Gastrointestinal Hormones and Neurotransmitters

Secretin became the first gastrointestinal (GI) hormone identified in humans in 1902. Since then, a myriad of GI peptides have been identified as hormones. Most have been verified, others have not; regardless, their functions are extremely important. GI hormones act in one of the following four ways:

• 1.

  Endocrine function. Epithelial cells secrete a substance into the circulation that acts at a distance.

• 2.

  Autocrine function. The substance secreted by the epithelial cell affects processes in the cell itself.

• 3.

  Paracrine signaling. The peptide secreted by the cell affects processes in adjacent cells.

• 4.

  Neurocrine function. Neurons secrete chemical transmitters with peptides into synapses or onto other cell types that signal neurotransmission.

Box 52.1 lists the peptides and hormonal actions identified in the GI tract. Fig. 52.1 demonstrates the diffuse and integrating effects of the peptide hormone substances gastrin, cholecystokinin (CCK), and serotonin.