GAST and Constipation: In addition, the regulation of gastrointestinal hormones can be considered as another mechanism that plays a role in constipation, since these hormones (including gastrin, motilin and somatostatin) regulate the secretion of gastric juice, constriction of gastrointestinal smooth muscles, the movement of gastrointestinal contents, and the excitation of gastrointestinal motility.