Regional Differences in Store Operated Ca²⁺ Entry in the Epithelium of the Intact Human Lens

¹ School of Biological Sciences, University of East Anglia, University of East Anglia, Norwich, NR4 7TJ, United Kingdom
² School of Biological Sciences, University of East Anglia, Norwich, United Kingdom
³ Ophthalmology, Norfolk and Norwich University Hospital, Norwich, United Kingdom

^* To whom correspondence should be addressed. E-mail: j.rhodes{at}uea.ac.uk.

	Abstract

PURPOSE. Elevated 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. We 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 using a Ca²⁺-indicator (Fluo4) and confocal microscopy. Gene expression was monitored by RT-PCR and immuno-blotting. RESULTS. 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 La³⁺ (0.5 µM) in both regions. Un-stimulated Ca²⁺-influx was greater in E than CA. Greater expression of Orai1 and STIM1 was detected in E than CA. CONCLUSIONS. Greater Ca²⁺ store capacity and Ca²⁺-influx in E compared to CA reflect underlying differences in proliferation and differentiation between the regions. The relatively small resting Ca²⁺-influx in CA epithelium suggests that store operated 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.

Key Words: Ca2+ influx, Ca2+ channels, lens epithelium, cataractogenesis, ATP receptors