Other Regulatory Peptides

Table 48-10 summarizes basic chemical characteristics of five of the major GI regulatory peptides and indicates their site of origin and major functions. More detailed descriptions of these five peptides are given below, followed by a listing of other regulatory peptides of the GI tract in Table 48-11. 

There is one known pathway for cellular iron export, involving the export of iron into the plasma from the basolateral membrane of duodenal enterocytes, from macrophages, hepatocytes and a number of other cell types. This involves the protein known as IREG1 or ferroportin described already in Chapter 7. We will discuss ferroportin in more detail in the next section on iron homeostasis, since ferroportin is the target of hepcidin, a recently described iron regulatory peptide. 

A number of peptides including leptin, glucagon-like peptide-I and peptide YY induce weight loss in experimental animals and in humans [124, 125] but are not currently approved for use by the FDA or other regulatory bodies in the clinical treatment of obesity. 

Polak, J.M. and Bloom, S.R, (1982). Regulatory peptides and neuron-specific enolase in the respiratory tract of man and other mammals. Exp. Lung Res. 3, 313-328. 

The gut is the largest endocrine organ in the body it is also a major target for many hormones, released locally and from other sites. GI regulatory peptides are released from the pancreatic islets (e.g., somatostatin) or from endocrine cells... 

TABLE 48-11 Brief Description of Other Gi Regulatory Peptides... 

Fig. 4. Four routes of cellular secretion of gastrin. Several other regulatory systems, wherein the same peptide acts as hormone, neurotransmitter, and growth factor, are also released as they occur in these four cell types. Autocrine and paracrine secretion are assumed to play decisive roles for the growth of malignantly transformed cells.

Recently it was demonstrated that LSAL is implicated in the regulation of 5-HTjb receptor activity [16]. It is quite possible that other small peptides do have a similar role in the regulatory mechanisms of other G-protein coupled receptors. 

Endogenous opioid peptide released both in the central nervous system and in other apparatuses of the body that have many regulatory functions, including inhibition of pain transmission. 

The secretion of extracellular matrix proteins is also a function of smooth muscle cells but, since it occurs concurrently with other activities, it does not seem to constitute a physiological state. However, the fraction of the cellular resources which are devoted to it must be regulated these regulatory mechanisms are virtually unknown. In addition, it should be anticipated that autocrine activity occurs as well, involving peptides, prostaglandins, cytokines, and nitric oxide. 

Hormone response elements (for steroids, T3, retinoic acid, peptides, etc) act as—or in conjunction with— enhancers or silencers (Chapter 43). Other processes that enhance or silence gene expression—such as the response to heat shock, heavy metals (Cd and Zn +), and some toxic chemicals (eg, dioxin)—are mediated through specific regulatory elements. Tissue-specific expression of genes (eg, the albumin gene in liver, the hemoglobin gene in reticulocytes) is also mediated by specific DNA sequences. 

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