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5 - The structure and regulation of the somatostatin gene
- Edited by Hiroshi Okamoto
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- Book:
- Molecular Biology of the Islets of Langerhans
- Published online:
- 10 December 2009
- Print publication:
- 22 February 1990, pp 87-106
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Summary
Introduction
Somatostatin is a tetradecapeptide that regulates the release of pituitary, pancreatic, and gastrointestinal hormones (Reichlin, 1983, 1987). Initially identified in the hypothalamus as an inhibitor of growth hormone secretion (Brazeau et al., 1973), somatostatin has subsequently been found in the extrahypothalamic brain, spinal cord, retina, gastrointestinal tract, pancreatic islets, and thyroid (Patel & Reichlin, 1978; Arimura et al, 1975; Rorstad et al, 1979; Hökfelt et al, 1975). In addition to inhibiting the secretion of a number of pep tide hormones, somatostatin has been proposed to act as a neurotransmitter and to modulate gastrointestinal motility (Barker, 1976; Gerich & Patton, 1978).
The diverse functions and the widespread distribution of the tetradecapeptide somatostatin (somatostatin-14) have focused attention on the biosynthesis of the hormone. Several studies have shown that somatostatin- 14 is synthesized as part of a larger precursor. A 28-amino acid form of the hormone (somatostatin-28) has been identified in extracts of porcine hypothalamus (Schally et al, 1980), gastrointestinal tract (Pradayrol et al., 1980) and bovine hypothalamus (Esch et al., 1980). The biological actions of somatostatin-28 are similar to those of somatostatin- 14 but in addition the larger peptide may have functions distinct from those of the tetradecapeptide (Meyers et al., 1980; Mandarino et al., 1981; Browne et al., 1981).
The amino acid sequence of somatostatin-14 has been conserved remarkably well throughout evolution; the sequences of the fish and mammalian (human and rat) peptides are identical (Fig. 5.1). The variety of somatostatin-related peptides present in the animal kingdom, however, is quite diverse.
4 - The structure and regulation of the glucagon gene
- Edited by Hiroshi Okamoto
-
- Book:
- Molecular Biology of the Islets of Langerhans
- Published online:
- 10 December 2009
- Print publication:
- 22 February 1990, pp 67-86
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- Chapter
- Export citation
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Summary
Introduction
Glucagon is a peptide hormone of 29 amino acids produced and secreted by the A-cells of the pancreatic islets (Unger & Orci, 1981). The hormone is one of a family of several peptides with similar primary structures that includes secretin (Mutt et al, 1970), vasoactive intestinal peptide (Mutt & Said, 1974), gastric inhibitory peptide (Brown, 1971), and growth hormone releasing hormone (Spiess et al, 1982). The secretion of glucagon is regulated by blood levels of glucose (Gerich et al, 1974) and amino acids (Assan et al., 1977), as well as by a variety of hormonal stimuli (Samols et al., 1983). The action of glucagon on its target tissues, particularly the liver, is an important factor in protein and carbohydrate metabolism (Aoki et al., 1974; Cherrington et al., 1976). Abnormal regulation of glucagon gene expression has been implicated in the pathogenesis of diabetes mellitus (Dobbs et al, 1975).
Peptides related immunologically to glucagon are produced in several extrapancreatic tissues such as brain (Tager et al., 1980), salivary glands (Lawrence et al, 1977) and intestine (Conlon, 1980; Parker et al, 1984; Hoshino et al, 1984). The principal hormonal function of glucagon is to regulate carbohydrate, fat and protein metabolism, but it is also possible that glucagon and glucagon-related peptides function as paracrine agents by way of communicating with adjacent cells, for example within pancreatic islet cells and as neurotransmitters within the nervous system. Glucagon and glucagon-related peptides have been isolated and characterized from catfish (Andrews & Ronner, 1985) and anglerfish (Andrews et al, 1986) endocrine pancreases.