Calcium dynamics in catecholamine-containing secretory vesicles

We have used an aequorin chimera targeted to the membrane of the secretory granules to monitor the free [Ca2+] inside them in neurosecretory PC12 cells. More than 95% of the probe was located in a compartment with an homogeneous [Ca2+] around 40 μM. Cell stimulation with either ATP, caffeine or high-K+ depolarization increased cytosolic [Ca2+] and decreased secretory granule [Ca2+] ([Ca2+]SG). Inositol-(1,4,5)-trisphosphate, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate were all ineffective to release Ca2+ from the granules. Changes in cytosolic [Na+] (0-140 mM) or [Ca2+] (0-10 μM) did not modify either ([Ca2+]SG). Instead, [Ca2+]SG was highly sensitive to changes in the pH gradient between the cytosol and the granules. Both carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) and nigericin, as well as cytosolic acidification, reversibly decreased [Ca2+]SG, while cytosolic alcalinization reversibly increased [Ca2+]SG. These results are consistent with the operation of a H+/Ca2+ antiporter in the vesicular membrane. This antiporter could also mediate the effects of ATP, caffeine and high-K+ on [Ca2+]SG, because all of them induced a transient cytosolic acidification. The FCCP-induced decrease in [Ca2+]SG was reversible in 10-15 min even in the absence of cytosolic Ca2+ or ATP, suggesting that most of the calcium content of the vesicles is bound to a slowly exchanging Ca2+ buffer. This large store buffers [Ca2+]SG changes in the long-term but allows highly dynamic free [Ca2+]SG changes to occur in seconds or minutes.