Multistep mechanism of polarized Ca²⁺ wave patterns in hepatocytes

The spatial organization of cytosolic Ca²⁺ (Ca(i)/²⁺) signals is thought to be important for regulation of cell function. In epithelial cells, the involvement of inositol 1,4,5,-trisphosphate (IP₃)-mediated Ca²⁺ release in evoking Ca(i)²⁺ signals is appreciated, but the location of IP₃-sensitive Ca²⁺ stores and the role of Ca²⁺-induced Ca²⁺ release (CICR) for Ca²⁺ signaling are less defined. Here, we demonstrate that IP₃ receptors are localized to the apical region in hepatocytes. We also show that hormone-induced Ca(i)²⁺ waves propagate across the cell at a rate that depends on mobilization of Ca²⁺ stores that are sensitive to caffeine, ryanodine, and dantrolene, and that these agents, at concentrations that inhibit CICR, decrease the magnitude of Ca(i)²⁺ signals. Furthermore, Ca(i)²⁺ wave speed is not reduced in Ca²⁺-free medium. These findings suggest that Ca(i)²⁺ signals in epithelial cells begin as apical-to-basal Ca(i)²⁺ waves that result from sequential release of Ca²⁺, first from IP₃-sensitive stores in the apex and then from Ca²⁺-sensitive stores distributed across the remainder of the cell.