Modes of propagation of Ca2+-induced Ca2+ release in bullfrog sympathetic ganglion cells

How depolarization-induced Ca2+ entry or caffeine activates Ca2+-induced Ca2+ release (CICR) in the cytoplasm and nucleoplasm was studied by recording intracellular Ca2+ ([Ca2+](i)) with a confocal microscope in cultured bullfrog sympathetic ganglion cells. The amplitude and propagation speed of voltage pulse-induced rises in [Ca2+](i) were greater in the submembrane (< 5 mu m depth) region than in the core region, and delayed and smaller, but significant, in the nucleus. Ryanodine and dantrolene reduced the rises in [Ca2+](i) in both the cytoplasm and nucleus. A rapid applica tion of high K+ solution induced global rises in [Ca2+](i) in both the cytoplasm and nucleoplasm, which were decreased by dantrolene. Caffeine produced a slow, small rise in [Ca2+](i) which grew into a global, regenerative rise both in the cytoplasm and nucleoplasm with some inward gradient in the cytoplasm. Each of the high [Ca2+](i) phases during caffeine-induced [Ca2+](i) oscillation began in the submembrane region, while low [Ca2+](i) phases started in the core region. These results suggest that CICR activated by Ca2+ entry or caffeine occurs predominantly in the submembrane region causing an inwardly spreading Ca2+ wave or [Ca2+](i) oscillations, and that the nuclear envelope can cause CICR in the nucleoplasm, which is delayed due to Ca2+ diffusion barrier at the nuclear pores. (C) 2000 Harcourt Publishers Ltd.