Interaction of adrenergic antagonists with prostaglandin E 2 and tetrahydrocannabinol in the eye. Keith Green and Keun Kim

Transcription

1 Interaction of adrenergic antagonists with prostaglandin E 2 and tetrahydrocannabinol in the eye Keith Green and Keun Kim Both a- and /3-adrenergic antagonists have been utilized in an attempt to discern the site of action of prostaglandin (PG) and tetrahydrocannabinol (THC) in the eye. Both a- and (1-adrenergic antagonists (a-antagonists, phentolamine and phenoxybenzamine; P-antagonists, propranolol and sotalol) caused a dose-dependent reduction in intraocular pressure and blood pressure and increased total outflow facility. The results are consistent with the concept that both a- and fi-adrenergic receptors are present in the anterior uvea and that vasomotor tone is essential to the maintenance of normal intraocidar pressure. No antagonist reduced the PG-induced elevation of intraocular pressure unless the blood pressure was severely lowered. All antagonists inhibit the normal PG-induced increase in total outflow facility, indicating that these agents protect the blood-aqueous barrier from breakdown without altering the vasodilatory response to PG. All antagonists reduced the fall in intraocular pressure produced by THC by approximately 50 per cent, except for sotalol which completely abolished the intraocular pressure fall. Only the a-adrenergic antagonists prevented the THC-induced increase in total outflow facility. The results indicate that true outflow facility may well be regulated exclusively by a-receptors. The data are consistent with the effect of THC being primarily a vasodilation of the efferent blood vessels of the anterior uvea. The partial inhibition by a-adrenergic antagonists may also suggest a lesser role of THC on the afferent vessels. Key words: rabbit, intraocular pressure, total outflow facility, blood pressure, adrenergic antagonists, propranolol, phenoxybenzamine, phentolamine, sotalol, prostaglandin Es, tetrahydrocannabinol. I ntravenous a- and ^-adrenergic blocking agents have been used extensively in From the Departments of Ophthalmology and Physiology, Medical College of Georgia, Augusta, Ga. Supported in part by Public Health Service Research Grants EY 0083 and EY 0113 from the National Eye Institute. Submitted for publication April 17, Reprint requests: Dr. Keith Green, 3 D 11, R & E Building, Medical College of Georgia, Augusta, Ga studies on aqueous humor dynamics 1 " to determine the relative roles of a- and /3- receptors in the control of aqueous humor production and outflow from the eye, however, there is a paucity of reports on the interactions of these drugs with either prostaglandins or tetrahydrocannabinol. The present study was performed to examine the possibility that either prostaglandin E L. or tetrahydrocannabinol may exert their ocular effects via the adrenergic innervation system of the eye. Prostaglandin (PG) effects in the rabbit Downloaded From: on 12/03/2017

2 Volume 15 Number 2 Interaction of adrenergic antagonists 103 eye are well known. 5 ' It has been shown that neither intracameral phenoxybenzamine (PBA) 7 nor intracameral propranolol (PR) S have any effect on the elevation of intraocular pressure caused by the intracameral administration of PG. The intravenous administration of PBA (.5 mg. per kilogram) produced severe depression of systemic blood pressure 7 which masked any intracameral PG effect. Neither PR nor PBA blocked the in vitro PGE, activation of ciliary process adenyl cyclase. 0 PG's have a profound effect on circulation through the anterior uvea 1012 and it would be unlikely that the concentration of antagonist in the circulation when administered via the intracameral route would reach that achieved with intravenous administration. The interrelationship between PG's and adrenergic antagonists has, therefore, been studied with all agents given intravenously. The active ingredient of marihuana, delta-1-tetrahydrocannabinol (THC), also produces changes in anterior uveal blood flow. 11 ' u The present study employs adrenergic antagonists to further examine this effect in detail in an attempt to determine the mode of action of THC in the eye. Two a- and two /?-adrenergic antagonists were utilized since it is accepted that although having primarily a similar effect different antagonists either have different actions, binding capacities, or varying specificity of binding. It was thought, therefore, that the use of two antagonists for each primary receptor site would be advantageous in revealing more exactly the effects of drugs on the eye. Materials and methods Experimental procedure. Adult albino rabbits, 2 to 3 kilograms, of either sex were anesthetized with 25 per cent urethane in 0.9 per cent NaCl solution and the head held securely in a metal stereotaxic holder. A new 22-gauge needle was inserted into each eye once a suitable depth of anesthesia was obtained: one needle was connected via a 3-way stopcock to a transducer and a capillary as described previously, while the other needle was connected directly to a transducer. Intraocular pressure (IOP) was recorded continuously and, in order to be accepted, had to remain stable for at least 5 minutes before intravenous injection of drugs. Total outflow facility (Ctot) was measured as described previously, 12 ' r! using the constant pressure perfusion method of Barany 15 with a pressure difference of 5 mm. Hg. Stable pre- and postfacility IOP had to be within 2 mm. Hg. otherwise the eye was discarded. IOP was, therefore, determined for both eyes and C t <,t for one eye of each pair; the nonfacility eye acted as a control against the possible effects of the facility determinations per se causing a change in the behavior of IOP. A femoral artery was cannulated to measure systemic blood pressure and the cannula was filled with heparinized saline (100 units per milliliter), which was replenished periodically via a 3-way stopcock. In view of a discrepancy between the effects of propranolol on IOP of the urethane-anesthetized rabbit and results reported on conscious animals, 1 - '-' some experiments were performed to determine the effect of the drug in conscious animals. Adult albino rabbits were restrained in canvas bags tied loosely at the neck and the IOP measured using an air applanation tonometer 10 suitably calibrated for the rabbit eye. The cornea was anesthetized with one drop of per cent tetracaine hydrochloride (Alcon Laboratories, Inc., Fort Worth, Texas) which was irrigated after 10 seconds with 0.9 per cent NaCl solution prior to the determination of IOP. Propranolol was given at a rate of 1 mg. per minute in a marginal ear vein to a final dose level of 5 mg. per kilogram. IOP was measured every 30 minutes for hours. Drugs used. Prostaglandin (PGE 2 ) was prepared as before 12 ' 1 and injected intravenously at a rate of 0.79 fig per 7.9 fi\ per minute for a period of 10 minutes; this rate of injection of PGEj has been shown previously to cause a 10 to 15 mm. Hg. elevation of IOP. 1 - Tetrahydrocannabinol (THC) was given intravenously at a concentration of mg. per 100 ml. of plasma volume; this concentration is sufficient to cause a 10 to 20 per cent fall in IOP " The adrenergic antagonists used were as follows: a-blockers, phentolamine HC1 (PH) (Regitine; Ciba Pharmaceutical Company, N. J.), and phenoxybenzamine HC1 (PBA) (Dibenzyline; Smith, Kline and French Laboratories, Pa.); (3-blockers, propranolol (PR) (Inderal; Ayerst Laboratories, N. Y., Lot #IKSY), and sotalol HC1 (SO) (Regis Chemical Company, 111.). When used in combination with both PGE- and THC, the four blocking agents were used at only the lowest two dose levels which produced the minimal effects on IOP and systemic blood pressure. All dose levels are given as the salt and drugs were injected via a marginal ear vein. Downloaded From: on 12/03/2017

3 10 Green and Kim Investigative Ophthalmology February 197 Table I. Total outflow facility of the urethane-anesthetized rabbit eye as a function of time, a-adrenergic antagonists and interactions with PG and THC Drug PH 5 Antagonist concentration (mg./kg.) PH + PGE 2 PH.+ THC PBA 0.1 PBA+PGE PBA + THC Total outflow facility (nl/mm. Hg/min.) Time (minutes) T fl T PG 0.201f 0.198H T H t If $ 0.00 THC PG 0.171H ][ THC f O.1831f ff J [ K [ f $ $ O.1911[ F $ $ [ [ $ $ f * t 0.25f t f f O.371f t $ $ * f f f * f * * The values are mean S.E.M. P < 0.001; fo.005 > P > 0.001; J > P > 0.005; 0.1 > P > ; fl Not significantly different. For explanation of abbreviations, see text. N Results a-adrenergic antagonists. Phentolamine (PH). PH caused a dose-dependent fall in intraocular pressure (IOP), blood pressure (BP) (Fig. 1), and a dose-related increase in total outflow facility (C to t) (Table I). BP fell rapidly within five minutes after injection thereafter being stable, whereas the IOP, although it fell at a rapid initial rate at high doses, continued a slower constant rate of fall during the experimental time period. PH did not reduce the peak IOP elevation caused by PGE 2, except when the BP was severely depressed ( mg. per kilogram PH). The normal increase in C^t 30 minutes after PGE, (i was not found after PH (Table I). The dose of THC used normally produces a 3 to 5 mm. Hg fall in IOP at 0 minutes in either anesthetized or conscious 1 normal rabbits: similar experiments here caused a 2. and.5 mm. Hg (n=) fall in IOP at 30 and 0 minutes after THC, with corresponding increases in C to t from a control value of to and (33 per cent increase from control) /x\ per millimeter of mercury per minute, respectively. PH significantly in- Downloaded From: on 12/03/2017

4 Volume 15 Number 2 Interaction of adrenergic antagonists TIME (minutes) Fig. 1. Effect of phenotolamine on intraocular pressure and blood pressure of the urethaneanesthetized rabbit., 5 mg. per kilogram; O O, mg. per kilogram; A A, mg. per kilogram. IOP, intraocular pressure, BP, systemic blood pressure. All intraocular pressure values are the mean of 12 eyes, and blood pressure values are the mean of six animals. The error bars represent the S.E.M. and are shown at the times indicated on the graph for clarity; the values shown are representative of those for the remaining points. Table II. Percentage fall in intraocular pressure caused by adrenergic antagonist alone and in combination with THC. For explanation of abbreviations, see text A ntagonist concentration (mg./kg.) IOP 30mln. - lop Wm lopio min (Antagonists THC)- (antagonist alone) Drug PH PH + THC PBA PBA + THC PR PR + THC SO SO + THC THC alone hibited the IOP reduction normally caused by THC (Table II). THC prevented the increase in C to t normally seen with PH (Table I), since the values for C to t at 90 minutes after PH were less after THC than with PH alone. Phenoxybenzamine (PBA). PBA caused a dose-dependent fall in IOP and BP similar to that found with PH (Fig. 1), although the increase in C to t did not demonstrate such an exact relationship to dose (Table I). The IOP and BP fell over the first 30 minutes, at a slower rate than with PH (cf. Fig. 1) before assuming a steady value. The effect of PG on IOP was relatively unaffected by PBA, since an 8 mm. Downloaded From: on 12/03/2017

5 10 Green and Kim Investigative Ophthalmology February TIME (minutes) Fig. 2. Interaction between phenoxybenzamine and tetrahydrocannabinol on intraocular pressure and blood pressure of the urethane-anesthetized rabbit. A A, 0.1 mg. per kilogram; A A, mg. per kilogram. Arrow ( ) indicates time of THC injection. IOP values are the mean S.E.M. of eight eyes and BP values are the mean S.E.M. of four animals TIME (minutes) Fig. 3. Effect of propranolol on intraocular pressure and blood pressure of the urethane-anesthetized rabbit., 5 mg. per kilogram; O O, mg. per kilogram; A A, mg. per kilogram. IOP values are the mean S.E.M. of eight eyes and BP values are the mean S.E.M. of four animals. See legend to Fig. 1 for further explanation. Hg rise in IOP was found relative to the normal of 10 to 12 mm. Hg as described previously. 5 - c - " 1 With PBA the increase in C to t 30 minutes following PG was only to the value that was normally found with PBA alone (Table I), thus PBA prevented the expected rise in C tot which is normally evoked by PG. Pretreatment of the animals with PBA significantly inhibited the IOP reducing effect of THC (Fig. 2 and Table II). Ctot increased after THC (Table I) but this change was no greater than that induced by PBA alone at the same time relative to the injection of the antagonist. Downloaded From: on 12/03/2017

6 Volume 15 Number 2 Interaction of adrenergic antagonists 107 Table III. Total outflow facility of the urethane-anesthetized rabbit eye as a function of time, /3-adrenergic antagonists and interactions with PG and THC. For further explanation, see legend to Table I PR Drug PR+PGE 2 PR + THC SO A ntagonist concentration (mg./kg.) Total outflow facility (nl/mm. Hg/min.) Time (minutes) T T If 0.1 If H f 0.19H IT If THC O.1881f f f PG f if 0.203U If l21f f 0.191f 0.21 f f $ f f O.l91f f f $ $ f O $ $ * f N SO + PGE 2 SO + THC If 0.01 O.l81f 0.171f If PG THC 0.197TT f ff $ if 0.233$ $ t f * For explanation of symbols, see Table I. (3-Adrenergic antagonists. Propranolol (PR). All concentrations of PR used here in the urethane-anesthetized rabbit caused a fall in IOP and BP in a dose-dependent manner (Fig. 3). The increase in C tot was not clearly dose-dependent (Table III). When PR was given at the rate of 1 mg. per minute to a final dose of 5 mg. per kilogram to three awake rabbits, little or no change was seen in IOP over six hours. No effect was seen of PR on the PGE 2 - induced IOP increase although the increase in C t ot normally seen 30 minutes after PGE 2 was not clearly evident (Table III). PR appeared to have an influence on the THC-induced IOP fall since the IOP decrease caused by THC was about half of that found in the absence of PR (Table II). The THC effect on C to t was uninfluenced by PR (Table III), since C to t increased more than caused by PR alone, when the two drugs were given together. Sotalol (SO). All three dose levels of SO caused a fall in IOP with only a minor change in BP (cf. Fig. 3). The decrease in IOP and BP at all dose levels were quan- Downloaded From: on 12/03/2017

7 108 Green and Kim Investigative Ophthalmology February TO 80 TIME (minutes) Fig.. Interaction between sotalol and tetrahydrocannabinol on intraocular pressure and blood pressure of the urethane-anesthetized rabbit. O O, mg. per kilogram; A A, mg. per kilogram;, THC alone. Arrow ( ) indicates time of injection of THC. IOP values are the mean S.E.M. of eight eyes and BP values are the mean S.E.M. of four animals. titatively very similar (Fig., 0 to 30 minutes) with no apparent dose-response relationship. C to t was found to increase significantly after SO treatment alone (Table III). The PG-induced IOP elevation appeared to be unchanged by SO, but the THC-induced IOP effect was completely inhibited, even reversed by SO (Fig. and Table II). C tot, after PG, was also unchanged from the value found with SO alone, thus SO appeared to inhibit the induction of an increase in C to t which is normally generated by PG (Table III). THC caused an increase in C to t over and above that induced by SO alone (Table III). Discussion To place the following discussion in perspective it is advantageous to outline, albeit briefly, the current concepts of the influence of adrenergic innervation on aqueous humor dynamics. With specific reference to the rabbit, the presence of a-receptors in the regulation of total outflow facility has been unequivocally demonstrated, as well as a smaller /?-effect. ir More detailed studies have indeed shown that true outflow facility is exclusively controlled by a-adrenergic innervation system, 18 thus the /3-adrenergic component would be an effect on pseudofacility. With regard to aqueous humor formation, the a- and /?-adrenergic receptors appear to play significant roles, the former (a) producing a minor increase and the latter (/?) producing a major fall in aqueous formation. 19 Their control may well be exerted in blood flow regulation through the ciliary body thereby influencing aqueous humor formation > 21 * Adrenergic antagonist effects alone. To the best of our knowledge a systematic dose-response study of the effect of the most widely used adrenergic antagonists simultaneously on IOP and BP has not been reported previously. Phentolamine (PH), phenoxybenzamine (PBA), and propranolol (PR) all caused significant dose-related decreases in IOP and BP within a few minutes after intravenous administration. Sotalol (SO) produced only a minor reduction in BP although the IOP fell significantly. All antagonists produced a dose-related reduction in IOP when the 90 minute and 30 minute IOP were compared (Figs. 1 and 3 for examples of a- and /^-antagonists). Presumably Downloaded From: on 12/03/2017

8 Volume 15 Number 2 Interaction of adrenergic antagonists 109 some of this fall is related to the interference of these agents with the maintenance of normal vasomotor tone of the ocular blood vessels. The a-antagonists produced a dose-related increase in C to t, whereas the /^-antagonists, although increasing Ctot, showed no dose relationship. PH, PBA, and PR have been the most widely used antagonists in studies on aqueous dynamics 1 " and while intravenous PBA 2 ' 7 and PH 2 have been reported to lower both IOP and BP, only one report exists that intravenous PR reduces IOP in the rabbit. 3 PBA was reported not to reduce IOP in conscious rabbits, but later reports confirmed that PBA did reduce IOP. 2 Previous authors have reported that PR has no effect on IOP in either anesthetized 2-22 or conscious 1 ' rabbits. In the urethane-anesthetized rabbit, Takats, Szilvassy, and Kerek 3 reported that the IOP fell from 20.8 to 18.8 mm. Hg at a dose of 0.3 mg. per kilogram PR, a concentration lower than that often utilized, viz. 5 mg. per kilogram. The experiments reported here indicate that PR caused a fall in IOP at concentrations from 5 to mg. per kilogram in the anesthetized rabbit, although the latter dose had little effect on BP. In the conscious rabbit little or no effect of PR was seen on IOP, confirming previous observations There was a difference in the behavior, therefore, of the PR response in the awake and anesthetized rabbit, although the response of both the awake and anesthetized rabbit to the same dose of PH and PBA were almost identical. (IOP reduction 0 minutes after PH in awake 2 and anesthetized [Fig. 1] rabbits was 5.5 and 7.5 mm. Hg, respectively, and for PBA in awake 2 and anesthetized rabbits the reductions were and.5 mm. Hg, respectively.) The difference in response to PR may relate to the fact that PR is known to have central effects as well as act as a local anesthetic; both could possibly be revealed in the anesthetized and not in the conscious animal. In addition, it is obviously important to relate reports of either an effect on IOP or lack of an effect to the time after administration of the drug. In the rabbit there is a- and /3-adrenergic control of fluid outflow from the eye. 17 C to t has two components, C tr or trabecular outflow and C ps or pseudofacility. 2 It is not possible in this study to distinguish the relative effects of the antagonists on these two components. In the present experiments, C to t is increased with either a- or ^-antagonists (Tables I and III), but the effect is greater with the a- than with the ^-antagonists. The formation of aqueous humor has been reported 1 to be under ^-adrenergic control, but both a- and /3-antagonists decrease IOP. 19 The ^-antagonists PR and SO, however, appear to be more active on a weightfor-weight basis than the a-antagonists (Table II). From the present data, it is not possible to assess the relative role of the a- and /?-adrenergic system on the various components which affect aqueous humor dynamics since the appropriate direct agonists were not employed. It is possible that both a- and /?-adrenergic receptors are located in the outflow pathways because of the action of the drugs on Ct,,t, although an effect on the formation of aqueous humor by altering ultrafiltration (C,, s ), secretion, or blood flow through the anterior uvea cannot be excluded. In view of the effect on C to t by all the antagonists in the presence of PG (see below), however, the major effector site of the antagonists is strongly suggested to be at the outflow channels regulating Ctr. Adrenergic antagonist interaction with PGE S. Both a-antagonists, even at low concentrations, decreased BP by about 20 to 25 per cent and, as described previously, following intracameral PG 7 the depression in BP markedly reduced the ocular PGE 2 effect in a dose-dependent manner. Neither a-antagonist had any effect on the characteristic PG-induced lowering of sys- Downloaded From: on 12/03/2017

9 110 Green and Kim Investigative Ophthalmology February 197 temic blood pressure. The PG-induced increase in Ctot, seen about 30 minutes after PG administration in normal animals, 0-12 is also inhibited by the antagonists. The early effect of PG is known to be on the blood vessels of the anterior uvea, 10 ' 12 followed, as the local PG concentration rises, by breakdown of the blood-aqueous barrier. 25 It appears, therefore, that the a-antagonists protect the blood-aqueous barrier from the breakdown without altering the vasodilatory effect of PG, since the PG-induced increase in C to t is thought to be due to an increase in C ps. G> 12 It is interesting, in this regard, to note that Hendley and Crombie 22 also found that PBA (a) and PR (p) prevented the leakage of protein into the aqueous humor, which is usually observed at the height of the ganglionectomy effect, despite the presence of hyperemic blood vessels in the iris. Takase 20 also found that /3-receptor blockade caused a fall in aqueous humor protein concentration. These findings are most strongly suggestive of an effect either on the ciliary epithelium permeability or the capillary vessels. Since the latter are very leaky to proteins even under normal conditions the most likely site of action would be the ciliary epithelium. The /^-antagonists, PR and SO, had similar effects on the ocular PGE 2 response. Neither PR nor SO influenced the PG effect on IOP, and SO had very little effect on BP. The characteristic fall in BP caused by PG was also unaffected by PR or SO. PG's are powerful vasodilators in the eye 10 " 12 and a /^-antagonist would be expected to block the PG effect if PG stimulated the /8-adrenergic system. Both PR and SO prevent the characteristic rise in C^t which is seen in normal eyes after Gi 12 PG, indicating that these agents also protect the blood-aqueous barrier from breakdown normally caused by PG, although the antagonists either themselves increase C tr (see above) or, by allowing dominance of the nonantagonized adrenergic system, cause the effect of this system to be revealed more completely especially when interactions where other drugs are considered. The present data, therefore, offer support for the mechanism of action of PG being a vasodilation of the afferent vessels to the anterior uvea. There is some mediation of at least part of the outflow system (C, )S ) by adrenergic receptor mechanisms through the protection of the blood-aqueous barrier from the action of PG provided by ^-adrenergic antagonists. Adrenergic antagonist interaction with THC. Both «- and /3-antagonists inhibit THC-induced IOP reduction whereas only the a-adrenergic antagonists strongly inhibit THC-induced increase in C to t- If the control of C tr is completely a-receptor dominated in the rabbit 1 s then these results certainly support this concept. THC is known to increase C to t, 13 ' 1 and it has been suggested that this, in part, is due to an increase in Ctr." The a-antagonist inhibition of this increase indicates that THC acts by increasing C fr per se; conversely, the lack of effect of ^-antagonists on the THC-induced increase in C to t suggests that ^-receptors play no role in regulating the THC-induced increase of Ctr- THC also causes an increase in the permeability of the ciliary epithelium 13 and, thereby increases C, )S in vivo. 27 The a-adrenergic antagonists may inhibit a THC-induced effect on vasoconstriction of afferent episcleral plexus vessels of the rabbit, which are involved in aqueous humor drainage and regulate C tr. 2S THC has been proposed to act either by vasodilating the efferent vessels or vasoconstricting the afferent vessels supplying the anterior uvea. 13 ' 2!) It is apparent from Table II that the percentage reduction in IOP caused by THC is approximately halved by the a-adrenergic antagonists and the /^-antagonist PR, whereas the /3-adrenergic antagonist SO inhibits the THCinduced IOP fall completely. Such findings may be related to those seen with other drugs by Macri and Cevario 20 ' 21 who found that different drugs produce dif- Downloaded From: on 12/03/2017

10 Volume 15 Number 2 Interaction of adrenergic antagonists 111 ferential actions on both afferent and efferent vessels of the arterially perfused cat eye. Indeed, pilocarpine has been shown to have stimulatory but contrary effects on the arteriolar and venous sites controlling blood flow through the anterior uvea. 21 The results indicate that THC acts in the normal eye primarily by vasodilating the efferent blood vessels of the eye thereby reducing the capillary pressure within the ciliary body thereby causing an IOP fall (since SO completely blocks the characteristic response). Such an action is also consistent with the known vasodilatory action on the conjunctival blood vessels. 30 The effect of other antagonists on the THC-induced reduction in IOP may also relate to the differential action of THC on the blood supply to the ciliary body. It is possible that THC also causes some vaso constriction of the afferent vessels since the a-antagonists reduce the THC-induced IOP reduction by half, while there is no further change in C to f PR appears to be in a unique position since the IOP falls by half the normal THC-induced amount and C to t also increases. Thus it would appear to be acting less specifically or less powerfully than SO to counteract the THC effect. The present data leave undetermined the question of the direct or indirect action of THC on the blood vessels of the eye, although mediation via the adrenergic system appears likely. We thank Ms. Deborah Hancock for her secretarial assistance, Mrs. Karen Bowman for performing the intraocular pressure measurements on conscious rabbits, Drs. J. F. Bigger and J. L. Matheny for their valuable comments on this paper, and Dr. J. E. Pike, The Upjohn Company, Kalamazoo, Mich, and Dr. Monique Braude, National Institute on Drug Abuse, respectively, for generously making the prostaglandin and tetrahydrocannabinol available for this study. REFERENCES 1. Langham, M. E.: The response of the pupil and intraocular pressure of conscious rabbits to adrenergic drugs following unilateral superior cervical ganglionectomy, Exp. Eye Res. : 381, Langham, M. E., Simjee, A., and Josephs, S.: The alpha- and beta-adrenergic responses to epinephrine in the rabbit eye, Exp. Eye Res. 15: 75, Takats, I., Szilvassy, I., and Kerek, A.: Intraocularer Druck and Kammerwasserzirkulations Untersuchungen an Kaninchenaugen nach intravenoser Verabreichung von Propranolol (Inderal), Arch. Klin. Exp. Ophthalmol. 185: 331, Radius, R., and Langham, M. E.: Cyclic- AMP and the other responses to norepinephrine, Exp. Eye Res. 17: 219, Beitch, B. R., and Eakins, K. E.: The effects of prostaglandins on the intraocular pressure of the rabbit, Br. J. Pharmacol. 37: 158, Kass, M. A., Podos, S. M., Moses, R. A., et al.: Prostaglandin Ei and aqueous humor dynamics, INVEST. OPHTHALMOL. 11: 1022, Waitzman, M. B.: Influences of prostaglandin and adrenergic drugs on ocular pressure and pupil size, Prostaglandin Symposium of the Worcester Foundation for Experimental Biology, Ramwell, P. W., and Shaw, J., editors. New York, 198, Interscience, p Waitzman, M. B.: Effects of prostaglandin and /3-adrenergic drugs on ocular pressure and pupil size, Am. J. Physiol. 217: 1593, Waitzman, M. B., and Woods, W. D.: Some characteristics of an adenyl cyclase preparation from rabbit ciliary process tissue, Exp. Eye Res. 12: 99, Whitelocke, R. A. F., and Eakins, K. E.: Vascular changes in the anterior uvea of the rabbit produced by prostaglandins, Arch. Ophthalmol. 89: 95, Masuda, K., and Mishima, S.: Effects of prostaglandins on inflow and outflow of the aqueous humor in rabbits, Jap. J. Ophthalmol. 17: 300, L2. Green, K., and Kim, K.: Pattern of ocular response to topical and systemic prostaglandin, INVEST. OPHTHALMOL. 1: 3, Green, K., and Pederson, J. E.: Effect of Aitetrahydrocannabinol on aqueous humor dynamics and ciliary body permeability in the rabbit, Exp. Eye Res. 15: 99, Creen, K., and Podos, S. M.: Antagonism of arachidonic acid-induced ocular effects by tetrahydrocannabinol, INVEST. OPHTHALMOL. 13: 22, Barany, E. H.: Simultaneous measurement of changing intraocular pressure and outflow facility in the vervet monkey by constant pressure infusion, INVEST. OPHTHALMOL. 3: 135, Walker, R. E., and Litovitz, T. L.: An experimental and theoretical study of the pneu- Downloaded From: on 12/03/2017

11 112 Green and Kim Investigative Ophthalmology February 197 matic tonometer, Exp. Eye Res. 13: 1, Sears, M. L., and Neufeld, A. H.: Adrenergic modulation of the outflow of aqueous humor, INVEST. OPHTHALMOL. 1: 83, Eakins, K. E.: The effect of intravitreous injections of norepinephrine, epinephrine and isoproterenol on the intraocular pressure and aqueous humor dynamics of rabbit eyes, J. Pharmacol. Exp. Ther. 10: 79, Chiou, C. Y., and Zimmerman, T. J.: Ocular hypotensive effects of autonomic drugs, INVEST. OPHTHALMOL. 1: 1, Macri, F. J., and Cevario, S. J.: A pharmacodynamic study of the inhibitory effects of 1-norepinephrine, 1-epinephrine, and d, 1- isoproterenol on aqueous humor formation in the enucleated, arterially perfused cat eye, INVEST. OPHTHALMOL. 13: 392, Macri, F. J., and Cevario, S. J.: The dual nature of pilocarpine to stimulate or inhibit the formation of aqueous humor, INVEST. OPHTHALMOL. 13: 17, Hendley, E. D., and Crombie, A. L.: The 2-hour ganglionectomy effect in rabbits. The influence of adrenergic blockade, adrenalectomy, and carotid ligation, Exp. Eye Res. : 152, Neufeld, A. H., Jampol, L. M., and Sears, M. L.: Cyclic-AMP in the aqueous humor: the effects of adrenergic agents, Exp. Eye Res. 1: 22, Barany, E. H.: A mathematical formulation of intraocular pressure as dependent on secretion, ultrafiltration, bulk outflow, and osmotic reabsorption of fluid, INVEST. OPHTHALMOL. 2: 58, Green, K.: Permeability properties of the ciliary epithelium in response to prostaglandins, INVEST. OPHTHALMOL. 12: 752, Takase, M.: Studies on the protein content in the aqueous humor of the living rabbits. IV. Effects of beta stimulant and beta-blockade, Acta Soc. Ophthalmol. Jap. 75: 25, Pederson, J. E., and Green, K.: Aqueous humor dynamics: a mathematical approach to measurement of facility, pseudofacility, capillary pressure, active secretion, and x c, Exp. Eye Res. 15: 25, Hart, R. W.: Theory of neural mediation of intraocular dynamics, Bull. Math. Biophys. 3: 113, Green, K., and Bowman, K.: Effect of marihuana and derivatives on aqueous humor dynamics in the rabbit, in, Pharmacology of Cannabis, Szara, S., and Braude, M. C., editors. New York, 1975, Raven Press, in press. 30. Hepler, R. S., Frank, I. M., and Ungerleider, J. T.: Pupillary constriction after marihuana smoking, Am. J. Ophthalmol. 7: 1185, Downloaded From: on 12/03/2017