Effect of inositol 1,4,5-trisphosphate receptor stimulation on mitochondrial [Ca2+] and secretion in chromaffin cells

Ca²⁺ uptake by mitochondria is a potentially important buffering system able to control cytosolic [Ca²⁺]. In chromaffin cells, we have shown previously that stimulation of either Ca²⁺ entry or Ca²⁺ release via ryanodine receptors triggers large increases in mitochondrial [Ca²⁺] ([Ca²⁺]_M) approaching the millimolar range, whose blockade dramatically enhances catecholamine secretion [Montero, Alonso, Carnicero, Cuchillo-Ibañez, Albillos, Garcia, Carcia-Sancho and Alvarez (2000) Nat. Cell Biol. 2, 57–61]. In the present study, we have studied the effect of stimulation of inositol 1,4,5-trisphosphate (InsP₃) receptors using histamine. We find that histamine produces a heterogeneous increase in [Ca²⁺]_M, reaching peak levels at approx. 1μM in 70% of the mitochondrial space to several hundred micromolar in 2–3% of mitochondria. Intermediate levels were found in the rest of the mitochondrial space. Single-cell imaging experiments with aequorin showed that the heterogeneity had both an intercellular and a subcellular origin. Those mitochondria responding to histamine with increases in [Ca²⁺]_M much greater than 1μM (30%) were the same as those that also responded with large increases in [Ca²⁺]_M following stimulation with either high-K⁺ medium or caffeine. Blocking mitochondrial Ca²⁺ uptake with protonophores or mitochondrial inhibitors also enhanced catecholamine secretion induced by histamine. These results suggest that some InsP₃ receptors tightly co-localize with ryanodine receptors and voltage-dependent Ca²⁺ channels in defined subplasmalemmal functional units designed to control secretion induced by different stimuli.