Rabbit eyes, in vivo and in vitro, were exposed to UV-B irradiation at 300 nm, from a mercury arc lamp with an 11 nm bandpass filter. Radiant exposure ranged from 0.1 J/cm2 to 0.5 J/cm2. In vivo, swelling of the cornea resulted over a 12 to 40 hr period, the extent and duration being directly related to exposure. Recovery of normal thickness was complete within four days. Corneas removed at 18 hr after exposure recovered normal thickness during a five hour perfusion period, except for those most heavily exposed. When removed at 42 hr post exposure all corneas thinned to almost normal thickness. SEM showed the endothelial cells of exposed eyes to have either exaggerated villi on the surface and a disorganized mosaic or, after higher exposures, to be devoid of villi and have loose, flap like cell borders and large "blebs." After exposure of isolated corneas mounted for perfusion, swelling again ensued and similar changes were observed in the appearance of the cells, except that "blebs" were not found. No significant changes were observed in the metabolic components ATP, ascorbate and glutathione, nor was there any indication of lipid peroxidation. At higher in vivo exposures, the aqueous humor did show a decrease in ascorbate concentration and an increase in protein content, which probably result from a breakdown of the blood-aqueous barrier. UV-B irradiation may cause or promote changes in the endothelium associated with aging, but the one time radiant exposures of the magnitude used in this study, appear to have no severe or permanently toxic effects.
The interaction of ascorbate, hydrogen peroxide and oxygen has been examined in order to understand the equilibrium between these compounds that exists in the aqueous humor of the eye and their influence on function of the corneal endothelium. Ascorbate was found to promote corneal swelling when isolated corneas were perfused with a medium lacking glucose. This was found to be due to the rapid oxidation of ascorbate in the medium, yielding H2O2 which is toxic to the endothelial cells. In the absence of oxygen, or if EDTA was added to the medium, no H2O2 was produced from ascorbate, but ascorbate reacted with any pre-existing H2O2. Oxidation of ascorbate in the aqueous humor is limited by the presence of glutathione (and, possibly, other compounds) and no significant increase in H2O2 concentration occurs on standing in air. Nevertheless, the concentration of H2O2 in the aqueous is directly dependent on the concentration of ascorbate secreted in the aqueous humor. Therefore, there must be a dynamic equilibrium in this fluid between ascorbate, H2O2 and oxygen, and it may be modulated by glutathione. Each of these substances is important in redox reactions, including free-radical production or scavenging. Consequently, when studying the effects on corneal or lenticular function of other agents which cause or relieve oxidant stress, it is critical that the modifying effects of ascorbate and H2O2, as they occur in vivo, be considered. A perfusion system is described which permits an approximation in vitro of stable concentrations of ascorbate, H2O2, GSH and O2 similar to those found in the aqueous humor.
PAH, p-aminohippurateNumerous studies of perinatal physiology, metabolism, nutrition, and pathologic conditions require estimation of the fluid content of various body compartments. As direct volumetric determinations are not possible in vivo, less direct methods must be relied upon to obtain an acceptable estimate. The most commonly used techniques involve a principle developed by Fick. Fick's principle states that the fluid content (V) of a given space may be calculated following the administration of a dye or chemical into that space if the exact amount (Q) of substance injected and its concentration (C) in the fluid of the space are Rece~ved August 15, 1988; accepted November 22, 1988 known, or V = Q/C. The dye or chemical must have five main properties: It must I ) be harmless to the organism; 2) be easily measurable; 3) diffuse evenly and within a predictable period of time throughout its vol of distribution; 4) have no effect on fluxes of fluids across membranes; and 5 ) neither be metabolized nor leave the space for which the vol is being measured for the duration of the study. This last requirement is seldom met. If, however, the decrease in dye concentration with time follows a straight line, extrapolation of that line to time 0 (y-intercept or Co) still allows a vol determination to be made, or V = Q/Co. In that case, the rate at which the substance escapes from its vol of distribution, be it by metabolism or excretion, may be also calculated (slope of the concentration versus time line).In cases where all four criteria are met, the marker diffuses as soon as it is administered to reach an even concentration throughout its vol of distribution. In cases where the fourth criterion is not met, two phenomena occur simultaneously as soon as the marker is administered. First, the marker diffuses to reach an even concentration throughout its vol of distribution; second it is being cleared from its assigned space. To interpret the data correctly, it is important to determine at what point in time equilibration is complete and the behavior of the marker concentration thereafter, i.e. the kinetics of the marker. Yet kinetic data have often been disregarded or remained buried in the investigators' laboratory data books and are scarce in the relevant literature. Also, in human perinatal medicine, there are ethical and practical limitations on the number and vol of body fluid samples that may be obtained. In this report, we have made use of the baboon (Papio cynocephalus), whose fetoplacentouterine unit and neonatal physiology and metabolism are very similar to those of Homo sapiens, to determine the kinetics of six markers: PAH (amniotic fluid vol), inulin (amniotic fluid vol), antipyrine (total body water), H2'" (total body water), bromide (extracellular water) and T-1824 (plasma vol). From these kinetics, precise recommendations may be made regarding optimal time and method of amniotic fluid or blood sampling. MATERIALS A N D METHODSPAIf and inulin dilution. The kinetics of PAH and inulin dilution in amniotic f...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.