Effects of neurotensin, substance P and methionine-enkephalin on colonic motility.

The influence of neurotensin, substance P and methionine-enkephalin (met-enkephaline) on proximal and distal colonic motility was studied in anesthetized cats. When administered i.v, at a dose of 14 pmol x kg-1 x min-1, neurotensin increased basal smooth muscle tone and antiperistaltic activity in the proximal colon. After higher doses of neurotensin the basal muscle tone also increased in the distal colon. Substance P at a dose of 17 pmol x kg-1 x min-1 selectively and powerfully stimulated the distal colon, thus causing a mass contraction pattern with a concomitant decrease in peristaltic amplitude. In the proximal part of the colon 17 pmol x kg -1 x min-1 of substance P exerted an inhibitory action on the peristaltic amplitude in animals with a high level of spontaneous activity. In addition, the basal muscle tone was slightly decreased. In animals with sluggish spontaneous activity, however, no effects were detected in the proximal colon. With increasing doses of substance P forceful contractions of the proximal colon were also registered. Met-enkephalin at a dose of 40 pmol x kg-1 x min-1 increased the smooth muscle basal tone with no effect on the peristaltic activity in the proximal or distal segments. The effect on basal tone was blocked by naloxone. Thus, neurotensin, substance P and met-enkephalin have distinct motor actions on the colon. At low doses neurotensin may stimulate the churning and mixing functions of the proximal colon. Substance P exerts its major effects on the distal part with a mass contraction response and met-enkepahlin increases the basal muscle tone equally in the proximal and distal parts of the colon.