Releasing Effect of Calcium and Phosphate on Catechol-amines,ATP, and Protein from Chromaffin Cell Granules

Addition of Ca²⁺3 mM to isolated bovine adrenal niedullary granules incubated in 130 mM K-phosphate buffer pH 6.8–7.2 causes a rapid initial release of noradrenaline (NA), adrenaline (A), ATP and soluble protein. The effect is increased by addition of 1–6 mg/ml of RNA and inhibited by 1–6 mM Mg²⁺. The effect of Ca²⁺is dependent on the presence of phosphate ions in the incubation medium and does not occur in sucrose, RCI or NaCl. Addition of freshly precipitated calcium phosphate is also effective in inducing release, but addition of CaHPO₄ or Ca₂P₂O₇ had no effect. CaCl₂ 0.3–1 mM did not increase the release but was effective together with 1–6 mg/ml of RNA. In the absence of phosphate ions Ca²⁺and Mg²⁺as well as the divalent ions Ba²⁺and Sr²⁺inhibited the release. The catecholamine releasing effect of Ca²⁺in the presence of phosphate is reduced by 2 mM ATP and almost completely prevented by ATP-Mg²⁺. It is suggested that the increased outflow of granular contents is due to membrane alterations induced by Ca-phosphate.