Calcium regulation in tissue-cultured human and bovine lens epithelial cells

To study calcium regulatory mechanisms in lens cells with particular reference to the relative contributions from the calcium adenosine triphosphatase of plasma and endoplasmic reticulum membranes, respectively.

The calcium-sensitive fluorescent dye, Fura 2, was incorporated into tissue-cultured human and bovine epithelial cells and internal calcium was calibrated using the ionomycin (1 microM) method. The dynamics of calcium release from the endoplasmic reticulum were also studied in digitonin-permeabilized bovine cells.

Tissue-cultured bovine and human lens cells have very similar resting calcium levels (235 +/- 22 nM and 216 +/- 12 nM, respectively). Thapsigargin caused an increase in cytoplasmic calcium both in the presence and absence of external calcium, but the calmodulin antagonist W7 only initiated an increase in the presence of external Ca2+. The effects of thapsigargin and W7 were additive. Exposing lens cells to Na(+)-free perfusing solutions caused a transient increase in internal Ca2+. Bovine lens cells permeabilized by digitonin-released Ca2+ when exposed to inositol (1,4,5) triphosphate and the effect was maximal at 1 microM.

Lens cytoplasmic calcium is controlled by calcium adenosine triphosphatases at the plasma and endoplasmic reticulum membranes. The former is inhibited by W7 and insensitive to thapsigargin whereas the latter is inhibited by thapsigargin, but insensitive to W7. The lens endoplasmic reticulum store is also controlled by an inositol (1,4,5) trisphosphate calcium-release mechanism. Na+/Ca2+ exchange plays a relatively minor role in calcium regulation, at least at resting calcium levels.