Adrenal medulla, catecholamines produced

Hi-receptors in the adrenal medulla stimulates the release of the two catecholamines noradrenaline and adrenaline as well as enkephalins. In the heart, histamine produces negative inotropic effects via Hr receptor stimulation, but these are normally masked by the positive effects of H2-receptor stimulation on heart rate and force of contraction. Histamine Hi-receptors are widely distributed in human brain and highest densities are found in neocortex, hippocampus, nucleus accumbens, thalamus and posterior hypothalamus where they predominantly excite neuronal activity. Histamine Hrreceptor stimulation can also activate peripheral sensory nerve endings leading to itching and a surrounding vasodilatation ( flare ) due to an axonal reflex and the consequent release of peptide neurotransmitters from collateral nerve endings. 

Catecholamines, such as epinephrine secreted by the chromaffin cells of the adrenal medulla or norepinephrine produced by the pancreas, have similar actions on metabolism to those of glucagon. 

The adrenal medulla synthesizes two catecholamine hormones, adrenaline (epinephrine) and noradrenaline (norepinephrine) (Figure 1.8). The ultimate biosynthetic precursor of both is the amino acid tyrosine. Subsequent to their synthesis, these hormones are stored in intracellular vesicles, and are released via exocytosis upon stimulation of the producer cells by neurons of the sympathetic nervous system. The catecholamine hormones induce their characteristic biological effects by binding to one of two classes of receptors, the a- and )S-adrenergic receptors. These receptors respond differently (often oppositely) to the catecholamines. 

When injected intravenously, kinins produce a rapid fall in blood pressure that is due to their arteriolar vasodilator action. The hypotensive response to bradykinin is of very brief duration. Intravenous infusions of the peptide fail to produce a sustained decrease in blood pressure prolonged hypotension can only be produced by progressively increasing the rate of infusion. The rapid reversibility of the hypotensive response to kinins is due primarily to reflex increases in heart rate, myocardial contractility, and cardiac output. In some species, bradykinin produces a biphasic change in blood pressure—an initial hypotensive response followed by an increase above the preinjection level. The increase in blood pressure may be due to a reflex activation of the sympathetic nervous system, but under some conditions, bradykinin can directly release catecholamines from the adrenal medulla and stimulate sympathetic ganglia. Bradykinin also increases blood pressure when injected into the central nervous system, but the physiologic significance of this effect is not clear, since it is unlikely that kinins cross the blood-brain barrier. 

Epinephrine and norepinephrine, hormones called catecholamines, are released from the adrenal medulla during exercise. Epinephrine and, to a lesser extent, norepinephrine stimulate effects in muscles and the liver similar to those produced by glucagon. Epinephrine activates adenylate cyclase, resulting in such events as the breakdown of glycogen and fatty acids. 

The adrenal medulla produces catecholamines in response to stressors. These are epinephrine and norepinephrine. Catecholamines increase the supply of oxygen and glucose to the brain and muscles. Digestion decreases. Heart rate increases. Blood vessels and bronchioles dilate. 

Although often considered a part of the sympathetic nervous system, the adrenal meduUa produces and secretes a different catecholamine, epinephrine, with different functions from the norepinephrme secreted by sympathetic nerves, The adrenal medulla and sympathetic nerves are also regulated separately, often in divergent directions in response to different forms of stress. 

Pheochromocytomas are catecholamine-producing tumors that arise from chromaffin cells of the adrenal medulla. Excluding neuroblastomas, about 10% of catecholamine-producing tumors arise from extraadrenal sympathochro-maffin tissue, usually in the abdomen, and are known as... 

The adrenal medulla forms part of the sympathetic nervous system and is the primary site for the production of the catecholamines—epinephrine (adrenaline) and norepinephrine (noradrenaline), which are primary hormones (also called biogenic amines). The cells of the medulla are arranged in lobules and the medulla contains chromaffin cells, which are modified postganglionic cells of the sympathetic nervous system. The medulla produces catecholamines from tyrosine and their structures contain catechol and amine groups (Figure 10.3.3). 

Which ofthe catecholamines is formed in acell, depends on its armamentarium of available enzymes. The adrenal medulla produces, besides noradrenaline mainly adrenaline the former is as well generated in particular sections ofthe brain stem. 

Another catecholamine, noradrenaline, is produced mainly at the nerve endings of the sympathetic system and also in small amounts in the adrenal medulla. Secretion from this gland appears to be under nervous control and occurs rapidly as an appropriate response to stressful situations. 

Intracellular metabolic activity may be modulated in response to extracellular conditions by hormones. Hormones are substances which act as chemical messengers between cells in different locations to alter the activity of the recipient cell. Three classes of hormones have been identified protein (polypeptide) hormones, steroid hormones and tyrosine-derivative hormones of which there are two distinct groups, the catecholamines and the thyroid hormones. The catecholamine hormones (adrenaline and noradrenaline) are produced by and secreted from the adrenal medulla. The thyroid hormones (thyroxine and triiodothyronine) are elaborated by and secreted from the thyroid gland. 

The naturally occurring catecholamines—dopamine (DA) (1), / -norepinephrine (/ -NE) (2), and / -epinephrine (/J-EPI) (3)— have many imiK)rtant biological functions. These catecholamines are produced in vivo from L-tyrosine. Tyrosine is first converted to dihydroxyphenylalanine (DOPA) by aromatic hydroxylation. L-DOPA is then decarboxylated to give DA, which is subsequently converted to / -NE by p-hydroxylation. DA is a vital neurotransmitter in the central nervous system (CNS) and has actions on the kidneys and heart. Norepinephrine is also present as a neurotransmitter in the CNS, and is the principal neurotransmitter of the peripheral sympathetic nervous system. Epinephrine, which is elaborated from / -NE by N-methylation in the adrenal medulla, has potent actions on the heart, smooth muscle, and other organs (/). 

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