Neurokinin A TFA (Substance K TFA), a peptide neurotransmitter of the tachykinin family, acts via the NK-2 receptor. Neurokinin A acts as a major mediator in human airway and gastrointestinal tissues.In Vitro: Neurokinin A (substance K) is a peptide neurotransmitter of the tachykinin family with potential as a major mediator in human airway and gastrointestinal tissues. Neurokinin A acts via the NK-2 receptor believed to be localized on smooth muscle cells and pharmacologically coupled to a GTP-binding protein. Neurokinin A is a member of a family of peptide neurotransmitters known as tachykinins. These peptides are associated with the central and peripheral nervous systems and display a wide tissue distribution. Tachykinins share the COOH-terminal structure Phe-X-Gly-Leu-Met-NH. The best known members of this family are Substance P and Neurokinin A or Substance K.