Chromaffin reaction

A characteristic feature of adrenal medullary chromaffin cells is the presence of numerous catecholamine storage granules ranging in size from 100 to 300 nm in diameter. These granules turn brown when exposed to potassium bichromate solutions, ammoniacal silver nitrate, or osmium tetroxide because of the oxidation and polymerization of epinephrine and norepinephrine. This process is known as the chromaffin reaction, hence the terms chromaffin cells and chromaffin granules. At least two kinds of adrenal medullary chromaffin cells have been identified in most animal species based on differences in chromaffin granules. Norepinephrine-producing chromaffin cells possess dense-... 

In humans, EC cells are found in the antrum and oxyntic mucosa. Their main secretory product is 5-hydroxytryptamine (5-HT). It is this monoamine that reduces silver and chromium, constituting the argentaffin and chromaffin reactions, respectively. D cells secrete somatostatin and are distributed throughout the antral and oxyntic mucosa but are more numerous in the antrum. They display argyrophilic staining (i.e., they accumulate silver predpitates from a silver nitrate solution when treated with an exogenous... 

In this lecture we will be concerned by exocytosis of neurotransmitters by chromaffin cells. These cells, located above kidneys, produce the adrenaline burst which induces fast body reactions they are used in neurosciences as standard models for the study of exocytosis by catecholaminergic neurons. Prior to exocytosis, adrenaline is contained at highly concentrated solutions into a polyelectrolyte gel matrix packed into small vesicles present in the cell cytoplasm and brought by the cytoskeleton near the cell outer membrane. Stimulation of the cell by divalent ions induces the fusion of the vesicles membrane with that of the cell and hence the release of the intravesicular content into the outer-cytoplasmic region. 

Takigami, T., Takeuchi, F., Nakagawa, M., Hase, T. and Tsubaki, M. Stopped-flow analyses on the reaction of ascorbate with cytochrome b561 purified from bovine chromaffin vesicle membranes. Biochemistry 42 8110-8118,2003. 

As described, the secretion of epinephrine in response to stress, trauma, extreme exercise, or hypoglycemia causes a rapid mobilization of energy stores, that is, glucose from the liver and fatty acids from adipose tissue. The reaction in which NE is methylated to form E is mediated by the enzyme phenylethanolamine-N-methyltransferase (PNMT). Although the enzyme occurs predominantly in the chromaffin cells of the adrenal medulla, it is also... 

Release of Catecholamines from the Adrenal Medulla Angll stimulates catecholamine release from the adrenal medulla by depolarizing chromaffin cells. Although this response usually is of minimal physiological importance, dangerous reactions have followed Angll administration to individuals with pheochromocytoma. 

Norepinephrine is biosynthesized in the neurons of both the central nervous system and the autonomio nervous system, whereas EPI is formed in the ohromaffin cells of the adrenal medulla. Both NE and EPI are derived from L-tyrosine by a series of enzyme-catalyzed reactions (Fig. 44.4 depicts the overall pathway). L-Tyrosine hydroxylase hydroxylates the meta position of L-tyrosine, producing L-dihydroxyphenylalanine (L-DOPA) and is the rate-limiting step. The L-DOPA is then decarboxylated by L-aromatic amino acid decarboxylase to form dopamine, which is converted to NE by the action of dopamine p-hydroxylase. Dopamine p-hydroxylase occurs in storage vesicles of the nerve ending, and the NE formed is stored there until it is released into the synaptic cleft. In the chromaffin cells, the formed NE is converted to EPI by N-methylation catalyzed by phenylethanolamine N-methyltransferase. 

Both hormones are pyrocatechol (o-dihydroxybenzene) derivatives, which are easily oxidized. This fact explains the histochemical reaction of the chromaffin tissue. The hormonal content of the adrenal medulla is relatively high (several mg per gm of gland). The two active substances of the adrenal medulla, epinephrine and norepinephrine, are both derivatives of phenyl ethylamine, which possesses strong pharmacologic effects. 

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