Tachykinins as transmitters of ascending contractile component of the peristaltic reflex

The role of tachykinin neurons of the myenteric plexus in the regulation of the intestinal peristaltic reflex was examined in rat colonic segments that enable separate measurement of ascending contraction and descending relaxation. Release of immunoreactive substance P (SP) and substance K (SK) increased (68% for SP; 70% for SK) during ascending contraction and decreased (37% for SP; 13% for SK) during descending relaxation. Exogenous SP and SK augmented ascending contraction and inhibited descending relaxation. Neither SP antiserum nor SK antiserum had any effect on descending relaxation. The antisera had no effect also on ascending contraction induced by low grades of stretch (2 g), but they inhibited significantly contraction induced by high grades (6-10 g) of stretch (52 +/- 8% inhibition with SP antiserum and 48 +/- 8% with SK antiserum). A combination of both antisera abolished ascending contraction induced by high grades of stretch. Atropine inhibited only partially ascending contraction at high grades of stretch. The pattern of inhibition by both antisera and by atropine implied that ascending contraction induced by high grades of stretch was mediated by tachykinin neurons acting directly on smooth muscle cells as well as indirectly via release of acetylcholine from cholinergic motor neurons.