Primitive organization of cytosolic Ca²⁺ signals in hepatocytes from the little skate Raja erinacea Available to Purchase

Cytosolic Ca²⁺ (Ca_i²⁺) signals begin as polarized, inositol 1,4,5-trisphosphate (InsP3)-mediated Ca_i²⁺ waves in mammalian epithelia, and this signaling pattern directs secretion together with other cell functions. To investigate whether Ca_i²⁺ signaling is similarly organized in elasmobranch epithelia, we examined Ca_i²⁺ signaling patterns and InsP3 receptor (InsP3R) expression in hepatocytes isolated from the little skate, Raja erinacea. Ca_i²⁺ signaling was examined by confocal microscopy, InsP3R expression by immunoblot, and the subcellular distribution of InsP3Rs by immunochemistry. ATP induced a rapid increase in Ca_i²⁺ in skate hepatocytes, as it does in mammalian hepatocytes. Unlike in mammalian hepatocytes, however, the Ca_i²⁺ increase in skate hepatocytes began randomly throughout the cell rather than in the apical region. In cells loaded with heparin ATP-induced Ca_i²⁺ signals were inhibited, but de-N-sulfated heparin was not inhibitory, suggesting that the increases in Ca_i²⁺ were mediated by InsP3. Immunoblot analysis showed that the type I but not the types II or III InsP3R was expressed in skate liver. Confocal immunofluorescence revealed that the InsP3R was distributed throughout the hepatocyte, rather than concentrated apically as in mammalian epithelia. These findings demonstrate that ATP-induced Ca_i²⁺ signals are mediated by InsP3 in skate hepatocytes, as they are in mammalian hepatocytes. However, in skate hepatocytes Ca_i²⁺ signals begin at loci throughout the cell rather than as an organized apical-to-basal Ca_i²⁺ wave, which is probably because the InsP3R is distributed throughout these cells. This primitive organization of Ca_i²⁺ signaling may in part explain the observation that Ca²⁺-mediated events such as secretion occur much less efficiently in elasmobranchs than in mammals.