In situ Ca(2+)-induced Ca2+ release from a ryanodine-sensitive intracellular Ca2+ store in corneal epithelial cells

1. In a number of tissues, Ca2+ signaling involves Ca(2+)-induced Ca2+ release (CICR) from ryanodine- and caffeine-sensitive intracellular Ca2+ stores. We sought evidence for such a mechanism in bovine corneal epithelial cells (BCE). 2. We have identified a microsomal fraction of BCE which possesses high-affinity [3H]-ryanodine binding sites indicating the presence of the ryanodine receptor Ca2+ channel. 3. Functional evidence for CICR is that in fura-2 loaded BCE the magnitude of Ca2+ transients induced by the addition of either the adenylate cyclase activator, forskolin, or the L-type Ca2+ channel agonist, BAY-K 8644, were both enhanced by preincubation with 5 microM ryanodine. This ryanodine enhancement provides evidence that Ca2+ release from a ryanodine-sensitive intracellular Ca2+ store also contributes to the Ca2+ transients. Therefore, Ca(2+)-induced Ca2+ release is a component of Ca2+ signaling in BCE.