Adrenal medullary hormones

Two PNS neurotransmitters, acetylchohne and norepinephrine, have particular chnical importance. Both are synthesized and stored primarily in the nerve terminals until released by a nerve impulse. It should be noted, to avoid confusion, that in the United States the transmitter in the sympathetic nervous system is referred to as norepinephrine and the major adrenal medullary hormone is referred to as epinephrine. In Europe and most of the world these two substances are called noradrenaline and adrenaline, respectively. 

General activation of the sympathetic system during stress, fear, or anxiety is accompanied by increased secretion of adrenal medullary hormones, which consist primarily of epinephrine in the human. The secretory activity of the adrenal medulla is regulated by the CNS. 

Adrenoceptors interact not only with norepinephrine but also with the adrenal medullary hormone epinephrine and a number of chemically related drugs. However, the responses produced by the drugs in different autonomic structures differ quantitatively or qualitatively from one another. 

The primary purpose of the endocrine system is to maintain homeostasis — that is, to maintain a relatively constant internal environment in the face of a constantly changing external environment. The endocrine system consists of hormones and the glands and tissues that produce the hormones. A hormone is a chemical substance released by certain cells to effect the function of other distant cells (endocrine function). Many compounds act as endocrine hormones as well as having paracrine and autocrine functions. Paracrine and autocrine describe actions on nearby cells and on other cells that produce the substance, respectively. There is considerable overlap between substances classified as hormones and other chemical messengers such as neurotransmitters and cytokines. Many substances function in more than one of these categories. For example, epinephrine and norepinephrine function as both neurotransmitters and adrenal medullary hormones. 

The hormones of (he adrenal glands are essential for survival. The adrenal cortex is the source of the two important steroid hormones, aldosterone and cortisol (Fig. 1). The adrenal medulla is embryologically and histologically distinct from the cortex anil is part of the sympathetic nervous system. Medullary cells synthesize, store and secrete adrenaline, along with noradrenaline and dopamine. The adrenal medullary hormones arc discussed further on pages 126-127. 

VON Euler, U. S., Noradrenaline (arterenol) adrenal medullary hormone and chemical transmitter of adrenergic nerves, Ergebn. Physiologic 46, 261 (1950). 

In cells that synthesize epinephrine, the final step in the pathway is catalyzed by the enzyme phenylethanolamine /V-methyltransferase. This enzyme is found in a small group of neurons in the brainstem that use epinephrine as their neurotransmitter and in the adrenal medullary cells, for which epinephrine is the primary hormone secreted. Phenylethanolamine N-methyltransferase (PNMT) transfers a methyl group from S-adenosylmethionine to the nitrogen of norepinephrine, forming a secondary amine [5]. The coding sequence of bovine PNMT is contained in a... 

The cells of the adrenal medulla, called chromaffin cells, are homologous with sympathetic postganglionic neurons. The adrenal medulla may in fact be considered a modified sympathetic ganglion. The adrenal medulla secretes two hormones. One is norepinephrine, which is also the primary neurotransmitter of sympathetic postganglionic neurons. The other medullary hormone is epinephrine. 

The synthesis of ADR from NA in the adrenal medulla also appears to be under complex control in vivo. The N-methyl transferase enzyme responsible for this conversion is potently inhibited by the product ADR (Kt = 50-100 /iM). Since the endogenous concentration of ADR in the adrenal medulla is very high, it is probable that the synthesis of ADR is controlled by product inhibition of the N-methyl transferase. In addition a long-term control mechanism exists in that the synthesis of the N-methyl transferase enzyme is dependent on continued exposure of the medullary tissue to high concentrations of glucocorticoid hormones secreted by the surrounding adrenal cortex. Enzyme activity falls after hypophysectomy. and can be restored by the administration of ACTH or glucocorticoids. 

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