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

The present study expands the contemporary view of mitochondria as important participants in cellular Ca²⁺ dynamics and provides evidence that mitochondria regulate the supply of release-competent secretory granules. Using pharmacological probes to inhibit mitochondrial Ca²⁺ import, the ability of mitochondria to modulate secretory activity in single, patch- clamped bovine chromaffin cells was examined by simultaneously monitoring rapid changes in membrane surface area (ΔC(m)) and cytosolic Ca²⁺ levels ([Ca²⁺](c)). Repetitive step depolarizations or action potential waveforms were found to raise the [Ca²⁺](c) of chromaffin cells into the 1 μM to tens of micromolar range. Inhibiting mitochondria by treatment with carbonyl cyanide p-(trifuoro-methoxy)phenylhydrazone, antimycin-oligomycin, or ruthenium red revealed that mitochondria are a prominent component for the clearance of Ca^2- that entered via voltage-activated Ca²⁺ channels. Disruption of cellular Ca²⁺ homeostasis by poisoning mitochondria enhanced the secretory responsiveness of chromaffin cells by increasing the amplitude of the transient rise and the time course of recovery to baseline of the evoked Δ[Ca²⁺](c). The enhancement of the secretory response was represented by significant deviation of the Ca²⁺-exocytosis relationship from a standard relationship that equates Ca^2- influx and ΔC(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 Ca^2--dependent activation of specific proteins that recruit or prime secretory granules for exocytosis.