Chromaffin granules release calcium on contact with annexin VI: implications for exocytosis

Chromaffin granules represent a substantial and exchangeable intracellular calcium pool which is thought to be regulated by a sodium/calcium exchange protein and also by a putative inositol trisphosphate-activated calcium channel. A family of calcium-binding proteins, called the annexins, has been shown to bind to chromaffin granules. We have therefore investigated the possible involvement of these proteins in the regulation of chromaffin granule sequestered calcium. Annexin VI (A-VI) produced a concentration-dependent release of 45Ca2+ from chromaffin granules; half-maximal release occurred at 1 microM A-VI, with near-maximum release being at 20 microM A-VI. The A-VI-induced release of 45Ca2+ was rapid, being essentially complete by our first time point of 7 s, and corresponded to 40% of the total sequestered 45Ca2+. A-VI-induced release occurred at extravesicular Ca2+ concentrations ranging from a pCa2+ of 4.12 to 6.86 and also appeared specific to this protein since neither annexin I nor annexin II (tetramer) could evoke any 45Ca2+ release. Given the predominant localization of A-VI to the apical plasmalemma, these results suggest that this protein could participate in the secretory event by mediating the localized release of Ca2+ at sites of contact between the chromaffin granule and plasma membrane.