Efflux and Hydrolysis of Phosphorylethanolamine and Phosphorylcholine in Stressed Cultured Rat Lenses

Abstract

Phosphorylcholine (P-choline), and phosphorylethanolamine (P-ethanolamine) are among the organic phosphate compounds that decrease in concentration in some cataracts and in lenses that have been subjected to cataractogenic osmotic or oxidative stresses. Decreases in the lenticular concentrations of these compounds could be caused by decreased synthesis, increased utilization or hydrolysis, or increased efflux, or by a combination of these mechanisms. To distinguish between these possibilities, the P-choline and P-ethanolamine pools of intact cultured rat lenses were radiolabeled with [3H]choline or [3H]ethanolamine, and the lenses were then subjected to oxidative or osmotic stress. The efflux and hydrolysis of P-[3H]choline and P-[3H]ethanolamine were then followed for up to 48 hr. Osmotic stress induced by incubation with 30 mM xylose caused increased P-choline and P-ethanolamine efflux from the lenses, but had little effect on the rate of hydrolysis of these compounds. For example, xylose-treated lenses lost 26% of their initial P-[3H]ethanolamine content into the medium during a 24 hr incubation, compared with only 6% for control lenses. Oxidative stress from singlet oxygen (generated by rose bengal and light) also increased lenticular P-choline efflux three to four-fold, with minimal effects on hydrolysis, and the increased efflux was accompanied by a decrease in the P-choline concentration. Permeability increases also were observed in lenses oxidatively stressed by H2O2 or by riboflavin and light. These results show that a variety of cataractogenic stresses, both oxidative and osmotic, cause increased permeability of rat lenses to organic phosphate compounds, including P-choline and P-ethanolamine. This increased permeability is a major factor in the decreased concentrations of these compounds in stressed and cataractous lenses.