Regional differences in store-operated Ca²⁺ entry in the epithelium of the intact human lens

PURPOSE. An elevated level of Ca²⁺ is an important factor in cataract, yet precisely how Ca²⁺ enters the lens is unknown. Lens epithelial cells contain a range of G-protein-coupled receptors and receptor tyrosine kinases that induce increases in intracellular Ca²⁺. Receptor-associated Ca²⁺influx is, therefore, likely to be an important route for Ca²⁺ influx to the lens. The authors investigated stimulated and passive Ca²⁺influx in in situ human lens epithelium. METHODS. Ca²⁺ changes in equatorial (E) and central anterior (CA) epithelial cells were monitored with the use of a Ca²⁺indicator (Fluo4) and confocal microscopy. Gene expression was monitored by RT-PCR and immunoblotting. RESULTS. Adenosine triphosphate (ATP) induced Ca²⁺ responses that were smaller in CA than E. Ca²⁺ store depletion, using ATP (100 μM) or thapsigargin (1 μM), revealed greater relative store capacity and Ca²⁺influx in E. Ca²⁺influx was blocked by La²⁺ (0.5 μM) in both regions. Unstimulated Ca²⁺ influx was greater in E than CA. Greater expression of Orai1 and STIM1 was detected in E than in CA. CONCLUSIONS. Greater Ca²⁺store capacity and Ca²⁺ influx in E compared with CA reflects underlying differences in proliferation and differentiation between the regions. The relatively small resting Ca²⁺ influx in CA epithelium suggests that storeoperated Ca²⁺ entry (SOCE) is the main route of Ca²⁺influx in these cells. Greater resting influx and SOCE in E cells suggests that these are a major route for Ca²⁺ influx into the lens. Increased expression of Orai1 and STIM1 in E could account for the differences in Ca²⁺entry. Receptor activation will modulate Ca²⁺influx, and inappropriate activity may contribute to cortical cataract.