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FIGURE 8-11 Amino acid sequences of bombesins and tachykinins. Neuromedin B is the mammalian homolog of the amphibian bombesin peptide ranatensin. * denotes the amide of the carboxyl of the C-terminal amino acid. ** denotes pyroglutamyl; see Figure 3-8. [The bulk of the information was abstracted with permission from Edmondson, S., Khan, N., Shriver, J., Zdunek, J., and Grâslund, A. (1991). The solution structure of motilin from NMR distance constraints, distance geometry, molecular dynamics, and an iterative full relaxation matrix refinement. Biochemistry 30,11271-11279. The sequence of the human neuromedin B is abstracted from Krane, I. M., Naylor, S. L„ Heilin-Davis, D., Chen, W. W„ and Spindel, E. R. (1988). "Molecular cloning of cDNAs encoding the human bombesin-like peptide neuromedin B". J. Biol. Chem. 263, 1331713323.]

FIGURE 8-11 Amino acid sequences of bombesins and tachykinins. Neuromedin B is the mammalian homolog of the amphibian bombesin peptide ranatensin. * denotes the amide of the carboxyl of the C-terminal amino acid. ** denotes pyroglutamyl; see Figure 3-8. [The bulk of the information was abstracted with permission from Edmondson, S., Khan, N., Shriver, J., Zdunek, J., and Grâslund, A. (1991). The solution structure of motilin from NMR distance constraints, distance geometry, molecular dynamics, and an iterative full relaxation matrix refinement. Biochemistry 30,11271-11279. The sequence of the human neuromedin B is abstracted from Krane, I. M., Naylor, S. L„ Heilin-Davis, D., Chen, W. W„ and Spindel, E. R. (1988). "Molecular cloning of cDNAs encoding the human bombesin-like peptide neuromedin B". J. Biol. Chem. 263, 1331713323.]

is effected by endocrine (gastrin), neuroendocrine (acetylcholine), and paracrine (histamine) pathways.

The parietal cell is known to have separate receptors for gastrin, histamine, and acetylcholine. Figure 8-13 presents a working model of the signal transduction pathways that are utilized to stimulate the secretion of acid. Figure 8-14 presents a model describing the production of HC1 by the parietal cell.

Pepsin is the principal proteolytic enzyme present in the gastric secretions. Pepsin is stored in an inactive form as pepsinogen in secretory granules of the chief cells of the oxyntic mucosa of the stomach. Smaller amounts of pepsinogen are also found in the cardiac and pyloric cells of the stomach and upper duodenal mucosa. The process of pepsin secretion is probably analogous to that of secretion of the proenzymes tryp-sinogen and chymotrypsinogen from the exocrine portion of the pancreas.

Pepsinogen secretion is stimulated by gastrin and related peptides, as well as by a vagal cholinergic stimulation that may be induced by feeding. Also, secretin

TABLE 8-9 Biological Actions of Vasoactive Intestinal Peptide (VIP)"

Location

Action

Esophagus

Relaxation of lower sphincter

Stomach

Relaxation of gastric fundus; inhibition of gastric

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