* bicarbonate derived from carbonic anhydrase (CA)-dependent hydration.
* CO metabolism
* Enters through membrane-transporting proteins or cystic fibrosis transmembrane conductance regulators.
* Calcium enters by voltage-dependent Ca channels or by release from the endoplasmic reticulum.
* Hydrogencarbonate and calcium activates sAC in the nucleus. 
* sAC inside mitochondria is activated by metabolically generated CO through carbonic anhydrase.