Are beta-adrenergic mechanisms involved in ocular hypotensive actions of adrenergic drugs?

Specific beta 1- and beta 2-adrenergic agonists and antagonists were studied for their effects on aqueous humor dynamics in a cat model. Tazolol (beta 1-agonist) reduced formation of aqueous humor more than outflow, while metoprolol (beta 1-antagonist) reduced outflow more than formation. Both salbutamol (beta 2-agonist) and butoxamine (beta 2-antagonist) inhibited aqueous humor formation and aqueous humor outflow to an equal extent. It is concluded that the adrenergic mechanism is functioning only slightly, if at all, in affecting the intraocular pressure. On the other hand, receptor binding experiments on iris-ciliary body showed clearly that there are 40-fold and 281-fold differences in binding affinity of D- and L-isomers of timolol and propranolol, respectively. These results indicate that the adrenergic receptor mechanism does exist in the eye tissues but that it does not control intraocular pressure significantly.

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.

Establishment of cell lines of uveal melanoma. Methodology and characteristics

Six continuous cell lines have been established from choroidal and ciliary body melanomas. These lines have been maintained in culture for at least 100 in vitro population doublings for periods over 1 year. They were established initially using a human diploid fibroblast strain MRC-5 as a feeder layer. Cells were grown in Ham's F-12 medium containing fetal bovine and horse sera and supplemented with glucose, cholera toxin, and epidermal growth factor. Culture doubling times ranged from 72-96 hr; cloning efficiencies ranged from 1-5% in the absence of a feeder layer. Six cell lines were studied in detail by electron microscopy, and all were found to have evidence of melanosomes and/or premelanosomes. The morphology of the cells was characteristic of melanomas as defined by the Callender classification, with cell types ranging from spindle A to epithelioid. Karyotypic studies revealed the presence of only human chromosomes with modal numbers ranging from 48-54 in the different lines.

Studies on the involvement of melatonergic mechanism in intraocular pressure regulation

It has been reported that diurnal changes in intraocular pressure (IOP) may be related to the fluctuation of melatonin levels in the eye. In order to test this hypothesis, effects of methysergide, tranylcypromine, and p-chlorophenylalanine (PCPA) on IOP recovery were studied in rabbits infused with hypertonic saline. Methysergide, a serotonin blocker at receptor sites, did not affect IOP recovery, presumably because it did not affect melatonin synthesis. Tranylcypromine, a monoamine oxidase inhibitor which preserves serotonin and eventually increases the level of melatonin in tissues, facilitated IOP recovery. PCPA, known to inhibit serotonin synthesis and to lower the melatonin level in tissues, markedly delayed IOP recovery. These results indicate that a chemical which decreases melatonin content in the eye also lowers the IOP and vice versa. It is suggested that some melatonergic mechanism is involved in the circadian rhythm of IOP.