Showing papers in "Investigative Ophthalmology & Visual Science in 1980"

Chronic experimental glaucoma in primates. II. Effect of extended intraocular pressure elevation on optic nerve head and axonal transport.

Abstract: Intraocular pressure (IOP) elevations lasting from 2 to 42 days were produced in 13 primate eyes by anterior chamber injections of autologous, fixed red blood cells. The retina, optic nerve head, and optic nerves were studied by electron microscopy, and ganglion cell rapid axonal transport was examined after IOP elevations for various durations. Transport of axonal material was blocked at the scleral lamina cribrosa by IOP elevations to 50 mm Hg. With IOP elevation for less than 1 week, return to normal IOP restored normal transport in some axons. However, in other axons IOP elevation for less than 1 week intiated ganglion cell degeneration. The process of cellular death involved a rapid ascending degeneration from nerve head to brain, followed 3 to 4 weeks later by descending degeneration of the ganglion cell body and its attached axon. Axons of the superior and inferior optic nerve head and nerve seem to be damaged more extensively than those in the nasal and temporal optic nerve. Two to four days after IOP elevation, axons of the superficial optic nerve head were swollen with accumulating axonal material, leading to histologic disk edema. In those eyes with IOP elevation longer than 1 week, the loss of anterior disk nerve fibers combined with posterior and lateral movement of the lamina cribrosa lead to an increase in optic disk cupping. Astrocytes and capillaries of the optic nerve head seem to tolerate elevated IOP well and were relatively spared.

Circadian nature of rod outer segment disc shedding in the rat.

Abstract: Rod outer segment disc shedding has been examined in the rat to determine whether it follows a circadian rhythm. The burst of shedding that occurs soon after the onset of light repeats daily in continuous darkness for at least 12 days. The disc shedding rhythm becomes slightly longer than 24 hr in continuous darkness, with an estimated period of 24 hr and 8 min to 24 hr and 19 min. In rats maintained in cyclic light, the disc shedding rhythm can be re-entrained to a new light schedule following a phase shift in the environmental lighting. After a 3 hr phase delay, at least 4 weeks is required for re-entrainment. Rod outer segment disc shedding in the rat can be considered to follow a circadian rhythm because (1) it is a self-sustaining oscillation; (2) it appears to free-run with a period close to 24 hr in the absence of lighting cues; and (3) it can re-entrain to shifts in the light cycle.

Growth characteristics and ultrastructure of human retinal pigment epithelium in vitro.

Abstract: The in vitro growth characteristics and morphology of human retinal pigment epithelium (RPE) cells from adult donor eyes (15 to 100 years of age) have been studied. Although RPE cells are viable in culture for several months, only a fraction of the cells actually divide. Rapid cell proliferation and confluency of a culture occur from 10 to 30 days after seeding. The time for confluency is both age- and media-dependent; the number of cells that are potential dividers and contribute to confluency decreases with increasing donor age. Since melanolipofuscin granules do not form in vitro and are diluted by cell division, stationary (nondividing) cells can be distinguished from the dividing cells by the presence of dense clusters of melanolipofuscin granules in the stationary cells. Confluent cultures contain a monolayer of relatively clear polygonal cells with densely pigmented stationary cells scattered throughout. Stationary cells can often represent as much as 95% of the original RPE cell population. Ultrastructurally all cells appear epithelioid, with apical-basal polarity, junctional complexes, and cytoplasmic organization characteristic of RPE cells in vivo. Stationary cells are extremely large, with melanolipofuscin granules clustered around the nucleus. Melanolipofuscin is a unique marker for nondividing cells and may facilitate studies of the age-dependent loss of replicability of human RPE.

Preservative cytotoxic threshold for benzalkonium chloride and chlorhexidine digluconate in cat and rabbit corneas.

Abstract: Benzalkonium chloride (BAC) and chlorhexidine digluconate (CDG) were applied to rabbit and cat corneal epithelium in clinically used concentrations Corneas were fixed 1/2 hr later and examined by scanning electron microscopy (SEM) BAC was found to produce a progressive increase in damage at concentrations between 0001% and 001% as determined by SEM CGD produced less damage than BAC at any concentration Cats lacrimated copiously and blinked frequently after instillation of drops; rabbits did not No significant difference was found between the two species, however, in their response to the preservative agents tested It is presumed that binding of these surface active agents occurs almost immediately and is unaffected by tear film dilution Studies measuring permeability increase in the human eye after preservative use are required to allow clinical interpretation of the data presented here

Immediate effect of epinephrine on aqueous formation in the normal human eye as measured by fluorophotometry.

Abstract: Epinephrine hydrochloride (1%) and an identical-appearing placebo were instilled into the right/left eye pairs (random assignment) of 24 young normal subjects. Aqueous humor flow (measured by fluorophotometry over a 7 hr period), intraocular pressure, and (in 10 subjects) tonographic facility of outflow were measured. The effects of epinephrine were to lower intraocular pressure (2.8 mm Hg +/- 1.9, mean +/- S.D.), raise the facility of outflow (37.4% +/- 21.7), and increase the rate of formation of aqueous humor (18.6% +/- 18.1). By comparing tonographic and fluorophotometric data in the same subjects, it was postulated that epinephrine increases the rate of outflow via the pressure-insensitive (uveoscleral, uveovortex) outflow pathway.

Diet-related macular anomalies in monkeys.

Abstract: Fundus color photographs and retinal fluorescein angiograms were obtained from 48 nonhuman primates of three macaque species. Yellow pigmentation of the macula was present in monkeys fed a standard laboratory diet containing xanthophylls but was absent in animals maintained on semipurified or liquid formula diets with no xanthophyll content. Plasma levels of xanthophylls ranged from 0.5 to 2.4 microliters/ml in monkeys receiving the standard diet but were undetectable in animals raised on semipurified or liquid formula diets. Fluorescein angiograms revealed foveal areas of hyperfluorescence in almost all monkeys; however, the degree of hyperfluorescence was significantly greater in monkeys maintained on the semipurified or liquid formula diets.

Disturbances of small-field horizontal and vertical optokinetic nystagmus in amblyopia.

Abstract: Horizontal and vertical small-field optokinetic nystagmus (OKN) were examined in persons with strabismic or anisometropic amblyopia. Reduced velocity for the slow phase of OKN driven by temporalward and upward target motion presented monocularly was observed in both the amblyopic and nonamblyopic eyes of some subjects. Several experiments were conducted in search of a sensory disturbance of perceived motion sensitivity which could account for the abnormal OKN. Comparisons between the frequency response for OKN and the contrast sensitivity function for perceived motion revealed that amblyopes with asymmetric OKN had equal sensitivity to nasal and temporal target motion. Contrast thresholds for driving the temporal slow phase of OKN were elevated by over 1 log unit above contrast thresholds for perceived temporal target motion, whereas contrast thresholds for stimulating nasal movement and driving the nasalward slow phase of OKN were equal. Contrast sensitivity to nasal and temporal target motion was symmetrical at thefovea and parafovea of the amblyopic eye. These studies did not reveal a sensory anomaly as the basis for abnormal OKN in amblyopia. Directional asymmetries in OKN were also observed in both eyes of persons with congenital strabismus without amblyopia and in the nondeprived eye in monocular congenital cataract. These observations suggest a relationship between directional asymmetries of OKN and the incomplete development of binocular vision.

Vasoactive intestinal peptide nerves in ocular and orbital structures of the cat.

Abstract: Vasoactive intestinal polypeptide (VIP), a neuronal peptide of ubiquitous occurrence in the body, is known to have strong vasodilatory effects and to promote secretion from many exocrine glands. Nerves displaying VIP immunoreactivity (VIP nerves) were detected in several orbital structures of the cat. Such nerves were numerous in the lacrimal glands and somewhat less numerous in the Harderian glands and the tarsal glands. The nerves surrounded glandular acini and small blood vessels. Intraocularly, VIP nerves were seen in the ciliary processes, in the posterior third of the ciliary muscle, and around small to medium-sized blood vessels in the posterior uvea. VIP nerve fibers were absent from vessels in the anterior uvea. This distribution may explain why intracranial stimulation in the oculomotor nerve exit region dilates the vessels of the choroid but not those of the iris. A large number of VIP-immunoreactive nerve cell bodies were observed in the pterygopalatine ganglion. Extirpation of this ganglion resulted in the disappearance of VIP nerves from the intraocular structures and from the lacrimal and Harderian glands. Removal of the superior cervical ganglion and the ciliary ganglion did not affect the VIP nerve supply. The results suggest that the VIP nerves originate in the pterygopalatine ganglion.

The mechanism of optic nerve damage in experimental acute intraocular pressure elevation

Abstract: We produced intraocular pressure (IOP) elevations in 32 primate eyes and studied retinal ganglion cell rapid axonal transport with autoradiography and electron microscopy Animals breathing room air at sea level pressure were compared to animals breathing 100% oxygen at 3 atm pressure in a hyperbaric chamber Despite major increases in arterial oxygen levels in the hyperbarically oxygenated animals, both groups had axonal transport blockade at the optic nerve head Anoxia appears not to be the most important cause of acute axonal damage induced by elevated IOP The pattern of axonal abnormality within individual fiber bundles at the optic nerve head provides support for mechanical compression as a more likely alternative cause for induced neural damage

The effect of metabolic inhibitors on retinal adhesion and subretinal fluid resorption.

Abstract: Retinal adhesion and the resorption of subretinal fluid were studied in Dutch rabbits, with two experimental techniques. In the first, small local detachments were made in vivo by injecting a small amount of fluid under the retina with a micropipette, and the time for resorption of the detachment was monitored. In the second, strips of eyecup were maintained in a physiologic bathing medium, and the force required to peel retina from the pigment epithelium was measured. Using these techniques, we have found that both the mechanisms for removing subretinal fluid and for maintaining retinal adhesion in vitro are enhanced by exposure to ouabain, are inhibited by exposure to cyanide, and are inhibited by the replacement of the physiologic medium with normal saline. These data suggest that metabolic systems may simultaneously hydrate and dehydrate the subretinal space and that some factors which are important to the maintenance of normal adhesion are missing from normal saline.

Contrast sensitivity in the presence of a glare light. Theoretical concepts and preliminary clinical studies.

Abstract: A method is presented for quantitative measurements of the glare effect of light scattered in the ocular media The contrast sensitivity function is measured with a television display system A bright light source is introduced into the field of vision, and the resultant decrease in contrast sensitivity is mea-ured It is further used to calculate a scattering factor which is a direct measure of the intraocular light scattering The scattering factor shows a marked increase in patients with early cataracts even if their visual acuity is not affected

Evidence for both local and central regulation of rat rod outer segment disc shedding.

Abstract: Rats entrained to a 7 A.M.:7 P.M. lighting cycle had one eye patched and then were placed in either constant light or in a 2 A.M.:2 P.M. lighting cycle. In both cases, the patched eyes maintained the original pattern of rod outer segment (ROS) disc shedding, whereas the open eyes responded to the ambient lighting conditions. When the optic nerves were severed at the optic chiasm, the shedding rhythm persisted, but it was no longer possible to shift the rhythm to a new lighting cycle. From these experiments we conclude that ROS disc shedding is regulated both by local oscillators within the orbit as well as by central elements.

Studies on retinal neovascularization. Friedenwald Lecture.

Abstract: Evidence supporting the original concept of Michaelson and Ashton of a vasoproliferative or angiogenesis factor responsible for retinal neovascularization is discussed. The oxygen model of retinal neovascularization is described as a key example in this evidence. Newer studies on experimental retinal neovascularization and its control are summarized.

Deficiencies of vitamins E and A in the rat. Retinal damage and lipofuscin accumulation.

Abstract: The interrelationships of vitamins E and A in maintaining various structural components of the retina were studied in four groups of weanling female rats fed purified diets adequate or deficient in each vitamin: +E, +A; -E, +A; +E, -A; and -E, -A. Groups deficient in retinol (-A) were supplemented with retinoic acid. After 14, 21, and 35 weeks, the retinas were examined histologically and ultrastructurally. At 35 weeks, the doubly deficient rats (-E, -A) had lost 92% of their rod nuclei, whereas rats deficient in vitamins A (+E, -A) or E (-E, +A) alone had lost only 34% and 20%, respectively. Vitamin E deficiency resulted in extensive lipofuscin deposits in the retinal pigment epithelium as early as 21 weeks, but the presence of vitamin A doubled the number of lipofuscin granules (-E, +A vs. -E, -A) and induced an even greater increase in their autofluorescence. Another clear influence of vitamin A was seen when +E, +A retinas autofluoresced not only much more than +E, -A retinas, which had similar numbers of granules, but also more than -E, -A retinas, which had about twice as many lipofuscin granules. In the retina, unlike the uterus, the lipofuscin-specific autofluorescence and lipofuscin granule number were not proportional. Moreover, the numbers of granules were influenced by both vitamins E and A, whereas the intensity of lipofuscin-specific autofluorescence was determined almost exclusively by vitamin A. Probably the accelerated loss of photoreceptor cells in -E, -A retinas resulted from both oxidation of membranes and oxidation of retinal vitamin A stores in the absence of vitamin E protection.

The role of ambient lighting in circadian disc shedding in the rod outer segment of the rat retina.

Abstract: Rats raised from birth on a 7 A.M.:7 P.M. light cycle exhibited a circadian burst of shedding 1 1/2 hr after the scheduled onset of light. This rhythmic event persisted even after the animals had been placed in total darkness for up to 2 weeks. When animals were placed in constant light (LL) for more than 24 hr, the shedding peak was abolished. A period of darkness of at least 2 hr, ending at the normal time for light onset, was necessary for the production of a shedding burst. When animals were placed in altered lighting cycles, the shedding rhythm shifted accordingly, although the speed of this shift was greatest with large advances in the light cycle and slowest with small delays in the light cycle. Animals exposed to LL and then returned to the dark exhibited the original shedding rhythm, a phenomenon not affected when the optic nerves were severed. A shedding peak could be induced at a noncircadian time by exposing the animals to more than 20 hr of LL followed by 2 hr of darkness and 1 1/2 hr of light, although the magnitude of the shedding was related to the circadian time.

Altering the course of cataracts in diabetic rats

Abstract: A potent new aldose reductase (AR) inhibitor was effective in preventing cataractous changes in diabetic rats. Untreated diabetic rats developed early lens changes by 3 weeks and dense nuclear opacities by 6 to 9 weeks. In contrast, diabetic rats treated with the AR inhibitor showed no lens changes during the 5-month period of the experiment.

Evidence for a role of the plasminogen activator-plasmin system in corneal ulceration

Abstract: Plasminogen is present in the cornea andcan be activated to plasmin by plasminogen activator. Plasmin is able, in turn, to activate latent collagenase. This system could initiate and perpetuate the collagen degradation of corneal ulceration. This report details evidence for such a system in the cornea. Plasmin has been found to activate latent collagenase from organ cultures of ulcerating rabbit corneas and from fibroblast cultures derived from such corneas. As in the case of activation by trypsin, activation by plasmin results in the conversion of the 40,000 MW latent form to an active species of 23,000 MW. Explants of normal or alkali-burned, ulcerating corneas demonstrated plasminogen-dependent lysis of fibrin clots; frozen sections of such corneas demonstrated that lysis begins in the superficial stroma near the periphery of the cornea. Multiply freeze-thawed ulcerating corneas, but not normal corneas, showed initial lysis, not peripherally but at the ulcer region containing polymorphonuclear leukocytes. The fact that the peripheral lytic pattern existed in corneas that were obtained from eyes prefrozen in liquid nitrogen before excision of the corneas would suggest that plasminogen activator is normally contained in cells in vivo and is not made only in response to tissue injury. There was no correlation between the location of blood vessels or the presence of the corneal endothelium and the plasminogen-dependent lysis. Plasminogen activator from the ulcerating cornea and from fibroblasts was characterized by sodium dodecyl sulfate--gel electrophoresis of its cleavage products of plasminogen. The activator cleaves plasminogen into heavy- and light-chain fragments similar to those produced from plasminogen by urokinase. Plasminogen activator activity was quantitated by a new assay that restricts diffusion of the enzyme to one dimension into a narrow bore tube. The addition of plasminogen daily to cultures of ulcerating corneas resulted in earlier rises of plasminogen activator, collagenase, and collagen degradation fragments in the culture media. Although total plasminogen activator levels were not increased by the addition of plasminogen to culture, levels of both collagenase and solubilized collagen were approximately doubled. It is concluded that the plasminogen activator--plasmin system might play an important role in the destruction of stromal matrix in corneal ulceration.

Optically induced concomitant strabismus in monkeys.

Abstract: In infant monkeys that view the world through dissociating prisms the binocular neurons of the striate cortex cease to function. The loss of binocular cortical neurons is proportional to the duration of prism wear. Virtually all binocular neurons will disappear from the striate cortex within 60 days, and the remaining monocular neurons will become equally divided, between the two eyes. Although the cortical neuron population is dramatically altered, this form of experimental strabismus failed to affect the size of principal cells of the lateral geniculate nucleus.

Sensitivity of photoreceptors to elevated levels of cGMP in the human retina.

Abstract: When isolated human retinas were cultured in the presence of a phosphodiesterase inhibitor or dibutyryl cyclic guanosine monophosphate (dbcGMP), degenerative changes occurred which were proportional to the concentration of drug used and the period of exposure. Low concentrations of either drug did not alter retinal morphology as compared to controls. Higher concentrations provoked vesiculation of rod inner segments and rounding up of cones. Numerous pyknotic nuclei were noted in the outer nuclear layer of those preparations. Combining IBMX and dbcGMP in the same medium destroyed virtually every rod in the specimen within 8 hr of incubation. Under those conditions, cones remained structurally intact although somewhat rounded. In all treatments, cells of the innerretinal layers maintained normal morphology. Our results suggest that elevated levels of cGMP in the human retina can alter certain metabolic processes in photoreceptors, which leads to degenerative changes and cell death uniquely in rod photoreceptors.

Chronic experimental glaucoma in primates. I. Production of elevated intraocular pressure by anterior chamber injection of autologous ghost red blood cells.

Abstract: Chronic elevations of intraocular pressure (IOP) were produced in rabbit and monkey eyes by anterior chamber injection of autologous fixed red blood cells The method confirms the possibility of secondary glaucoma due to trabecular obstruction by ghost cells in eyes with intraocular hemorrhage In primates, decreased aqueous outflow may result from direct obstruction by free cells and macrophages as well as swelling of trabecular endothelium following phagocytosis of cellular debris IOP elevations for from 2 days to greater than 1 month were produced in order to study the effects of elevated IOP on ocular tissues The model has the advantages of producing IOP elevation easily and without associated intraocular inflammation The extensive filling of the anterior chamber necessary to produce IOP rises in healthy animal eyes leads to the disadvantage of poor visibility of the optic disk In rabbit eyes, chronic IOP elevation leads to corneal enlargement and ectasia, making IOP measurements difficult

Inferred natural history of uveal melanoma.

Abstract: We have observed that the fatality rate of patients with posterior uveal melanomas rapidly increased from a very low rate prior to enucleation to a maximum of 8% per year during the second year after enucleation. These data suggest to us that events related to enucleation have a deleterious effect on survival, and we have postulated that these events are either the iatrogenic dissemination of tumor cells or an adverse effect on the immune-defense system, or both. Others have proposed that the relationship between enucleation and increased mortality is only happenstance. They would attribute the onset of symptoms to the rapid growth of the malignant tumor and thus claim that this brings the patient to enucleation soon after the tumor becomes malignant. We have studied 2105 cases on file in the REgistry of Ophthalmic Pathology and have found that enucleation was not always performed shortly after the onset of visual disturbance. In one third of the cases, enucleation was delayed until onset oment was not obtained until the tumor measured larger than 15 mm in diameter. Using Zelen's method to infer the natural history of uveal melanoma, we found that the mean time it took for a small tumor (less than 10 mm in diameter) to become a large tumor (greater than 15 mm) was approximately 7 years. The average age of patients treated for medium-sized tumors was 5 years less than that of patients treated for larger tumors. This is interpreted as evidence of an average delay of 5 years in the treatment of large melanomas. If this delay is taken into consideration, then the patients treated with medium-sized tumors had a worse survival during the first 7 years than did patients whose treatment was delayed until the tumor became large. After the seventh year, however, the survival was better. These findings support our hypothesis that the postoperative increase in mortality during the first several years, particularly among patients with tumors of medium size, was related to enucleation.

Eye movements of the blind.

Abstract: We investigated a group of patients who were blind because of disease affecting the anterior visual pathways. All subjects showed an inability to maintain steady eye position, with a consequent jerk nystagmus. Blindness from birth was associated with an impaired vestibuloocular reflex and inability to voluntarily initiate saccades, although quick phases of nystagmus were maintained. Acquired blindness was associated with relatively preserved vestibulo-ocular responses and the ability to initiate voluntary saccades and smoothly track self-moved targets. Certain features of the eye movements of the blind are similar to those due to cerebellar dysfunction.

Beta-adrenergic receptors in ciliary processes of the rabbit.

Abstract: Identification and characterization of beta-adrenergic receptors were attempted in particulate membrane fractions derived from isolated ciliary processes (CP) of rabbit eyes. High-affinity binding sites for 125I-hydroxybenzylpindolol (125I-HYP), a beta-adrenergic antagonist, were identified in particulate membrane fractions of homogenized CP that were recovered from discontinuous sucrose density gradients. Adenylate cyclase activity was recovered in the same fraction as the 125I-HYP binding sites. The dissociation constant of 125I-HYP for the high affinity site is 0.25 nM, with a minimum capacity of about 35 fmol/mg of protein. Adrenergic agonists and antagonists, including timolol, 1-alprenolol, d,1-propranolol, 1-isoproterenol, 1-epinephrine, and phentolamine, were examined for their ability to displace 125I-HYP from its binding site. The results were consistent with the identification of the high-affinity 125I-HYP binding sites as beta-adrenergic receptors. This is the first report which identifies by ligand binding techniques beta-adrenergic receptors in CP exclusive of iridial or other uveal tissue and supports the possibility of direct action of beta-adrenergic agents on the formation of aqueous humor.

Assessment of vision in young children: a new operant approach yields estimates of acuity.

Abstract: Behavorial assessment of visual function is now possible in 6- to 24-month-old children through the use of an operant preferential looking technique. Use of the technique to measure grating acuity shows that acuity develops from 6 to 8 min arc at 6 months of age to near adult levels by 2 years.

The pathophysiological effects of nitrogen mustard on the rabbit eye. II. The inhibition of the initial hypertensive phase by capsaicin and the apparent role of substance P.

Abstract: hovers near venous pressure. A study of glaucoma patients, in whom intraocular pressure is much higher, might yield different results. It appears that aand /3-adrenergic drugs have effects on the ciliary body in its production of aqueous humor. Beta antagonists seem to decrease aqueous formation, and beta agonists seem to increase formation. Unblocked alpha agonists appear to decrease aqueous formation slightly, but the alpha effect does not seem to be as important as the beta effect. To further clarify the effects of adrenergic drugs on aqueous formation, a similar experiment using an alpha blocker would be helpful because it is not known whether timolol blocks all the beta effects of epinephrine on the eye or not. It should be remembered that the acute, single-dose effect of epinephrine on the normal human eye probably differs from the effects of long-term administration. Most published studies in which the effect of epinephrine is measured somewhat later or after long-term administration indicate that aqueous formation is reduced.\" We do not know from this study whether timolol has any influence on the late or long-term effect of epinephrine on aqueous formation. Likewise, it is not known from these studies whether patients with abnormal aqueous humor dynamics respond differently to these drugs than do normals. To know, it will be necessary to measure aqueous humor flow directly in such patients before and after the test drug instillation.

Actin filaments in retinal pericytes and endothelial cells.

Abstract: The contractile protein actin was identified within retinal capillary pericytes and endothelial cells. In capillary cross-sections, circumferential pericyte processes showed numerous parallel bundles of actin filaments forming a cap over the adjacent endothelial cells. The latter contained few actin filaments but a considerable number of intermediate filaments. Pericytes and endothelial cells communicated via gaps in the adluminal basal lamina. These special anatomic relationships are interpreted as evidence for a contractile role of retinal capillary pericytes and against a contractile role of retinal capillary endothelial cells

Investigations into whether 5-hydroxytryptamine is a neurotransmitter in the retina of rabbit and chicken.

Abstract: A system of indoleamine-accumulating neurons exists in the retina of several species, but the exact transmitter of these neurons is not known. 5-Hydroxytryptamine (5-HT) is a likely candidate, but there are conflicting reports on its amount in the retina. We have determined the amounts of 5-HT with two sensitive, specific, and independent methods. Because the content was found to be lower than what was expected for a neurotransmitter, the rate-limiting enzyme in the 5-HT synthesis was also estimated, as was the effect of 5-HT on cyclic AMP content of the retina. In the rabbit the 5-HT content was found to be 25 to 35 ng/gm wet weight, with not difference between light- and dark-adapted animals. There was no detectable activity of the rate-limiting enzyme tryptophan hydroxylase. It was not possible to raise the 5-HT concentration by treating the rabbits with a monoamine oxidase inhibitor and L-tryptophan. 5-HT induced no change in the cyclic AMP content of the rabbit retina. In both newly hatched and older chicken retina, the 5-HT concentration was higher than in rabbit. The 5-HT concentrations in all retinas were lower than would be expected for a monoamine neurotransmitter, and these results argue against 5-HT being the neurotransmitter of the indoleamine-accumulating neurons.

Pharmacological effects of topical timolol in the rabbit eye.

Abstract: The ability of timolol to act as a beta-adrenergic antagonist in the cornea and in the iris--ciliary body of albino rabbits is reported. In vitro, timolol potently blocks the isoproterenol-stimulated synthesis of cAMP. In the iris-ciliary body, the apparent inhibition constant (K1) for timolol is 0.6 nM, indicating that timolol is approximately seven times more potent than propranolol. Topical timolol (0.5% and 4%) rapidly inhibits the beta-adrenergic--stimulated synthesis of cAMP in both corneal and iris--ciliary body tissues. Within 3 hr, however, the iris--ciliary body tissue regains its ability to produce large amounts of cAMP when challenged in vitro with isoproterenol. The washout of timolol from corneal tissue is more protracted. Intraocular pressure measurements by anterior chamber cannulation of anesthetized rabbits indicate that topical timolol (0.5% and 4%) has no significant influence on intraocular pressure. These concentrations of timolol significantly decrease heart rate but do not affect femoral arterial blood pressure. Because of the marked potency of timolol clinically, we conclude that the effects of the drug in the rabbit do not completely account for its therapeutic efficacy in humans, which must depend on more complex pharmacological actions.

Collagen production by cultured retinal capillary pericytes.

Abstract: The cells that proliferate from microvessels isolated from calf retinas and placed in tissue culture have been found to derive from the intramural pericytes. When these cells were cultured in medium supplemented with ascorbate and pulsed for 24 hr with [14C]proline, about 2% of the [14C]proline in the nondialyzable protein secreted into the culture medium by these cells was hydroxylated, and about 10% of the incorporated [4C]proline was solubilized by purified collagenase. Agarose gel chromatography of the postculture media showed that unreduced [14C]collagen polypeptides were recovered in large-molecular-weight aggregates (greater than 300,000 daltons) which were largely converted to chains of approximately 95,000 molecular weight and some dimers and trimers of MW approximately 180,000 and 270,000 when the medium was subjected to reduction and alkylation prior to chromatography. The findings indicate that cultured retinal pericytes elaborate collagen and suggest the production of type III collagen. Retinal microvessel cells in culture may facilitate study of abnormalities in retinal pericyte function and collagen synthesis that occur in retinal vascular diseases.