The effect of thermal burns on the release of collagenase from corneas of vitamin A--deficient and controls rats.

A mild trauma in the form of a thermal burn was applied to corneas of vitamin A--deficient rats and their pair-fed controls. The control corneas routinely showed rapid re-epithelialization without stromal changes. The corneas of deficient rats recovered more slowly, frequently exhibiting stromal edema, leukoma, and sometimes ulceration. Because collagenase is thought to initiate collagen destruction in corneal ulceration, the relationships among vitamin A status, severity of trauma, and collagenase levels were determine. Mild thermal burns were found to cause corneas from less severely deficient rats to ulcerate rarely but no release increased levels of collagenase, mainly on the first day of culture, as in the case of nonburned, severely deficient rats. Comparable burns of corneas of pair-fed control rats resulted in no ulceration and in very little collagenase release. Severe burns of either pair-fed control or normal rat corneas caused ulceration and collagenase release, but collagenase activity was maximal on the second and third days of culture. Differences in vitamin A status at time of burning gave rise to different patterns of collagenase. By following the development of the vitamin deficiency, it was determined that little active collagenase is released after mild burns of corneas in animals in the pre--weight plateau stage but that much more active enzyme is released when animals are in weight plateau or 5% weight loss stages. Studies of the effect of recovery from vitamin A deficiency on the response to mild thermal burn indicated that the longer the interval between feeding vitamin A and the burn, the lower the postburn level of collagenase in the day 1 medium. Thus it would appear that restitution of vitamin A status decreased the level of active collagenase after the mild thermal burn. The system developed here can be used to study the biochemical basis for ulceration in vitamin A deficiency, and the possibility exists that the ulceration characteristic of keratomalacia in people can be initiated by an environmental trauma.