CHROMAFFIN GRANULES RELEASE CALCIUM ON CONTACT WITH ANNEXIN-VI - IMPLICATIONS FOR EXOCYTOSIS

Chromaffin granules represent a substantial and exchangeable intracell ular calcium pool which is thought to be regulated by a sodium/calcium exchange protein and also by a putative inositol trisphosphate-activa ted calcium channel. A family of calcium-binding proteins, called the annexins, has been shown to bind to chromaffin granules. We have there fore investigated the possible involvement of these proteins in the re gulation of chromaffin granule sequestered calcium. Annexin VI (A-VI) produced a concentration-dependent release of Ca-45(2+) from chromaffi n granules; half-maximal release occurred at 1 mu M A-VI, with near-ma ximum release being at 20 mu M A-VI. The A-VI-induced release of Ca-45 (2+) was rapid, being essentially complete by our first time point of 7 s, and corresponded to 40% of the total sequestered Ca-45(2+) A-VI-i nduced release occurred at extravesicular Ca2+ concentrations ranging from a pCa(2+) of 4.12 to 6.86 and also appeared specific to this prot ein since neither annexin I nor annexin II (tetramer) could evoke any Ca-45(2+) release. Given the predominant localization of A-VI to the a pical plasmalemma, these results suggest that this protein could parti cipate in the secretory event by mediating the localized release of Ca 2+ at sites of contact between the chromaffin granule and plasma membr ane.