Intracellular Ca²⁺ stores in permeabilized sheep lens cells were imaged with mag-fura 2 to characterize their distribution and sensitivity to Ca²⁺-releasing agents. Inositol 1,4,5-trisphosphate (IP₃) or cyclic ADP-ribose (cADPR) released Ca²⁺ from intracellular Ca²⁺ stores that were maintained by an ATP-dependent Ca²⁺ pump. The IP₃ antagonist heparin inhibited IP₃- but not cADPR-mediated Ca²⁺ release, whereas the cADPR antagonist 8-amino-cADPR inhibited cADPR- but not IP₃-mediated Ca²⁺ release, indicating that IP₃ and cADPR were operating through separate mechanisms. A Ca²⁺ store sensitive to IP₃, cADPR, and thapsigargin appeared to be distributed throughout all intracellular regions. In some cells a Ca²⁺ store insensitive to IP₃, cADPR, thapsigargin, and 2,4-dinitrophenol, but not ionomycin, was present in a juxtanuclear region. We conclude that lens cells contain intracellular Ca²⁺ stores that are sensitive to IP₃, cADPR, and thapsigargin, as well as a Ca²⁺ store that appears insensitive to all these agents.

inositol 1,4,5-trisphosphate; cyclic ADP-ribose; calcium pools; mag-fura 2; epithelium

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