Substance P is a vasoactive hormone in the Atlantic hagfish Myxine glutinosa (Cyclostomata).

The in vitro effects of synthetic substance P (SP) on rings of the ventral aorta and on the systemic heart of Myxine glutinosa were investigated by the use of an organ bath. Increasing molar amounts (10(-16) to 10(-8) M) of SP were applied and the reaction of the aortic rings was recorded with isometric transducers. SP was found to contract the Myxine aorta in a dose-dependent manner starting at a threshold concentration of 10(-14) M and reaching a plateau at 10(-11) M. Therefore, the effective dose of SP seems to be in a physiological range. On the other hand, the heart was found to be insensitive to SP even in doses as high as 10(-5) M. By means of immunohistochemistry, SP-immunoreactive endocrine cells were found throughout the Myxine intestine while no SP-immunoreactive nerve fibers were detected at the Myxine aorta. Thus, under physiological conditions the observed contractile effect of SP on the ventral aorta may be raised by SP-like material released from enteroendocrine cells.