Mitochondria regulate the Ca2+-exocytosis relationship of bovine adrenal chromaffin cells

The present study expands the contemporary view of mitochondria as importan t participants in cellular Ca2+ dynamics and provides evidence that mitocho ndria regulate the supply of release-competent secretory granules. Using ph armacological probes to inhibit mitochondrial Ca2+ import, the ability of m itochondria to modulate secretory activity in single, patch-clamped bovine chromaffin cells was examined by simultaneously monitoring rapid changes in membrane surface area (Delta C-m) and cytosolic Ca2+ levels ([Ca2+](c)). R epetitive step depolarizations or action potential waveforms were found to raise the [Ca2+](c) of chromaffin cells into the 1 mu M to tens of micromol ar range. Inhibiting mitochondria by treatment with carbonyl cyanide p-(tri fuoro-methoxy)phenylhydrazone, antimycin-oligomycin, or ruthenium red revea led that mitochondria are a prominent component for the clearance of Ca2+ t hat entered via voltage-activated Ca2+ channels. Disruption of cellular Ca2 + homeostasis by poisoning mitochondria enhanced the secretory responsivene ss of chromaffin cells by increasing the amplitude of the transient rise an d the time course of recovery to baseline of the evoked Delta[Ca2+](c). The enhancement of the secretory response was represented by significant devia tion of the Ca2+-exocytosis relationship from a standard relationship that equates Ca2+ influx and Delta C-m. Thus, mitochondria would play a critical role in the control of secretory activity in chromaffin cells that undergo tonic or repetitive depolarizing activity, likely by limiting the Ca2+-dep endent activation of specific proteins that recruit or prime secretory gran ules for exocytosis.