by biochemical pharmacology and cytochemical electron microscopy. Adrenergic effects on the blood flow and the vascular permeability of the anterior uvea were discussed. The effects of adrenergic drugs on aqueous humor dynamics differ considerably among rabbits, monkey and the human subjects, and they were discussed separately for these species. The mechanism underlying the hypotensive effects of beta-adrenergic drugs is complex, but the available data indicate that in the human eye the hypotensive effects of epinephrine are attributed mainly to a decrease in the outflow resistance and the effects of timolol are principally to a reduction of aqueous humor formation.