Subcellular organization of agonist-evoked Ca2+ waves in the blowfly salivary gland

We have studied the subcellular organization of intra- and intercellular Ca2+ waves elicited by the neuro-hormone 5-hydroxytryptamine (5-HT) in intact blowfly salivary glands by using Ca2+-sensitive fluorescent probes and confocal microscopy. 5-HT (3 nM) elicited repetitive Ca2+ waves (1) that were initiated at Ca2+-release sites close to the basal plasma membrane, (2) that sequentially spread to the cell apex and (3) that, after a delay of 0.7 +/- 0.20 s at the cell boundaries, spread into adjacent cells. [Ca2+](i) increases in the adjacent cells were first detectable at those portions of the lateral plasma membrane that faced a previously activated cell. Electron microscopy revealed that the sites of Ca2+ wave transmission between the cells are correlated with the distribution of gap junctions that cluster in the basal cell portions. The ensuing intracellular Ca2+ wave propagated at constant velocity (27 +/- 7.3 mu m/s) in the lateral cell plane. Moreover, a basally to apically propagating wavefront was detectable at the cell membrane that bordered on the neighbor that provided the excitatory signal, whereas [Ca2+](i) increased simultaneously both apically and basally at the opposite lateral cell border. Overall, the subcellular patterns of Ca2+ wave propagation differed from the patterns observed in mammalian secretory epithelial cells. The findings impose some constraints on the functional significance of intra- and intercellular Ca2+ waves and potential mechanisms underlying 5-HT-evoked fluid secretion. (C) 2000 Harcourt Publishers Ltd.