Peptide hormones circulation

Lectins, proteins with highly specific carbohydrate-binding domains, are commonly found on the outer surface of cells, where they initiate interaction with other cells. In vertebrates, oligosaccharide tags read by lectins govern the rate of degradation of certain peptide hormones, circulating proteins, and blood cells. 

Somatostatin. Somatostatin is an endogenous peptide hormone involved in e.g. the control of the release of Somatomedin, Insulin and Pancreatin. Due to its biological role, Somatostatin has a very low biological stability. The half-life in the rabbit after intravenous administration has been determined to approximately 90 seconds in this investigation. After sc or im administration, the apparent half-life is somewhat longer, close to 10 minutes, probably due to the absorption of the peptide from the injection site into the systemic circulation. 

Protein/peptide hormones are derived from amino acids. These hormones are preformed and stored for future use in membrane-bound secretory granules. When needed, they are released by exocytosis. Protein/peptide hormones are water soluble, circulate in the blood predominantly in an unbound form, and thus tend to have short half-lives. Because these hormones are unable to cross the cell membranes of their target tissues, they bind to receptors... 

Some proteins are imported into cells from the surrounding medium examples in eukaryotes include low-density lipoprotein (LDL), the iron-carrying protein transferrin, peptide hormones, and circulating proteins destined for degradation. The proteins bind to receptors in invaginations of the membrane called coated pits, which concentrate endocytic receptors in preference to other cell-surface proteins. The pits are coated on their cytosolic side with a lattice of the protein clathrin, which forms closed polyhedral structures (Fig. 27-40). The clathrin lattice grows as more recep-... 

To date, the presence of receptors in the circulation has not been convincingly demonstrated and, even if such receptors exist, it is likely that they represent structurally altered forms of the receptor compared to the functional receptor present in membranes. There has, however, been a report of autoantibodies to a peptide hormone receptor spontaneously developing as the anti-idiotype to the hormone antibody itself [26]. 

After secretion, the hormones circulate in blood for periods ranging from minutes (insulin) to a few hours (glycoprotein hormones). The protein hormones, being water-soluble, circulate in free form and are not bound to specific proteins. Their concentrations in blood are very low, the resting concentrations being 1CT11 M to 10 10 M. Under stimulated conditions the concentrations of peptide hormones in blood may rise 5- to 100-fold. 

Another case in point is the hypophyseal gland, which like the adrenal gland combines entodermal and neural gland cells in one organ both cell types secrete peptide hormones into the circulation. 

Recently, a peptide hormone known as hepcidin has been imphcated as a major circulating molecule that coordinates iron metabolism responses in the reticuloendothehal system and the duodenum. The presence of a circulating factor helps to explain how sites that are physically distant from one another, such as the duodenum and spleen, can coordinately regulate the amount of iron that they release into the bloodstream. Hepcidin negatively regulates iron absorption through the small intestine, iron transport across the placenta,... 

GRF has been isolated and characterized from a human tumor of the pancreas (61) as well as from rat (62), porcine (63), bovine (64), caprine (65), ovine (65), and human (65) hypothalamus stalk-median eminence. It is a 40-44-amino-acid peptide hormone produced in the arcuate nucleus of the hypothalamus and released from neurosecretory nerve terminals of these arcuate neurons, and it is carried by the hypothalamo-hypophysial portal circulation to the anterior pituitary gland where it stimulates growth hormone (GH) secretion (66). 

Cellular debris in the epididymis or in ejaculated semen can also signal testicular toxicity impacting gametogenesis. Measurements of circulating steroids and LH/FSH as well as inhibin B (a peptide hormone produced by Sertoli or granulosa cells) can also help characterize impacts on the reproductive system and sometimes lead to hypotheses about the underlying mechanism(s) of the alterations. 

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