Studies on the eye lens in poikilothermal animals. I. Comparative studies on cation maintenance systems in rainbow trout and rat lenses

Abstract

The response of the poikilothermal lens to various incubation temperatures in vitro was compared with that of the homothermal lens. The rainbow trout lens was used as the poikilothermal lens and the rat lens as the homothermal lens. In contrast to rat lenses, cataract developed at 37°C in rainbow trout lenses, which was called ‘warm cataract’. Warm cataract developed not only when lenses were incubated in vitro but also when rainbow trout were kept in water at 37°C. Water, Na +, Ca 2+ and insoluble protein increased and K + and Mg 2+ decreased in warm cataract lenses, but GSH and soluble protein sulfhydryl levels did not change. This cataract was irreversible after only 5 min incubation at 37°C. On the other hand, rainbow trout lenses remained transparent without the change of cation balance at 0–25°C while cold cataract developed in rat lenses. Na,K-ATPase activity was detected at 0°C in rainbow trout lens homogenates, but not in rat lens homogenates. Na +K + ratio (Na +/K +) increased when the rainbow trout lens was treated with ouabain at 0°C. In the rainbow trout lens, lactic acid was produced continuously for 30 days at 0°C while it was not in the rat lens between 1 hr and 10 days after. These results strongly suggest that Na,K-ATPase acts as a cation pump at 0°C and that ATP is supplied by glycolysis in the rainbow trout lens in order to maintain the transparency. The above results also suggest that enzymes and membrane structures in rainbow trout lens are adapted to a cold-temperature habitat and that Na,K-ATPase and anaerobic glycolysis are important for the maintenance of lens transparency at low temperatures.