Relationship between neuropeptides and gastrointestinal smooth muscle activity

The actions of substance P (SP) and met-enkephalin (ME) on the motility of stomach and small intestine were compared in conscious dogs. In stomach the two peptides induced different effects on the motility whereas in small intestine SP and ME caused similar actions. The effects of SP were not influenced significantly by ME or naloxone applied before. Furthermore, SP desensitization did not influence the typical response to ME. Our results indicate no interactions between SP and ME in modulation of gastrointestinal motility. The presence of two peptidergic systems existing independently is discussed.