TACHYKININ RECEPTORS IN THE EQUINE PELVIC FLEXURE

Tachykinins, of which substance P (SP) is the prototype, are neuropept ides which are widely distributed in the nervous systems, In the equin e gut, SP is present in enteric nerves and is a powerful constrictor o f enteric muscle; in other species, SP is also known to have potent va sodilatory and pro inflammatory effects, The specific effects of SP ar e determined by the subtype of receptor present in the target tissue, There are 3 known subtypes of tachykinin receptors, distinguished by t heir relative affinities for SP and other tachykinins, The distributio n of SP binding sites in the equine pelvic flexure was determined usin g I-125-Bolton Hunter SP (I-BHSP) autoradiography, Most I-BHSP binding sites were determined to be saturable and specific, therefore presuma bly representing tachykinin receptors, The greatest degree of I-BHSP b inding occurred over very small vessels, and over the muscularis mucos ae; I-BHSP binding was also intense over the circular muscle of the mu scularis externa and mucosa, and present, although less intense, over the longitudinal muscle of the muscularis externa. Competition of I-BH SP with specific receptor agonists for binding sites in the equine pel vic flexure were used to determine the subtypes of tachykinin receptor s present, The neurokinin-1 receptor subtype predominated in the equin e pelvic flexure, followed by the neurokinin-3 receptor subtype.