Localized Ca²⁺ uncaging reveals polarized distribution of Ca²⁺-sensitive Ca²⁺ release sites : mechanism of unidirectional Ca²⁺ waves

Ca²⁺-induced Ca²⁺ release (CICR) plays an important role in the generation of cytosolic Ca²⁺ signals in many cell types. However, it is inherently difficult to distinguish experimentally between the contributions of messenger-induced Ca²⁺ release and CICR. We have directly tested the CICR sensitivity of different regions of intact pancreatic acinar cells using local uncaging of caged Ca²⁺. In the apical region, local uncaging of Ca²⁺ was able to trigger a CICR wave, which propagated toward the base. CICR could not be triggered in the basal region, despite the known presence of ryanodine receptors. The triggering of CICR from the apical region was inhibited by a pharmacological block of ryanodine or inositol trisphosphate receptors, indicating that global signals require coordinated Ca²⁺ release. Subthreshold agonist stimulation increased the probability of triggering CICR by apical uncaging, and uncaging-induced CICR could activate long-lasting Ca²⁺ oscillations. However, with subthreshold stimulation, CICR could still not be initiated in the basal region. CICR is the major process responsible for global Ca²⁺ transients, and intracellular variations in sensitivity to CICR predetermine the activation pattern of Ca²⁺ waves.