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

Cytosolic Ca2+ (Ca-i(2+)) signals begin as polarized, inositol 1,4,5-trisph osphate (InsP3)-mediated Ca-i(2+) waves in mammalian epithelia, and this si gnaling pattern directs secretion together with other cell functions. To in vestigate whether Cai2+ signaling is similarly organized in elasmobranch ep ithelia, me examined Ca-i(2+) signaling patterns and InsP3 receptor (InsP3R ) expression in hepatocytes isolated from the little skate, Raja erinacea, Ca-i(2+) signaling was examined by confocal microscopy, InsP3R expression b y immunoblot, and the subcellular distribution of InsP3Rs by immunochemistr y. ATP induced a rapid increase in Ca-i(2+) in skate hepatocytes, as it doe s in mammalian hepatocytes. Unlike in mammalian hepatocytes, however, the C a-i(2+) increase in skate hepatocytes began randomly throughout the cell ra ther than in the apical region. In cells loaded with heparin ATP-induced Ca -i(2+) signals were inhibited, but de-N-sulfated heparin was not inhibitory , suggesting that the increases in Ca-i(2+) were mediated by InsP3, Immunob lot analysis showed that the type I but not the types II or III InsP3R was expressed in skate liver, Confocal immunofluorescence revealed that the Ins P3R was distributed throughout the hepatocyte, rather than concentrated api cally as in mammalian epithelia. These findings demonstrate that ATP-induce d Ca-i(2+) signals are mediated by InsP3 in skate hepatocytes, as they are in mammalian hepatocytes. However, in skate hepatocytes Ca-i(2+) signals be gin at loci throughout the cell rather than as an organized apical-to-basal Ca-i(2+) save, which is probably because the InsP3R is distributed through out these cells. This primitive organization of Ca-i(2+) signaling may in p art explain the observation that Ca2+-mediated events such as secretion occ ur much less efficiently in elasmobranchs than in mammals.