Wang Z, Hess J L, Bunce G E
Department of Biochemistry and Nutrition, Virginia Polytechnic Institute and State University, Blacksburg 24061-0308.
Curr Eye Res. 1992 Jul;11(7):625-32. doi: 10.3109/02713689209000735.
Sodium gradient-dependent 45Ca2+ transport occurred across the lens membrane both in the direction of Ca2+ uptake by inside-out vesicles and Ca2+ efflux after Ca2+ loading of right-side-out vesicles. Using the calcium ionophore, A23187, greater than 90% of the Na+ gradient-dependent Ca2+ uptake was estimated to be free Ca2+. A normal Na+ gradient was also required to maintain calcium homeostasis in the intact lens. The Na+ gradient contributed to Ca2+ efflux from lenses pre-loaded in medium containing 15 mM CaCl2. Therefore, a Na/Ca-exchange functions to control Ca efflux in rat lens, in addition to the Ca-ATPase. In the preweanling rat mature nuclear cataracts occurred by 96 h after subcutaneous injection of sodium selenite (30 nmol/g animal wt). A 3-5 fold increase of Ca2+ accompanied cataract formation. The loss of Ca2+ homeostasis can be detected by 48 h after treatment selenite treatment. At this time the initial rate of Na+ gradient-dependent Ca2+ uptake was 30% lower in lens vesicles from selenite-treated rats compared to controls. No significant reduction of Na+,K(+)-ATPase activity was detected. Altered Na/Ca-exchange may contribute directly to the loss of Ca2+ homeostasis that leads to nuclear cataract.
钠梯度依赖性的45Ca2+转运在晶状体膜上双向发生，既发生在由内向外囊泡摄取Ca2+的方向，也发生在右侧向外囊泡加载Ca2+后的Ca2+外流过程中。使用钙离子载体A23187，估计超过90%的钠梯度依赖性Ca2+摄取是游离Ca2+。完整晶状体维持钙稳态也需要正常的钠梯度。钠梯度有助于从预先加载在含有15 mM CaCl2培养基中的晶状体中排出Ca2+。因此，除了钙ATP酶外，钠/钙交换在控制大鼠晶状体中的钙外流方面也发挥作用。在断乳前的大鼠中，皮下注射亚硒酸钠（30 nmol/g动物体重）后96小时出现成熟核性白内障。白内障形成伴随着Ca2+增加3 - 5倍。亚硒酸钠处理后48小时可检测到钙稳态的丧失。此时，与对照组相比，亚硒酸钠处理大鼠的晶状体囊泡中钠梯度依赖性Ca2+摄取的初始速率降低了30%。未检测到Na+,K(+)-ATP酶活性有显著降低。改变的钠/钙交换可能直接导致导致核性白内障的钙稳态丧失。
