Classical hormones, release

In the trans Golgi compartment the peptide is sorted via secretory vesicles into a regulated pathway. In contrast to vesicles of the constitutive pathway, vesicles of the regulated pathway are stored in the cytoplasm until their stimulated release. Membrane depolarisation as well as a wide range of substances such as intracellular mediators, neuropeptides, neurotransmitters, classical hormones, cytokines, growth factors, ions and nutrients induce somatostatin secretion. General inhibitors of somatostatin release are opiates, GABA, leptin and TGF- 3. 

In contrast, much is known about the catabolism of catecholamines. Adrenaline (epinephrine) released into the plasma to act as a classical hormone and noradrenaline (norepinephrine) from the parasympathetic nerves are substrates for two important enzymes monoamine oxidase (MAO) found in the mitochondria of sympathetic neurones and the more widely distributed catechol-O-methyl transferase (COMT). Noradrenaline (norepinephrine) undergoes re-uptake from the synaptic cleft by high-affrnity transporters and once within the neurone may be stored within vesicles for reuse or subjected to oxidative decarboxylation by MAO. Dopamine and serotonin are also substrates for MAO and are therefore catabolized in a similar fashion to adrenaline (epinephrine) and noradrenaline (norepinephrine), the final products being homo-vanillic acid (HVA) and 5-hydroxyindoleacetic acid (5HIAA) respectively. 

Hormones released from the endocrine glands and those secreted by the organs other than the classical endocrine glands as well as their main functions are listed in Table 7.2. It should be borne in mind that numerous hormones and hormone-like substances might not have been mentioned in Table 7.2. 

Hormones Released by Classical Endocrine Glands and by Nonclassical Hormone-Producing Tissues... 

In addition to the classical stress hormones already reviewed, several other hormones are augmented in response to stress. Stress-induced prolactin release is one of the most frequently studied examples. There is no doubt about the causal relationship between stress and increased pituitary prolactin release, but the biological meaning is much less clear (G2). This phylogenetically old hormone has been shown to have more than 85 different functions in all vertebrate species. However, besides its role in the induction of maternal lactogenesis, the physiological importance of prolactin is at present not fully established. Experimental and clinical evidence supports the view that prolactin is also an immunoregulating hormone (M44, R18). Prolactin receptors are present on human T and B lymphocytes (R18), and T lymphocytes depend on prolactin for maintenance of immunocompetence (B19). In addition, it has been shown that prolactin is able to influence the devel-... 

Once released from the cell of origin, the signal ligand must travel to its site of action. For the classical endocrine hormones this means via the bloodstream. Given that blood plasma is approximately 94% water, the physical nature of the hormone is important. Peptides are hydrophilic and so circulate unbound to any other molecule whereas... 

Release of the pituitary gonadotropins (LH, FSH) is regulated by gonadotropin-releasing hormone (GnRH) from the hypothalamus.21,144 A classic... 

Only the first type of neurotransmitter release mediates the fast point-to-point synaptic transmission process at classical synapses (sometimes referred to as wiring transmission). All of the other types of neurotransmitter release effect one or another form of volume transmission whereby the neurotransmitter signal acts diffusely over more prolonged time periods (Agnati et al., 1995). Of these volume transmitter pathways, the time constants and volumes involved differ considerably. For example, diffusible neurotransmitters such as nitric oxide act relatively briefly in a localized manner, whereas at least some neuropeptides act on the whole brain, and can additionally act outside of it (i.e., function as hormones). There is an overlap between wiring and volume neurotransmission in that all classical neurotransmitters act as wiring transmitters via ionotropic receptors, and also act as volume transmitters via G-protein-coupled receptors. Moreover, neuromodulators in turn feed back onto classical synaptic transmission. 

For a hormone to have a specific effect on gene activity, any increase in enzyme activity must result from de novo synthesis by newly formed mRNA. This increase in enzyme activity may or may not precede any general increase in metabolic activity. From the foregoing discussion on chromatin activity, it is clear that plant hormones largely either increase the activity of polymerase I or increase the synthesis of total RNA s. Claims that the hormones "activate" chromatin-bound polymerases and "modulate" the number of active sites on the chromatin (21) have not been substantiated. There are only two known examples of hormone-induced synthesis of specific mRNA s. The classic example is the barley aleurone cells, in which GA treatment induces de novo synthesis and release of K-amylase (58, 59, 60), protease (61), and possibly as many as ten proteins (62). 

The adrenal gland produces cortisol in response to intermediate hormones, called adrenocorticotropin hormone (ACTH) and corticotropin-releasing hormone (CRH) via the humoral stress pathway, which extends from the brain to the adrenals (Figure 50-2). Once the body has received an environmental stress signal, it is detected by neurons in the cerebral cortex and transmitted to the hypothalamus. The hypothalamus then releases CRH via the classic secretory pathway into the anterior pituitary. CRH stimulates release of... 

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