Novel mammalian endogenous peptide, homolog of substance P, that is a high affinity and selective agonist at the tachykinin NK1 receptor (Ki values are 0.175 and 560 nM for NK1 and NK2 receptors respectively). Has proliferative and antiapoptotic actions on B-cells in vitro and is antihypertensive in vivo. MW (observed): 1413.66.