text: * 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.