Structure of the macroglia of the retina: sharing and division of labour between astrocytes and Müller cells.

A detailed comparison is made between astrocytes and Müller cells of the cat's retina, with emphasis on their structural specialisations. Evidence is presented that astrocytes and Müller cells both contribute to the formation of the inner glia limitans of the retina, the glia limitans of vessels, and the glial sheaths of neurones. In particular, it was noted that both astrocytes and Müller cells wrap bundles of ganglion cells axons, that both contribute processes to the glial convergence on the initial segments and node-like structures of axons, and that both wrap the somas of neurones in the ganglion cell layer. Further, it was noted that adherent junctions form between astrocytes, between Müller cells, and between astrocytes and Müller cells, but not between these cells and neurones, or among neurones. These similarities suggest that astrocytes and Müller cells function interchangeably in many respects, and we suggest that they be regarded as variants of macroglia. Quantitative differences between astrocytes and Müller cells were noted in their ensheathment of neurones. In particular, the glial sheaths around the somas of ganglion cells are formed predominantly by Müller cells, and the glial processes attached to node-like specialisations of their axons are formed mainly by astrocytes. One qualitative difference was noted between the two cell classes. The gap junctions which form between astrocytes do not form between Müller cells or between cells of the two classes. From these differences, and previously established features of their shape, orientation, distribution and origin, a hypothesis is developed of the specialisation of macroglia represented by Müller cells.

Development of retinal vasculature in the cat: processes and mechanisms.

Two principal processes can be distinguished in the development of the retinal circulation in the cat. One process, which forms most of the inner layer of vasculature, involves three stages. First, beginning prior to E (embryonic day) 26, spindle cells of mesenchymal origin spread over the inner surface of the retina. Second, beginning at approximately E48, a network of coarse capillaries forms, apparently derived from spindle cells. Third, major vessels differentiate from the capillary plexus, and the capillaries become thinner and more widely spaced. All three stages begin at the optic disc and spread towards the margin of the retina. The other process involves budding of capillary sized vessels from existing vasculature. This process forms the inner layer of vasculature at the area centralis, the outer layer of vasculature, and the radial peripapillary capillaries. It begins between P (postnatal day) 7 and P10 at the area centralis and spreads to the margins of the retina. The radial peripapillary capillaries form at a later stage (P20). The different topographies of the two processes suggest that they are controlled by distinct mechanisms. In the first process, the formation of vessels follows a pattern set by the early migration of spindle cells. In the second process, the vessels form in a pattern determined by the metabolic needs of the developing retina.

Origin of retinal astrocytes in the rat: evidence of migration from the optic nerve.

To test recent ideas on the origin of retinal astrocytes, we have studied the spread of astrocytes in the developing retina of the albino rat. Astrocytes were identified with antibodies to their intermediate filaments, glial fibrillary acidic protein (GFAP). Astrocytes were first detected at E(embryonic day) 18, forming a corona of processes around the optic disc. Over subsequent days, astrocytes extended over the retina, covering approximately 35% of the retina at birth (typically E21-22) and reaching the edge of the retina by P(postnatal day)8. As they spread, astrocytes were closely associated with the developing vasculature, spreading ahead of patent vessels by a small but distinct margin. The most peripheral astrocytes assumed a bipolar morphology and extended processes towards the margin of the retina. Astrocytes nearer the optic disc showed the stellate shape characteristic of mature cells. The appearance of astrocytes at the optic disc at E18, 2 days after the appearance of type-1 astrocytes in the optic nerve (Miller et al.: Dev. Biol. 111:35-41, '85), suggests that retinal astrocytes may be type-1 astrocytes generated in the optic nerve. Watanabe and Raff (Nature 332:834-837, '88) have recently reported an independent study supporting the same conclusions.

Pharmacokinetics of ketorolac and p-hydroxyketorolac following oral and intramuscular administration of ketorolac tromethamine.

Ketorolac tromethamine (KT), a potent analgesic with cyclooxygenase inhibitory activity, was administered in an open, randomized, single-dose study of Latin-square design to 12 healthy male volunteers. Doses of 30 mg oral (po) and 30, 60, and 90 mg intramuscular (im) KT were administered in solution. Plasma samples were analyzed for ketorolac (K) and its inactive metabolite, p-hydroxyketorolac (PHK), by reversed-phase high-performance liquid chromatography (HPLC). The 30-mg im dose was found to be similar to the 30-mg po dose with respect to total AUC values for both K and PHK. The amount of PHK circulating in plasma was very low as judged by AUC ratios (PHK/K x 100) of 1.9 and 1.5% for the 30-mg po and im doses, respectively. The rate of absorption of K and formation of PHK, as determined by Cmax and Tmax values, was significantly slower following the im doses. Total AUC and Cmax for K and PHK increased linearly with dose after im administration of 30, 60, and 90 mg of KT. The mean plasma half-life of K was remarkably consistent between po and im administration and was independent of dose, ranging from 5.21 to 5.56 hr. The plasma metabolic profile was similar following both routes of administration and graded im doses.

The development of astrocytes in the cat retina: evidence of migration from the optic nerve.

To test recent ideas of the origin of retinal astrocytes we have studied the distribution of astrocytes, identified by anti-GFAP antibodies, in the developing retina of the cat. GFAP+ cells first appeared at the optic disc at E53 (embryonic day 53). At subsequent ages, GFAP+ cells covered successively larger regions surrounding the optic disc, and were found at the edge of the retina by P35 (postnatal day 35). During development, the GFAP+ cells near the optic disc were strongly related to blood vessels and axon bundles; in a more peripheral zone they were closely associated with the immature capillary net; while the most peripheral GFAP+ cells appeared to extend exploratory processes towards the margin of the retina. The velocity at which the 'front' of GFAP+ cells spread over the retina was estimated at 170-240 microns/day. At no time during development were GFAP+ cells observed in the area centralis. Except at the area centralis, the spread of GFAP+ cells preceded the formation of capillaries, by a small but distinct margin. GFAP+ cells also extended for a short distance from the optic disc along the proximal part of the hyaloid artery. These results support the view that retinal astrocytes migrate into the retina from the optic disc, in close association with the formation of retinal vasculature.

Long-term changes in corneal endothelial morphology following wounding in the cat.

The cat eye was used to determine the long-term morphological changes in the corneal endothelium that occur after central endothelial wounding. Central corneal thickness was measured using ultrasonic pachometry. Specular microscopy and computer-assisted morphometry was used to quantify central and peripheral endothelial cell density (ECD), coefficient of variation (COV) and the mean and standard deviation of the shape factor (S) over an 18-month period. After endothelial wounding, there was a rapid increase in corneal thickness followed by a rapid nonlinear decline, reaching presurgical levels 35 days after wounding. Central cell density had decreased by 25% at 4 weeks after wounding. During the following 18 months, endothelial cell density in the central cornea increased slightly. The coefficient of variation had increased by 60% at 4 weeks after wounding. This recovered slowly and had reached control levels by 18 months. The mean shape factor was higher in the wounded eye throughout the 18 months, whereas the standard deviation of the shape factor recovered after 12 months. Peripheral ECD had decreased significantly by 12 months after wounding, while COV and the mean shape factor was not significantly affected. The standard deviation of the shape factor had also increased significantly in the peripheral cornea after 18 months. These findings suggest that following endothelial wounding in the cat, changes in endothelial morphology occur over the entire cornea. Endothelial cell density and the shape factor have not recovered to control values, even 18 months after wounding. This pattern of endothelial repair supports the mechanisms of cell movement suggested by Honda et al and confirm the similarities in endothelial response between cat and man.

A multiple-dose pharmacokinetic comparison of naproxen as a once-daily controlled-release tablet and a twice-daily conventional tablet.

The bioequivalence and absorption kinetics of naproxen in a new controlled-release tablet (750 mg or 1,000 mg naproxen) administered once daily were determined relative to an equivalent dose of the conventional naproxen tablet (375 mg or 500 mg naproxen) administered q12h. Naproxen was well absorbed from the controlled-release tablet (about 90%) compared with the conventional tablet. Absorption was dependent on drug release from the tablet matrix. The mean absorption time of naproxen averaged 8.4 hours for the 750-mg controlled-release tablet and 9.2 hours for the 1,000-mg controlled-release tablet. Once-daily administration of the controlled-release tablet resulted in equivalent trough concentrations of naproxen, and steady-state plasma concentrations were maintained within narrower limits than with twice-daily naproxen.

Ocular bioavailability and tissue distribution of [14C]ketorolac tromethamine in rabbits.

The ocular bioavailability of 0.5% [14C]ketorolac tromethamine administered topically (50 microL) to the eye was determined. The ocular bioavailability of ketorolac was 4% in anesthetized rabbits and was determined by comparing drug concentrations in the aqueous humor after topical application with those obtained after intracameral injection of an equivalent dose of 0.25 mg of ketorolac tromethamine per eye. Although ketorolac administered to the eye was completely absorbed systemically, concentrations of ketorolac (AUC) were, on the average, 13 times higher in the aqueous humor than in plasma after topical administration. In a separate ocular distribution study, peak concentrations of radioactivity were achieved in the ocular tissues and in plasma within 1 h post instillation. Concentrations of total radioactivity were highest in the cornea and sclera and lowest in the lens.

Single intravenous dose and steady-state oral dose pharmacokinetics of nicardipine in healthy subjects.

Nicardipine HCl oral doses (10-40 mg) were administered sequentially to six healthy subjects. For each regimen the capsule dose was administered every 8 hours (q 8 h) for 3 days and the plasma profiles of nicardipine and its pyridine analogue (M5) were determined following the last dose on day 4. Steady-state plasma concentrations of nicardipine for each subject were fitted very well by the Michaelis-Menten equation. An intravenous tracer dose (0.885 mg nicardipine HCl) was administered simultaneously with the final oral dose on the fourth day of the 30 mg q 8 h regimen. The steady-state bioavailability of nicardipine was shown to be dose-dependent and averaged 19 per cent (10 mg), 22 per cent (20 mg), 28 per cent (30 mg), and 38 per cent (40 mg). Nicardipine undergoes linear first-pass metabolism to M5. Other metabolic pathways are responsible for the saturable first-pass metabolism observed for nicardipine.