Tomlinson J, Bannister S C, Croghan P C, Duncan G
School of Biological Sciences, University of East Anglia, Norwich, Norfolk, U.K.
Exp Eye Res. 1991 May;52(5):619-27. doi: 10.1016/0014-4835(91)90065-m.
The influx and efflux kinetics of 45Ca2+ were studied in the rat lens in vitro. Both data sets could be fitted by a multi-compartment mathematical model and were interpreted in terms of extracellular, cytosolic and slowly-exchanging (bound) components. At the end of a 16-hr influx period, when uptake into the extracellular and cytosolic compartments is complete, the 45Ca2+ exchanged fraction is less than 20% of the total calcium determined by atomic absorption. The bound compartment is therefore by far the largest in the lens. The efflux rate constant determined from the model for the cytosolic compartment was approximately 8 x 10(-3) min-1 and its origin was confirmed by its sensitivity to temperature, absence of external sodium and presence of the amiloride-analogue, dichlorobenzamil. A 55% reduction in efflux was obtained in sodium-free solution, indicating that Na(+)-Ca2+ exchange is responsible for a large proportion of calcium movement from the lens against its electrochemical gradient. This was confirmed in influx studies where, reduction of the lens sodium gradient by either exposure to sodium-free medium or 0.1 mM ouabain significantly elevated the 45Ca2+ content of the lens relative to the control level. Exposure to sodium-free conditions also rendered the lens opaque, which did not occur in the absence of external calcium. These experiments suggest a critical role for Na(+)-Ca2+ exchange in maintaining a low internal Ca2+ and hence transparency.
在体外对大鼠晶状体中45Ca2+的流入和流出动力学进行了研究。这两组数据都可以用多室数学模型进行拟合，并根据细胞外、细胞质和缓慢交换（结合）成分进行解释。在16小时的流入期结束时，当细胞外和细胞质区室的摄取完成时，45Ca2+交换部分小于通过原子吸收测定的总钙的20%。因此，结合区室是晶状体中迄今为止最大的区室。从模型确定的细胞质区室的流出速率常数约为8×10(-3) min-1，其来源通过其对温度的敏感性、外部钠的缺乏以及氨氯地平类似物二氯苯甲酰胺的存在得到证实。在无钠溶液中流出减少了55%，表明Na(+)-Ca2+交换是晶状体中大量钙逆其电化学梯度移动的主要原因。这在流入研究中得到了证实，即通过暴露于无钠培养基或0.1 mM哇巴因降低晶状体钠梯度，相对于对照水平显著提高了晶状体的45Ca2+含量。暴露于无钠条件下也使晶状体变得不透明，而在没有外部钙的情况下不会出现这种情况。这些实验表明Na(+)-Ca2+交换在维持低细胞内Ca2+从而维持透明度方面起着关键作用。
