Characterization of /?-Adrenergic Receptors in Cultured Human Trabecular Cells and in Human Trabecular Meshwork

Transcription

1 Investigative Ophthalmology & Visual Science, Vol. 30, No. 1, January 1989 Copyright Association Tor Research in Vision and Ophthalmology 4* Characterization of /?-Adrenergic Receptors in Cultured Human Trabecular Cells and in Human Trabecular Meshwork Martin D. Wox,*f Perry B. Molinoff,f Jorge Alvarodo,^ ond Jon Polonsky^ The characterization of /S-adrenergic receptors on cultured human trabecular cells and trabecular meshwork from human autopsy eyes was carried out by radioligand binding utilizing ( l25!)-iodopindolol. In cultured cells, the observed binding of ( I2S I)-iodopindolol was of high affinity (Kj = 43 pm) and saturable. Scatchard plots were linear and revealed a B max of 33 ± 7 fmol/mg of protein. Competition studies with a series of agonists and antagonists revealed that human trabecular cells contain a single class of 0-adrenergic receptors of the 0 2 subtype. Similarly, the IQo of ICI 89,406 (176 nm) in human trabecular meshwork from autopsy eyes supports the presence of /3 2 -adrenergic receptors in this tissue. Invest Ophthalmol Vis Sci 30:51-57,1989 The trabecular meshwork is the primary structure through which aqueous humor must flow during its egress from the anterior chamber of the eye. It is composed of branching, interlacing collagenous beams, each enclosed by a layer of endothelial cells. The trabecular meshwork is one of several structures in the anterior chamber angle through which filtration may occur. These include the juxtacanalicular tissue, the canal of Schlemm, and the efferent venous collector channels. 1 Impaired drainage of aqueous humor through these structures is thought to underlie the primary pathophysiologic abnormality in openangle glaucoma, namely, diminished outflow facility of aqueous humor from the anterior chamber. 2-3 Only limited success has been achieved in finding histologic or morphologic changes in the trabecular meshwork or its adjacent structures that would account for the alterations in outflow of aqueous humor that accompany glaucoma. The possibility that changes in the mucopolysaccharide composition of the trabecular meshwork, or in the mechanism by which endothelial pinocytotic vesicles apparently transport materials (including aqueous humor) into From the Departments of fpharmacology and 'Ophthalmology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, and the ^Department of Ophthalmology, University of California, San Francisco, California. Supported by USPHS Grants NS 18479, EY-06810, MH14654, EY-03980, EY-02068, and EY Submitted for publication: May 7, 1987; accepted April 19, Reprint requests: Martin Wax, MD, Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA the canal of Schlemm, has been studied in an effort to explain the reduced outflow of aqueous humor, hence elevated intraocular pressure, in glaucoma. The trabecular meshwork/filtration apparatus is also thought to be the site at which pharmacologic agents act to improve the outflow facility of the eye. Epinephrine, used for the treatment of glaucoma since 1894, 4 is thought to improve aqueous outflow although the precise mechanism that underlies this effect is not known. 5 " 7 Specifically, it remains unclear whether this effect is mediated primarily by a-adrenergic 5 ' 8 " 12 or 0-adrenergic receptors These discrepancies may be partially due to species variability. It has been suggested that in rabbits, both a- and (8-adrenergic agonists increase outflow facility, whereas in humans and monkeys there is no apparent effect from stimulation of a-adrenergic receptors although an increase does occur after stimulation of /3-adrenergic receptors Further understanding of the pharmacologic properties and characteristics of the trabecular meshwork has come from results of the in vitro study of human trabecular cells. 15 " 17 Trabecular cells obtained from humans and grown in primary culture have been used to study changes in intracellular camp production in response to catecholamines. 15 Furthermore, possible morphologic alterations have been observed in these cells in response to prolonged administration of epinephrine. 18 These functional effects are thought to be initiated following occupancy of j3-adrenergic receptors by epinephrine on the trabecular cells. We have characterized the pharmacologic properties of the /3-adrenergic receptors of human trabecular cells grown in culture, and have compared their prop- 51

2 52 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / January 1989 Vol. 30 v Fig. 1. Photomicrograph of the trabecular meshwork region of a human eyefixed24 hr postmortem shows the region from which trabecular tissue was obtained. After iris was removed from its insertion, gentle scraping was performed to obtain the tissue from Schwalabe's line to the scleral spur (between the black arrows), and medial to the inner wall of the canal of Schlemm (SC) (X50). erties to those observed in studies of trabecular meshwork tissue that was obtained from human eyes shortly after death. Our findings indicate that the /Sadrenergic receptors of human trabecular cells in vitro and in tissue from the trabecular meshwork in vivo are characterized by the presence of/3-adrenergic receptors of the /82 subtype. Materials and Methods Tissue Preparation Freshly enucleated eyes from human donors (ages 56 to 75) were obtained within 24 hr of death due to cardiorespiratory failure and were dissected on ice in tissue buffer (20 mm Hepes buffer, ph 7.5, containing 145 mm NaCl). The pars plana was incised 4 mm posterior to the corneascleral limbus, and the anterior segment was removed intact by surgical separation. After removal of the iris/ciliary body, the corneoscleral remnant, which included the trabecular meshwork, was scraped from the scleral spur to Schwalabe's line and placed in tissue buffer for further processing. Sections of the corneoscleral/trabecular meshwork remnant were also sent for histopathologi- cal analysis to confirm the presence of trabecular meshwork (Fig. 1). Tissues from human eyes were pooled in tissue buffer, homogenized with a Brinkmann (Westbury, NY) Polytron (five times for 1 sec at setting 6-7), and centrifuged at 20,000 g. Membranes were resuspended in tissue buffer and stored frozen at 70 C until they were thawed for binding assays. Tissues from seven eyes were thawed and pooled for an additional wash at 20,000 g and resuspended in a suspension of 25% Percoll (Pharmacia Fine Chemicals, Uppsala, Sweden) in 20 mm Hepes buffer, ph 7.5, containing 145 mm NaCl. The Percoll suspension containing tissue was placed in a Beckman Polyallomer (Palo Alto, CA) centrifuge tube (16X76 mm) and centrifuged at 20,000 rpm in a 70.1 Ti rotor to form a self-generated isopychnic gradient. Centrifugation resulted in the separation of plasma membranes from the melanin pigment-containing pellet at the bottom of the tube.19 The properties of/3-adrenergic receptors were then characterized in binding assays that used the radioligand (l25i)-iodopindolol (125I-IPIN). (I25I)-IPIN was prepared by a modification of the method of Barovsky and

3 No. 1 0-ADRENERGIC RECEPTORS IN TRADECULAR CELLS / W a x er al 53 Fig. 2. Human trabecular cells grown postconfluency as described in Methods (X100). Brooker,20 as described by Wolfe and Harden,21 and used in binding assays to assess the properties of &adrenergic receptors in human eye tissue as previously described.19 Cell Culture Human trabecular meshwork cells were obtained from the eyes of a 30-year-old patient with no known ocular disease and grown for 2 to 4 weeks postconfluency. This was approximately 1 week longer than required to obtain optimally stable, morphologically differentiated monolayers.15"17 Fourth-passage cells from frozen stock were plated at 5000 cells/cm2 in 100 X 15 mm dishes (Falcon Labware, Oxnard, CA), and grown in Dulbecco's modified Eagle's medium (Gibco Laboratories, Grand Island, NY) containing 2 mm glutamine, 15 ng/rs\\ gentamycin, and 2.5 mg/ml Fungizone. Medium was changed every 2-3 days and was supplemented with 15% fetal calf serum (Sterile Systems Inc., Logan, UT) during the growth phase to confluency in an atmosphere of 10% CO2-90% air at 37 C. In addition, 250 ng/ml of fibroblast growth factor (FGF) was added every other day until confluency was achieved. At confluency, cells were subsequently maintained in 10% fetal calf serum and grown in the absence of FGF for days prior to harvest (Fig. 2). The medium was changed hr prior to harvesting at which time cells were washed twice with phosphate-buffered saline and once with 20 mm Hepes buffer, ph 7.4, containing 145 mm NaCl. Cells were removed with a rubber policeman and centrifuged at 1500 g for 10 min. The supernatant was discarded and the pellet was stored at 70 C. Before use, cells were thawed and resuspended in 20 mm Hepes buffer, ph 7.4, containing 145 mm NaCl. Membranes were prepared by homogenizing the cells with a Brinkmann Polytron at setting 5-6 for 5 sec and centrifuging at 20,000 g for 15 min. The supernatant was discarded, and the membranes were resuspended in buffer as above prior to their addition to the binding assays. Assay of /9-Adrenergic Receptors Properties of /?-adrenergic receptors were assessed by adding aliquots of crude membranes to assay tubes containing, in a final volume of 0.25 ml, 12 mm Hepes buffer and 0.5% NaCl, bovine serum albumin (0.0004%) and ascorbic acid (1.1 mm). Assays were carried out in triplicate in the presence of 100 //M GTP. Specific binding was denned as the amount of (125I)-IPIN bound in the absence of a competing ligand minus the amount bound in the presence of 100 nm (-)-isoproterenol. Equilibrium binding was performed for 20 min at 37 C in disposable polypropylene tubes (Walter Sarstedt, Princeton, NJ). Reactions were terminated by dilution with 10 ml of buffer (10

4 54 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / January 1989 Vol. 30 mm Tris HC1, ph 7.5, containing 154 mm NaCl) at room temperature. Samples were then filtered over Schleicher and Schuell (Keene, NH) glass-fiber filters (No. 30) on a millipore vacuum filter manifold. The filters were washed with an additional 10 ml of buffer at room temperature and dried with suction. Radioactivity was determined in an LKB 1274 or a Beckman 4000 gamma counter. Protein was determined by the method of Bradford 22 using bovine serum albumin as a standard. Drugs The following drugs were kindly provided as gifts: ICI 118,551 and 89,406 (Imperial Chemical Industries America, Wilmington, DE). The 1-stereoisomers of isoproterenol, epinephrine, and norepinephrine were purchased from Sigma Chemical Co. (St. Louis, MO). Data Analysis Saturation data were analyzed by the method of Scatchard 23 to provide an estimate of the density of receptors and the affinity of the receptor (KJ for ( I25 I)-IPIN. Dose-response curves for the inhibition of the binding of ( 125 I)-IPIN were analyzed by the computer-aided curve-fitting program NEWFIT- SITES on the NIH-sponsored PROPHET computer system. Using nonlinear regression analysis, the mathematical modeling program MLAB was used to identify the presence of one or two classes of binding sites by comparing the sum of squares of the residuals for one- and two-site models. A two-site model was considered to fit the data significantly better than a one-site model if an.ftest of the sum of squares of the residuals yielded a significance level of improvement of fit of P < Kd values for the binding of a competing drug were calculated from the IC 5 o values using the method of Cheng and Prusoff. 24 Results Direct Binding of Cultured Human Trabecular Cells Direct binding of ( 125 I)-IPIN to membranes from cultured human trabecular cells was carried out with six to eight concentrations of ( 125 I)-IPIN ranging from 10 to 400 pm. The observed binding of ( 125 I)-IPIN was of high affinity (K<i = 43 ± 4 pm) and saturable (Fig. 3). Scatchard plots were linear and revealed a Bmax of 33 ± 7 fmol/mg of protein. Analysis of Indirect Binding The ability of several agonists and antagonists to compete for binding sites for ( I25 I)-IPIN was determined. The affinities of drugs were assessed by incubating homogenates of cultured human trabecular cells with 150 pm ( l25 I)-IPIN in the presence of 16 to 20 concentrations of competing drug. The relative proportions of 0 r and ft-adrenergic receptors were determined by computer-assisted analysis of dose-response curves resulting from the inhibition of the binding of ( 125 I)-IPIN with 20 concentrations of the j3 r selective antagonist ICI 89,406 or the fo-selective antagonist ICI 118,551. In the cultured trabecular meshwork cells, the inhibition curves were best explained by the presence of a single population of binding sites. ICI 118,551 was approximately 90-fold more potent in displacing ( 125 I)-IPIN from the (3 2 - adrenergic receptors in human trabecular cells than was ICI 89,406 (Fig. 4). The observed order of potency for the inhibition of the binding of ( I25 I)-IPIN in vitro by agonists was isoproterenol > epinephrine > norepinephrine (Fig. 5, Table 1). A similar assay for ( I25 I)-IPIN binding sites was performed in postmortem human trabecular tissue pooled from seven patients. The results obtained were similar to those found in vitro since the inhibition curve was best fit by a model that assumes the presence of one class of binding sites. The IC 50 value of ICI 89,406 in displacing specifically bound ( 125 I)- IPIN from postmortem human trabecular tissue was 176 nm and is similar to that obtained in the human trabecular cells grown in tissue culture (Fig. 6). Using the assumption that the affinity of the radioligand ( I25 I)-IPIN is similar in both human trabecular tissue and cultured cells, we may estimate that the density of receptors in the human trabecular meshwork is approximately 70 fmol/mg. Discussion The ability to propagate human trabecular cells in serial culture facilitates the study of receptor-mediated hormone interactions at the level of the cell membrane that cause subsequent biochemical responses in this tissue. The presence of saturable specific binding of ( I25 I)-IPIN demonstrates the conservation of membrane-bound /3-adrenergic receptors in broken cell preparations of human trabecular cells grown in tissue culture. 8-Adrenergic receptors have been classified into two subtypes based on the pharmacologic specificity of adrenergic receptor-mediated responses to selective drugs. 25 Analysis of the subtypes of membranebound /3-adrenergic receptors can be carried out by using radioligand binding assays in which selective antagonists compete for specifically bound ( I25 I)- IPIN. 26 In our experiments, 0-adrenergic receptors on trabecular cells grown in culture had a high affinity

5 No. 1 /3-ADRENERGIC RECEPTORS IN TRADECULAR CELLS / Wax er al 55 Fig. 3. Density of /3- adrenergic receptors in human trabecular cells. A representative saturation curve for binding of membranes of human trabecular cells grown in culture to ( 125 I)-IPIN is shown. Total binding ( ), specific binding (A), and nonspecific binding ( ) are shown in addition to the linear Scatchard transformation (inset). LU o d Q_ TOTAL 5X IPIN (M) for the ft-selective antagonist ICI 118,551, and a low affinity for the /?,-selective antagonist ICI 89,406. This is consistent with the reported properties of the /3 2 subtype of the /3-adrenergic receptor. The potency of ICI 89,406 at /3-adrenergic receptors in human trabecular tissue from postmortem eyes was similar to that which we observed in vitro. Furthermore, in both tissues, only a single homogeneous class of binding sites was found. Although considerably more efforts are necessary to define the mechanism(s) by which adrenergic drugs facilitate outflow, our findings provide some potentially useful information. Physiological studies in monkeys suggest that outflow facilitated by adrenergic agonists is mediated by /3-adrenergic receptors since cyclic AMP was elevated in aqueous humor with agonist stimulation and the response was blocked by propranolol. 67 Similarly, clinical investigation in humans suggested that timolol was able to block the ability of epinephrine to increase outflow facility. 13 The use of the nonselective /3-adrenergic antagonists propranolol and timolol, however, does not provide useful information concerning the /3- adrenergic receptor subtype that mediates their ef o ICI O o LOG DRUG (M) Fig. 4. Inhibition of the binding of ( I25 I)-IPIN by selective antagonists in membranes of cultured human trabecular cells. Assays were carried out with 100 pm ( I25 1)-IPIN. The IC 50 value and Hill coefficient were 3.5 nm and 0.94 for ICI! 18,551, and 270 nm and 0.90 for ICI 89,406. The data are from a representative experiment performed in triplicate. -LOG DRUG (M) Fig. 5. Inhibition of the binding of ( I25 I)-IPIN by selective agonists in membranes of cultured human trabecular cells. Assays were carried out with approximately 140 pm ( 125 I)-IPIN. The IC50 values for isoproterenol (0.9 nm), epinephrine (4.7 /im), and norepinephrine (18.7 tim) are the means of triplicate determinations. The Hill coefficients for the agonists were each approximately 0.9. The data are from a representative experiment performed in triplicate.

6 56 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Jonuory 1989 Vol. 30 Table 1. Apparent equilibrium constants for drugs acting at /3-adrenergic receptors in human trabecular cells Drug Antagonists ICI 118,551 ICI 89,406 Agonists Isoproterenol Epinephrine Norepinephrine Selectivity ft /3, nonselective nonselective Kj ± SEM (nm) 1.0 ± ± ± ± ± 270 Competition experiments for binding sites for ( 17! I)-IPIN were performed as described in Materials and Methods. All assays were performed in the presence of 100 nm GTP and represent the average of at least two experiments performed in triplicate. The order of potency is characteristic of the fe subtype of the /3-adrenergic receptor. fects. Recent reports have suggested that the /Si-selective antagonist betaxolol may not fully block epinephrine's effect on outflow facility in humans. 27 This observation may be partially explained by our finding that the /3-adrenergic receptors in trabecular cells and trabecular meshwork are of the /3 2 subtype, and is consistent with the finding that betaxolol (10~ 6 M) does not fully block epinephrine stimulation of cyclic AMP production in cultured trabecular cells. 28 The relative affinities of drugs selective for /3- adrenergic receptors in these tissues may have an important bearing on the efficacy and safety of topically administered /3-adrenergic antagonists used in the treatment of glaucoma. The finding that the /3-adrenergic receptors in human trabeculum are of the 182 subtype is consistent with results of previous investigations. 19 It is reasonable to assume that side effects due to systemic absorption of topically applied /?- adrenergic receptor antagonists may be minimized by using the lowest concentration of drug that effectively -LOG ICI 89,406 (M) Fig. 6. Inhibition of the binding of ( 125 I>IPIN to /3-adrenergic receptors by ICI 89,406 in human trabecular tissue obtained from postmortem eyes. Assays were carried out with approximately 70 pm of ( 12S I)-IPIN. The observed IC 50 value of 176 nm represents the results of triplicate determinations from tissue pooled from seven eyes. The Hill coefficient was 1.07, and the data were best fit to a model suggesting the presence of a single class of binding sites. occupies the ocular receptors responsible for mediating the hypotensive effect and results in effective lowering of intraocular pressure. Certainly there are multiple factors in addition to potency, such as systemic absorption, protein binding, volume of distribution, and rate of elimination which may influence ocular bioavailability as well as the observed systemic side effects of these drugs. Apparently /3-adrenergic antagonists such as timolol effectively reduce intraocular pressure by decreasing aqueous inflow. This response is presumably the result of antagonist occupancy of /3 2 -adrenergic receptors located on the ciliary epithelium of the ciliary processes. The use of drugs, therefore, which have a high potency for /3 2 -adrenergic receptors in the eye may allow effective reduction of intraocular pressure while minimizing cardiovascular side effects such as heart block and congestive failure which are thought to be due primarily to /3,-adrenergic receptor blockade. On the other hand, when we speculate as to the role of j8-adrenergic receptor occupancy on outflow facility we must consider the possibility it may not be therapeutically desirable to achieve /3-adrenergic blockade in the trabecular meshwork which could result in decreased outflow facility when epinephrine is used concomitantly. Towards that objective, it may be advantageous to use a jsi -selective antagonist at a concentration which results in incomplete occupancy of ft-adrenergic receptor sites in the outflow pathways. Unfortunately, it is difficult to explain how a /?i -adrenergic antagonist, at a concentration which fails to occupy /3 2 -adrenergic receptors in the outflow pathway, could thus effectively occupy the /3 2 -adrenergic receptors on the ciliary epithelium to effect inflow. It is certain that further work is necessary to fully explain the cellular mechanisms by which occupancy of specific receptors effect the regulation of aqueous secretion by the ciliary epithelium and outflow via the trabecular meshwork. Such considerations, however, are certainly important in understanding the rationale for the current and future use of/3-adrenergic receptor agonists or antagonists in the treatment of glaucoma. In conclusion, we have demonstrated that trabecular cells grown in culture possess /3-adrenergic receptors of the /3 2 subtype which are similar to those found in trabecular tissue in vivo. As such, the further study of trabecular cells grown in culture appear to be worthy of pursuit in the hope of providing insight into the mechanism by which several families of drugs modulate the function of trabecular cells, thus clarifying the rationale for their use in glaucoma. Key words: trabecular cells, trabecular meshwork, j8-adrenergic receptors, l25 I-iodopindolol

7 No. 1 /8-ADRENERG1C RECEPTORS IN TRABECULAR CELLS / Wax er al 57 References 1. Hamming A and Apple D: Anatomy and embryology of the eye. In Principles and Practice of Ophthalmology, Vol. 1, Peyman GA, Sanders DR, and Goldberg MF, editors. Philadelphia, W. B. Saunders, 1980, pp Grant WM: Further studies on the facility offlowthrough the trabecular meshwork. Arch Ophthalmol 60:523, Sears ML: Pressure in the canal of Schlemm and its relation to the site of resistance to outflow of aqueous humor in the eyes of Ethiopian green monkeys. Invest Ophthalmol 5:610, Darier A: Uber das Nebennieren. Augenheilkd Ophthalmol Klin 4:193, Sears ML and Barany EH: Outflow resistance and adrenergic mechanisms: Effects of sympathectomy, N(2-chloroethyl) dibenzylamine hydrochloride (dibenamine) and dichloroisoproterenol on the outflow resistance of the rabbit eye. Arch Ophthalmol 64:839, Bill A: Effects of norepinephrine, isoproterenol and sympathetic stimulation on aqueous humor dynamics in Vervet monkeys. Exp Eye Res 10:31, Neufeld AH: Influences of cyclic nucleotides on outflow facility in the Vervet monkey. Exp Eye Res 27:387, Hendley ED and Cromble AL: The 24-hour ganglionectomy effect in rabbits: The influence of adrenergic blockade, adrenalectomy and carotid ligation. Exp Eye Res 6:152, Norton AL and Viernstein LJ: The effect of adrenergic agents on ocular dynamics as a function of administration site. Exp Eye Res 14:154, Becker B: Annual reviews: Glaucoma Arch Ophthalmol 56:898, Lorenzetti OJ: Dose-dependent influence of topically instilled adrenergic agents on intraocular pressure and outflow in the rabbit. Exp Eye Res 12:80, Potter DE and Rowland JM: Adrenergic drugs and intraocular pressure: Effects of selective beta-adrenergic agonists. Exp Eye Res 27:615, Thomas JV and Epstein DL: Timolol and epinephrine in primary open-angle glaucoma. Arch Ophthalmol 99:91, Sears ML: Perspectives in glaucoma research. Invest Ophthalmol Vis Sci 17:6, Polansky JR, Weinreb RN, Baxter JD, and Alvarado J: Human trabecular cells: I. Establishment in tissue culture and growth characteristics. Invest Ophthalmol Vis Sci 18:1043, Alvarado JA, Wood I, and Polansky JR: Human trabecular cells: II. Growth pattern and ultrastructural characteristics. Invest Ophthalmol Vis Sci 23:464, Polansky JR, Wood IS, Maglio MT, and Alvarado JA: Trabecular meshwork cells culture in glaucoma research: evaluation of biological activity and structural properties of human trabecular cells in vitro. Ophthalmology 91:580, Tripathi BJ and Tripathi RL: Effect of epinephrine in vitro on the morphology, phagocytosis, and miotil activity of human trabecular endothelium. Exp Eye Res 33:731, Wax MB and Molinoff PB: Distribution and properties of p- adrenergic receptors in human iris/ciliary body. Invest Ophthalmol Vis Sci 28:420, Barovsky K and Brooker G: (-)-( 125 I)-Iodopindolol, a new. highly selective radioiodinated 0-adrenergic receptor antagonist: Measurement of ^-receptors on intact rat astrocytoma cells. J Cyclic Nucleotide Res 6:297, Wolfe BB and Harden TK: Guanine nucleotides modulate the affinity of antagonists at /3-adrenergic receptors. J Cyclic Nucleotide Res 7:303, Bradford MM: A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal Biochem 72:248, Scatchard G: The attractions of proteins for small molecules and ions. Ann NY Acad Sci 51:660, Cheng Y-C and Prusoff WH: Relationship between the inhibition constant (K;) and the concentration of inhibitor which causes 50% inhibition (I 50 ) of an enzymatic reaction. Biochem Pharmacol 22:3099, Molinoff PB: a- and /3-adrenergic receptor subtypes: Properties, distribution and regulation. In Pharmacology of Combined a-0-blockade and Its Clinical Implications. Serninar-in- Print, Prichard BNC, editor. Drugs 28 (Suppl. 2):1, Neve KA, McGonigle P, and Molinoff PB: Quantitative analysis of the selectivity of radioligands for subtypes of beta adrenergic receptors. J Pharmacol Exp Ther 238:46, Allen RC and Epstein DL: Additive effect of betaxolol and epinephrine in primary open angle glaucoma. Arch Ophthalmol 104:1178, Polansky JR, Kurtz R, Friedman Z, Bloom E, and Alvarado J: Differences between ophthalmic /3-blockers: Effects on cultured human trabecular cells and plasma radioreceptor assays. ARVO Abstracts. Invest Ophthalmol Vis Sci 28(Suppl):267, 1987.