THE EFFECT OF BARIUM ON [H-3] NORADRENALINE RELEASE FROM THE HUMAN NEUROBLASTOMA SH-SY5Y

Replacement of Ca2+ with Ba2+ in HEPES-buffered saline stimulated [H-3]noradrenalin release in the human neuroblastoma clone SH-SY5Y by up to 20% of the cell content in the absence of other secretory stimuli. The Ba2+-evoked release was inhibited by 85% by 3 mu M tetrodotoxin and 95% by 5 mu M nifedipine. Ba2+ also increased the potency of K+-evoked release of [H-3]noradrenalin, as maximal release was observed with 60 mM K+ compared with the 100 mM K+ necessary to achieve maximal release in the presence of Ca2+. In contrast, replacing Ca2+ with Ba2+ had little effect on carbachol- and bradykinin-evoked release of [H-3]noradrenalin. No evidence was obtained from studies on changes in [Ca2+](i) ( in response to 100 mu M carbachol) using fura-2 that Ba2+ could enter intracellular stores in SH-SY5Y cells. Whole-cell patch-clamp studies showed that Ba2+ depolarizes SH-SY5Y cells as well as enhancing inward Ca2+ channel currents and shifting their voltage dependence to more negative values. These results are discussed in terms of the hypothesis that Ba2+ blocks K+ channels, leading to depolarization followed by opening of voltage-sensitive Na+ channels. This in turn opens voltage-sensitive L-type Ca2+ channels, which are coupled to the release of [H-3]noradrenalin in SH-SY5Y cells.