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

An elevated level of Ca(2+) is an important factor in cataract, yet precisely how Ca(2+) 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 Ca(2+). Receptor-associated Ca(2+) influx is, therefore, likely to be an important route for Ca(2+) influx to the lens. The authors investigated stimulated and passive Ca(2+) influx in in situ human lens epithelium.

Ca(2+) changes in equatorial (E) and central anterior (CA) epithelial cells were monitored with the use of a Ca(2+) indicator (Fluo4) and confocal microscopy. Gene expression was monitored by RT-PCR and immunoblotting.

Adenosine triphosphate (ATP) induced Ca(2+) responses that were smaller in CA than E. Ca(2+) store depletion, using ATP (100 microM) or thapsigargin (1 microM), revealed greater relative store capacity and Ca(2+) influx in E. Ca(2+) influx was blocked by La(3+) (0.5 microM) in both regions. Unstimulated Ca(2+) influx was greater in E than CA. Greater expression of Orai1 and STIM1 was detected in E than in CA.

Greater Ca(2+) store capacity and Ca(2+) influx in E compared with CA reflects underlying differences in proliferation and differentiation between the regions. The relatively small resting Ca(2+) influx in CA epithelium suggests that store-operated Ca(2+) entry (SOCE) is the main route of Ca(2+) influx in these cells. Greater resting influx and SOCE in E cells suggests that these are a major route for Ca(2+) influx into the lens. Increased expression of Orai1 and STIM1 in E could account for the differences in Ca(2+) entry. Receptor activation will modulate Ca(2+) influx, and inappropriate activity may contribute to cortical cataract.