Various factors enhance the likelihood of MPTP opening. In some mitochondria, such as those in the central nervous system, high levels of Ca within mitochondria can cause the MPT pore to open. This is possibly because Ca binds to and activates Ca binding sites on the matrix side of the MPTP.
MPT induction is also due to the dissipation of the difference in voltage across the inner mitochondrial membrane (known as transmembrane potential, or Δψ).
In neurons and astrocytes, the contribution of membrane potential to MPT induction is complex, see.
The presence of free radicals, another result of excessive intracellular calcium concentrations, can also cause the MPT pore to open.
Other factors that increase the likelihood that the MPTP will be induced include the presence of certain fatty acids, and inorganic phosphate. However, these factors cannot open the pore without Ca, though at high enough concentrations, Ca alone can induce MPT.
Stress in the endoplasmic reticulum can be a factor in triggering MPT.
Conditions that cause the pore to close or remain closed include acidic conditions, high concentrations of ADP, high concentrations of ATP, and high concentrations of NADH.  Divalent cations like Mg also inhibit MPT, because they can compete with Ca for the Ca binding sites on the matrix and/or cytoplasmic side of the MPTP.