Just as redox signalling amplifies Ca2þ signalling, an elevation of
Ca2þ can enhance redox signalling. For example, a persistent
elevation in Ca2þ can strongly promote mitochondrial ROS formation.
When Ca2þ builds up within the mitochondrion, it increases
mitochondrial metabolism resulting in an increased formation of
O
2 and H2O2. In addition, O
2 formation within the mitochondria
can act synergistically with Ca2þ to open the mitochondrial
permeability transition pore (mPTP) to trigger apoptosis. Another
action of Ca2þ is to stimulate NOS to increase the formation of NO
that contributes to the generation of ONOO (Fig. 4).