Effect of inositol 1,4,5-trisphosphate receptor stimulation on mitochondrial [Ca2+] and secretion in chromaffin cells.
Ca(2+) uptake by mitochondria is a potentially important buffering system able to control cytosolic [Ca(2+)]. In chromaffin cells, we have shown previously that stimulation of either Ca(2+) entry or Ca(2+) release via ryanodine receptors triggers large increases in mitochondrial [Ca(2+)] ([Ca(2+)](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 (InsP(3)) receptors using histamine. We find that histamine produces a heterogeneous increase in [Ca(2+)](M), reaching peak levels at approx. 1 microM 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 [Ca(2+)](M) much greater than 1 microM (30%) were the same as those that also responded with large increases in [Ca(2+)](M) following stimulation with either high-K(+) medium or caffeine. Blocking mitochondrial Ca(2+) uptake with protonophores or mitochondrial inhibitors also enhanced catecholamine secretion induced by histamine. These results suggest that some InsP(3) receptors tightly co-localize with ryanodine receptors and voltage-dependent Ca(2+) channels in defined subplasmalemmal functional units designed to control secretion induced by different stimuli.