Adrenergic drugs and intraocular pressure

Facility changes mediated by cAMP in the bovine anterior segment in vitro

The aim of this study was to investigate the influence of substances that increase intracellular cAMP levels on the aqueous humor outflow facility (C) of isolated bovine anterior segments. Anterior segments were perfused in vitro at a constant pressure of 10 mmHg for 270 min with a general protocol as follows: 90 min control perfusion with DMEM, 90 min of experimental perfusion with DMEM containing the test drug(s), and 90 min of postdrug-perfusion with DMEM. C was calculated as the ratio between the rate of medium inflow (microliter/min) and the perfusion pressure (mmHg). Anterior segments can be perfused in vitro for up to 5 hr without significantly modifying their C. The addition of epinephrine, forskolin, dibutyryl-cAMP or isobutylmethylxanthine to the control perfusion medium elicited a significant increase of C. If, during isobutylmethylxanthine perfusion, forskolin or epinephrine was added, C increased significantly. Finally, perfusion with indomethacin prior to addition of epinephrine prevented the increase of C induced by epinephrine. Epinephrine, the adenylate cyclase activator forskolin, the cAMP analog dibutyryl-cAMP, and the phosphodiesterase inhibitor isobutylmethylxanthine all increase aqueous facility. It seems reasonable to suspect that the cAMP system is involved in epinephrine's effects on bovine trabecular meshwork cells. Moreover, the complete inhibition by indomethacin of the outflow facility increase induced by epinephrine suggests that prostaglandins may be involved in the outflow facility mechanisms related to adrenoreceptor stimulation of trabecular meshwork cells.

Adrenergic and imidazoline receptor-mediated responses to UK-14,304-18 (brimonidine) in rabbits and monkeys. A species difference

Brimonidine is a relatively selective alpha-2 adrenoceptor agonist that is being developed for the treatment of glaucoma. Because brimonidine is chemically related to clonidine and has affinity for the nonadrenergic imidazoline receptor, its ocular effects may be unrelated to alpha-2 receptor activation. The objective of this study was to determine the pharmacology of the intraocular pressure (IOP) response to brimonidine in rabbits and monkeys and the side effects (miosis, cardiovascular depression) in monkeys. Conscious albino rabbits and cynomolgus monkeys were pretreated topically with the following receptor antagonists: rauwolscine (alpha-2), idazoxan (alpha-2, imidazoline), SKF 105854 (vascular postjunctional alpha-2), and prazosin (alpha-1). Intraocular pressure, pupil size, or blood pressure/heart rate was monitored noninvasively for 6 hours following dosing. Binding experiments were performed using [3H]brimonidine in membrane preparations from rabbit iris/ciliary body and from monkey cerebral cortex and brain stem. In rabbits, the ocular hypotensive response to brimonidine was unilateral and was inhibited by rauwolscine > idazoxan >> SKF 105854 = prazosin; this ranked order of potency correlated with displacement of [3H]brimonidine in the rabbit iris/ciliary body. In monkeys, brimonidine decreased IOP bilaterally and suppressed cardiovascular function suggesting a CNS site of action. Intraocular pressure and cardiovascular responses to brimonidine were inhibited by idazoxan >> rauwolscine > SKF 105854 = prazosin; a similar profile was obtained for displacement of [3H]brimonidine in monkey brain tissue. Both rauwolscine and idazoxan inhibited the miotic response to brimonidine in monkeys. Taken together, these results indicate that brimonidine stimulates an ocular alpha-2 adrenoceptor to decrease IOP in the rabbit and a CNS imidazoline receptor to decrease IOP, blood pressure, and heart rate in the cynomolgus monkey. The miotic response in the monkey is mediated by an alpha-2 adrenoceptor. The alpha-1 and vascular postjunctional alpha-2 adrenoceptors do not appear to play a role in mediating these responses.

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.

Alpha-adrenergic antagonists: correlation of the effect on intraocular pressure and on alpha 2-adrenergic receptor binding specificity in the rabbit eye

Six alpha-adrenergic antagonists, which have a range of selectivities for alpha 1- and alpha 2-adrenoreceptor subtypes, were compared with respect to their ability to reduce intraocular pressure (IOP) after topical application to the rabbit eye, and their affinity and selectivity for alpha 2-adrenoreceptors, as determined by binding to membranes prepared from rabbit iris-ciliary body. A routine assay for alpha 2-adrenoreceptors using [3H]-rauwolscine was developed for this purpose. ICB contained 200-300 fmol (mg protein)-1 alpha 2-adrenoreceptors which represents approximately two-thirds of the total number of alpha-adrenoreceptor sites present in this tissue. All six antagonists bound at alpha 2-adrenergic receptor sites in an apparently simple competitive manner. The Kd for three of the drugs was about 10 nM (rauwolscine, yohimbine, WB-4101) and the Kd for the other three was greater than 3500 nM (prazosin, corynanthine, thymoxamine). However, all six antagonists were effective ocular hypotensive agents when given topically in a 50 microliter dose of 1% (w/v) concentration. The ability of alpha-adrenergic antagonists to lower IOP in the rabbit did not correlate with a single alpha-receptor subtype and appears to involve at least two separate mechanisms, one mediated by alpha 2-adrenergic receptors and one mediated by alpha 1-adrenergic receptors.

Adrenergic receptor subtypes in rabbit iris-ciliary body membranes: classification by radioligand studies

Receptor subclasses in iris-ciliary body cell membranes were determined by direct binding of radioactive dihydroalprenolol (DHA), yohimbine (YOH), WB-4101 WB) and prazosin (PRZ), classified respectively as beta 1 + beta 2, alpha 2, alpha 1, and alpha 1 subtype selective ligands (based on binding to brain adrenergic receptors). Specific binding was defined with appropriate unlabelled agonist and antagonist drugs in each case. Binding data were analysed by library programs of the PROPHET computer system. Subclass specificity was also determined indirectly by binding competition of the labelled ligand (at a three- to 10-fold Kd concentration) with increasing concentrations of 'cold' agonist or antagonist. Ligand binding parameters of unlabelled drugs were obtained from Dixon and Scatchard plots of the data. The adrenergic receptor density of iris-ciliary body is approx. 600 fmol mg-1 protein, of which 20-25% are primarily beta 2 receptors. Three distinct subpopulations of alpha-receptors, representing 10, 25 and 40-45% of the total, bind PRZ, WB and YOH, respectively, each with high specificity for its corresponding ligand but with 10- to 1000-fold lower specificity for the other two ligands. The majority of alpha-adrenergic receptors are of the alpha 2-subtype. A small population of receptors are similar to vascular postsynaptic alpha 1-receptors, while a larger subpopulation may have characteristics intermediate between alpha 1- and alpha 2-subtypes.

Effect of epinephrine in vitro on the morphology, phagocytosis, and mitotic activity of human trabecular endothelium

Epinephrine exerts a direct effect on cell morphology, phagocytosis and mitotic activity of human trabecular endothelium in primary culture. Its action is probably mediated through both beta and alpha adrenoceptors in a dose-time dependent manner. Younger cells and cells that were loosely attached to the substrate were found to be affected more rapidly and severely than were older cells and those in confluent regions where cell-to-cell attachment and stress fibers were well established. Continuous exposure to epinephrine at a concentration of 10(-5) M led to cessation of the normal cytokinetic cell movements, inhibition of mitotic and phagocytic activity, marked cell retraction, separation from the substrate, and, by 4-5 days, degeneration of cells. Similar, but less marked changes were seen with a concentration of 10(-6) M, the cell degeneration becoming apparent after one week of exposure. A still weaker concentration of epinephrine, 10(-7) M, did not result in cell degeneration even after 10 days of exposure and observation. On complete withdrawal of the drug, the cellular effects were reversible even after 3 days' exposure to 10(-5) M and 5-7 days' exposure to 10(-6) M epinephrine. The action of epinephrine was partially blocked by pretreatment of cultured trabecular cells with the beta-blocker, timolol. Available evidence suggests that the mechanism of action of epinephrine is mediated through both beta and alpha adrenoceptors, and that it intimately involves the cytoskeletal system of the cells. Extrapolation of our findings in vitro suggests that use of maximal doses of epinephrine over a prolonged time may contribute to tissue damage in certain conditions of glaucoma.

Effects of norepinephrine and other pharmacological agents on prostaglandin E2 release by rabbit and bovine irides

We have investigated: (a) synthesis and release of prostaglandin E2 (PGE2) and the effects of norepinephrine (NE) and other pharmacological agents thereon in rabbit and bovine irides; (b) the role of Ca2+ and the type of adrenergic receptors involved in PGE2 release by rabbit iris; (c) the structural requirement for catecholamine stimulation of PGE2 release by rabbit iris. Irides were incubated in Krebs-Ringer buffer (pH 7.4) in the absence and presence of pharmacological agents at 37 degrees C for 20 min. PGE2 in the medium was quantitated by radioimmunoassay; or by means of radiometric and chromatographic methods when [1-14C]-arachidonic acid was employed as precursor. Analysis of tissue PGE2 at the start of incubation revealed that rabbit and bovine irides contained about 185 and 18 ng/g tissue, respectively. During 15 min of incubation the release of PGE2 from rabbit and bovine irides was approximately 1100 and 14 ng/g tissue, respectively. We found that the rabbit iris and iris microsomes synthesize PGE2, both from endogenous and exogenous arachidonate pools, at a rate several times as high as that of bovine. Release of PGE2 by irides from both species is time-dependent; the stimulatory effects of NE on PGE2 release by irides from rabbit are more pronounced than those of the bovine; and the NE-induced release of PGE2 was abolished by low concentrations of indomethacin (1.5 microM). Synthesis and the effect of NE on PGE2 release by irides from albino and pigmented rabbit eyes are similar. It is concluded that the differences observed in the synthesis of PGE2 and the effect of NE on its release by rabbit and bovine irides are due to species differences. NE stimulation of PGE2 release by rabbit iris is probably mediated through alpha-adrenoceptors and maximal adrenergic stimulation requires the presence of Ca2+. Ca2+-ionophore A23187 significantly increased PGE2 release. These data suggest that the control step in PG synthesis, that is the release of free arachidonate from membrane phospholipids, could be involved in the NE-induced PG synthesis in this tissue. The structural studies revealed that both the catechol nucleus and the ethylamine polar-side chain are required for catecholamine activation of PG release by the rabbit iris. Thus normetanephrine significantly stimulated PGE2 release by the rabbit iris and iris microsomes; 3-methoxy, 4-hydroxy mandelic acid had no effect; and catechol at 50 microM inhibited PGE2 release by more than 50%. Catecholamines and adrenergic drugs are routinely employed therapeutically to lower intraocular pressure in the eye, and a catecholamine-induced increase in PG synthesis may play a significant role in mediating the pharmacological effects of these therapeutic agents.