Metallothionein Levels in the Neural Retina of Young Pigs

Preliminary Studies on Effect of Light Exposure and Dietary Zinc Levels

Abstract

This study documents the presence of metallothionein (MT) in retinal tissues of pigs. To evaluate any effects of dietary zinc level and light stress on retinal MT, pigs were fed low normal or high levels of zinc, and eighteen animals were exposed to high intensity intermittent or continuous light stress. Four animals served as unexposed controls. Effects of dietary zinc level and/or light stress on metallothionein levels in kidney, liver and retina were evaluated. The neural retinas were separated from the retinal pigment epithelium/choroid of both eyes from each animal immediately after sacrifice. A cadmium binding assay was used to quantitate metallothionein in kidney, liver, and retinal tissues. The assay was successfully applied to the neural retinal tissues. There was no significant effect of diet or light on neural retinal metallothionein levels. The mean neural retinal metallothionein in young pigs is 40±20ng/mg protein (n=22). The 95% confidence interval was 35-52ng/mg protein. Ocular metallothionein was not significantly correlated with kidney or liver metallothionein nor with the degree of light-induced retinal damage.

The neural retina of the pig possesses much less MT than has been found in human retinal pigment epithelium (RPE) cell cultures. This may represent a species-specific difference but is probably due to the common finding of abundant metallothionein in epithelial tissues like the RPE. This study documents the presence of metallothionein in neural retina which suggests that this ubiquitous protein may also participate in zinc metabolism in the inner retinal layers.