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1. We examined the effects of cations that bind to phospholipid bilayers on the calcium requirement for exocytosis in vitro. 2. Tetracaine and trifluoperazine, two monovalent cations that bind hydrophobically to lipid bilayers, decreased the zeta-potential of lipid vesicles with a phospholipid composition similar to sea urchin plasma membranes; the decrease in the magnitude of the negative zeta-potential was consistent with the Gouy-Chapman-Stern theory. 3. Trifluoperazine and tetracaine also increased the concentration of calcium required to produce half-maximal exocytosis (Ca50) in isolated fragments of plasma membranes from sea urchin eggs. 4. The effects of trifluoperazine and tetracaine on the Ca50 of egg fragments can be explained quantitatively from the zeta-potential data obtained with phospholipid vesicles if we assume the calcium binding sites responsible for triggering exocytosis lie within a Debye length (less than 1 nm) of the surface of the plasma membranes, such that the concentration of calcium at the sites is affected by the surface potential of the membrane. 5. The divalent cation magnesium, which binds specifically to the phosphate group of lipids, affected Ca50 in a manner that can also be explained quantitatively from its effects on the zeta-potential of phospholipid vesicles. 6. The polyvalent cation neomycin, which adsorbs to the lipid phosphatidylinositol 4,5-bisphosphate with high affinity, also binds co-operatively to phosphatidylinositol; its effect on Ca50 can be explained quantitatively from its effects on the zeta-potential of vesicles containing the latter lipid. 7. A decrease in the ionic strength increases the magnitude of the surface potential of membranes; the effect of ionic strength on Ca50 can be explained qualitatively from its effect on the zeta-potential of phospholipid vesicles. 8. All our results are consistent with the hypothesis that calcium binding sites lie within a Debye length of the plasma membrane; they also indicate that drugs with specific, high-affinity sites may affect exocytosis by a non-specific mechanism.
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