The tachykinins (rapid kinins), which mediate rapid contractions of smooth muscle, are so named to distinguish them from bradykinins, which have a much slower action on smooth muscle. Substance P (SP, colored green) has a peptide sequence of 11 amino acids; see Figure 1. Neurokinin A (blue) and neurokinin B (purple) each have 10 amino acids. Each of these three peptides has a C-terminal amidation. SP shares four identical residues #7, #9, #10, and #11 gold residues) with the neurokinins A (NKA) and B (NKA).
Fig1. The amino acid sequences of three tachykinin family members are illustrated. This includes Substance P (colored green), Neurokinin A (colored blue), and Neurokinin B (colored purple). Neurokinins A and B have a total peptide sequence of only 10 amino acids; of these six share identical amino acid sequences for residues #4 and #6, #7, #8, #9, and #10. Substance P (SP) has a peptide sequence of 11 amino acids and shares four identical residues (see #7 and #9, #10, and #11 gold residues) with the Neurokinins A and B. Each of these three peptides has a C-terminal amidation.
Neurokinins A and B have a total peptide sequence of only 10 amino acids and share 6 identical amino acid sequences (#4, #6, #7, #8, #9, #10) that are colored gold.
Tachykinins have been shown to function both as a vasoconstrictor and as a vasodilator, depending upon the circumstances. In the intestines the tachykinins are expressed largely in extrinsic afferent nerves fibers and, also, in the intrinsic enteric neurons. The biological responses of the three takykinins, SP, NKA, and NKB are mediated through binding to one of three G protein-coupled receptors.
The pattern of biological responses generated by the tachykinins is complex and includes smooth muscle contraction, hyperalgesia, release of histamine from salivary cells, and involvement with asthma, hay fever, migraine headache, and inflammatory bowel disease.
