Evidence for a divalent cation dependent catecholamine storage complex in chromaffin granules.

Chromaffin granules, the secretory vesicles of the adrenal medulla, are stable in isotonic sucrose solutions at room temperature; however, when low concentrations of the ionophore A23187 are added, rapid lysis ensues which is dependent on the presence of a divalent cation chelator and is prevented by the addition of either Ca2+ or Mg2+. As little as 10 microM Ca2+ totally inhibit lysis of chromaffin granules by A23187, while 28 mM KCl have no effect. Lysis by A23187 at 4.7 microM is almost 100% in the presence of EDTA in isotonic sucrose in 1 h and can be suppressed by raising the osmotic strength of the medium with half maximal inhibition at 0.57 M sucrose, demonstrating that A23187 causes osmotic lysis of chromaffin granules as a consequence of the withdrawal of divalent cations from the core solution. Our results strongly suggest that divalent cations are involved in the formation of a ionic complex in the core solution which lowers its effective osmotic pressure.