The effect of ultraviolet radiation on the metabolism of the corneal epithelium of the rabbit
Metabolic activity of the corneal epithelium has been believed to be unaffected by visible and near-visible radiation. The present research was directed, in contrast to his popular belief, at quantifying possible alterations in corneal epithelial metabolic activity secondary to exposure to ultraviolet radiation (UVR). A micropolarographic electrode was used as an in vivo measure of the corneal oxygen uptake rate in order to evaluate oxidative metabolic activity immediately before, and two minutes after, exposure to UVR. Microfluorometric metabolite assays on microgram tissue samples were used as an in vitro means of. assessing overall metabolic activity under the same conditions. The specific epithelial metabolite assays, paired to the corneal oxygen uptake experiment, were for glucose, glycogen, adenosine triphosphate (ATP), and phosphocreatine (PCr). Experimental UVR exposure conditions that served to decrease the corneal oxygen uptake rate also served to increase glucose and glycogen concentrations. Although the epithelial ATP concentration was unchanged, the epithelial PCr concentration (a high energy phosphate bond reservoir) decreased as a result of UVR exposure. A correlative plot of the in vivo and in vitro data inferentially suggests a differential effect of UVR exposure on the glycolytic and oxidative metabolic systems. Beyond a certain decrease in the oxygen uptake rate (-0.3 mmHg 02/sec) the glycolytic system appears to progressively increase in activity as the oxidative system progressively decreases in activity. Alternative data plots of both the in vivo and in vitro studies indicate that susceptibility of the corneal epithelium to metabolic stress may extend into the visible regions of the electromagnetic spectrum. The micropolarographic data, the glucose concentration data, and the glycogen data suggest susceptibility to metabolic stress may extend to 450 nm. Glucose concentration profiles of the entire rabbit cornea were performed under control and UVR-exposed conditions in order to evaluate traditional theories concerning corneal nutrition. While the control data appears to support passive diffusion theories of glucose passage within the cornea, the UVR-exposed data suggests an active mechanism capable of delivering glucose into the corneal epithelium against a concentration gradient. Overall, the data demonstrates a decrease in corneal epithelial metabolic activity as a result of UVR exposure. The decreased activity effect is wavelength-specific, both in the extent and in the process of UVR-tissue interaction; the shorter wavelength radiation being most effective in altering epithelial metabolic activity. This wavelength specificity suggests the presence of several different UVR mechanisms of action.