BIOCHEMICAL ASPECTS O F CONTACT L E N S W E A R RICHARD A. THOFT, M.D.,
AND JUDITH FRIEND,
M.A.
Boston, Massachusetts
The results indicate that glycogen is mobi lized when contact lenses are applied to the eye, even after prolonged adaptation, and that the epithelium does not return to normal under lenses despite long periods of wear. Furthermore, the glycogen is mobilized in response to trauma even in the presence of adequate oxygenation. There were no signifi cant changes in enzymes of the anaerobic glycolytic or tricarboxylic acid pathways. The A T P content of the epithelium, how ever, was depleted parallel to the depletion of glycogen stores and to the development of edema, indicating that the edema is probably the result of inadequate metabolic energy. While inadequate oxygenation probably al ways leads to edema, edema should not al ways be taken as a sign of inadequate oxy genation.
Some biochemical changes in the corneal epithelium as a result of contact lens wear have been thought to be secondary to anoxia. For example, the epithelial edema seen after prolonged wearing of poorly fitted lenses is a well-recognized indicator of cellular dys function. Glycogen depletion1"4 and changes in adenosine triphosphate ( A T P ) levels3 have been used as indicators of the adequacy of oxygenation under lenses. Burns, Robert, and Rich8 supported this association of glycogen depletion with anoxia by showing no glycogen fall under oxygen permeable lenses such as silicone lenses. Conversely, Hill, Augsburger, and Uniacke5 showed that although the glycogen depletion under con ventional hard lenses was more pronounced, glycogen levels in epithelium underlying sili cone lenses were still only about 80% of nor mal. Although oxygenation under lenses is necessary for the maintenance of glycogen Stores, such results Would indicate that ade quate oxygenation may not be sufficient, by itself, to protect the glycogen level.
MATERIALS AND METHODS
Previous experiments in our laboratory indicated that epithelial trauma alone may \ead to epithelial glycogen mobilization.* Our purpose here is to confirm the changes in glycogen content that occur under contact lenses in rabbits, and to relate these changes to hydration, adenosine phosphate content, and alterations in anaerobic and aerobic en zyme activity. The effect of minor trauma on rabbit corneal epithelium glycogen stores was also evaluated. From the Department of Cornea Research, Eye Research Institute of Retina Foundation, Boston, Massachusetts. This study was supported by re search grant EY-00208 and training grant EY-00043 from the National Eye Institute; in part by a Fight for Sight, Inc., grant-in-aid, G-495 (Dr. Thoft) ; and in part by the Massachusetts Lions Eye Research Fund, Inc. Reprint requests to Richard A. Thoft, M.D., Eye Research Institute of Retina Foundation, 20 Stam ford St., Boston, MA 02114.
Animal studies—Rabbits weighing be tween 2.5 and 3.5 kg were used in all experi ments, Conventional methyl methacrylate, hard contact lenses, polymacon (Soflens), crofilcon A (CS-151), and silicone lenses were used. The hard and soft lenses were 13 mm in diameter and 0.5 mm thick. The crofilcon A lenses were 13 mm in diameter and varied in thickness, 0.07, 0.3, or 0.6 mm thick. The polymacon and crofilcon A are both hydrogel lenses. They are both rela tively permeable to oxygen; the thinnest one (for example, crofilcon A, 0.07 mm) is the most permeable. The silicone lenses were simple silicone membranes, 7 mm in diameter and 0.12 mm thick. Both the crofilcon A and silicone lenses are oxygen permeable. All lenses had a radius of curvature of 7.5 mm. After the lenses were worn for six or 18 hours, either after adaptation or without it, the rabbits were anesthetized with pentobarbital sodium. Epithelia that were to have enzyme or glycogen levels (micromoles
139
140
AMERICAN JOURNAL OF OPHTHALMOLOGY
JULY, 1975
TABLE 1 GLYCOGEN LEVELS AND HYDRATION OF RABBIT CORNEAL EPITHELIUM AFTER CONTACT LENS WEAR*
Lens Type, Thickness, mm Adaptation Time (days)/ Wearing Time (hrs) None Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Crofilcon A, 0.07 Crofilcon A, 0.30 Crofilcon A, 0.60 Polymacon, 0.5 • •. Silicone, 0.12
— 0/1 0/6 0/18 5/6 11/6 26/6 0/6 0/6 0/6 0/6 0/6
GlycoKen
P
AND JUDITH FRIEND,
M.A.
Boston, Massachusetts
The results indicate that glycogen is mobi lized when contact lenses are applied to the eye, even after prolonged adaptation, and that the epithelium does not return to normal under lenses despite long periods of wear. Furthermore, the glycogen is mobilized in response to trauma even in the presence of adequate oxygenation. There were no signifi cant changes in enzymes of the anaerobic glycolytic or tricarboxylic acid pathways. The A T P content of the epithelium, how ever, was depleted parallel to the depletion of glycogen stores and to the development of edema, indicating that the edema is probably the result of inadequate metabolic energy. While inadequate oxygenation probably al ways leads to edema, edema should not al ways be taken as a sign of inadequate oxy genation.
Some biochemical changes in the corneal epithelium as a result of contact lens wear have been thought to be secondary to anoxia. For example, the epithelial edema seen after prolonged wearing of poorly fitted lenses is a well-recognized indicator of cellular dys function. Glycogen depletion1"4 and changes in adenosine triphosphate ( A T P ) levels3 have been used as indicators of the adequacy of oxygenation under lenses. Burns, Robert, and Rich8 supported this association of glycogen depletion with anoxia by showing no glycogen fall under oxygen permeable lenses such as silicone lenses. Conversely, Hill, Augsburger, and Uniacke5 showed that although the glycogen depletion under con ventional hard lenses was more pronounced, glycogen levels in epithelium underlying sili cone lenses were still only about 80% of nor mal. Although oxygenation under lenses is necessary for the maintenance of glycogen Stores, such results Would indicate that ade quate oxygenation may not be sufficient, by itself, to protect the glycogen level.
MATERIALS AND METHODS
Previous experiments in our laboratory indicated that epithelial trauma alone may \ead to epithelial glycogen mobilization.* Our purpose here is to confirm the changes in glycogen content that occur under contact lenses in rabbits, and to relate these changes to hydration, adenosine phosphate content, and alterations in anaerobic and aerobic en zyme activity. The effect of minor trauma on rabbit corneal epithelium glycogen stores was also evaluated. From the Department of Cornea Research, Eye Research Institute of Retina Foundation, Boston, Massachusetts. This study was supported by re search grant EY-00208 and training grant EY-00043 from the National Eye Institute; in part by a Fight for Sight, Inc., grant-in-aid, G-495 (Dr. Thoft) ; and in part by the Massachusetts Lions Eye Research Fund, Inc. Reprint requests to Richard A. Thoft, M.D., Eye Research Institute of Retina Foundation, 20 Stam ford St., Boston, MA 02114.
Animal studies—Rabbits weighing be tween 2.5 and 3.5 kg were used in all experi ments, Conventional methyl methacrylate, hard contact lenses, polymacon (Soflens), crofilcon A (CS-151), and silicone lenses were used. The hard and soft lenses were 13 mm in diameter and 0.5 mm thick. The crofilcon A lenses were 13 mm in diameter and varied in thickness, 0.07, 0.3, or 0.6 mm thick. The polymacon and crofilcon A are both hydrogel lenses. They are both rela tively permeable to oxygen; the thinnest one (for example, crofilcon A, 0.07 mm) is the most permeable. The silicone lenses were simple silicone membranes, 7 mm in diameter and 0.12 mm thick. Both the crofilcon A and silicone lenses are oxygen permeable. All lenses had a radius of curvature of 7.5 mm. After the lenses were worn for six or 18 hours, either after adaptation or without it, the rabbits were anesthetized with pentobarbital sodium. Epithelia that were to have enzyme or glycogen levels (micromoles
139
140
AMERICAN JOURNAL OF OPHTHALMOLOGY
JULY, 1975
TABLE 1 GLYCOGEN LEVELS AND HYDRATION OF RABBIT CORNEAL EPITHELIUM AFTER CONTACT LENS WEAR*
Lens Type, Thickness, mm Adaptation Time (days)/ Wearing Time (hrs) None Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Methyl methacryiate, 0.5 Crofilcon A, 0.07 Crofilcon A, 0.30 Crofilcon A, 0.60 Polymacon, 0.5 • •. Silicone, 0.12
— 0/1 0/6 0/18 5/6 11/6 26/6 0/6 0/6 0/6 0/6 0/6
GlycoKen
P
