RESPONSIVENESS IN VERTEBRATES 



The adrenal glands consist of two kinds of cells which in the higher verte- 

 brates are arranged as a central medulla surrounded by a cortex. Both 

 medulla and cortex produce hormones. 



The adrenal medulla was identified as an endocrine gland in 1901. It is now 

 recognized that the medulla secretes two closely related compounds known as 

 epinephrine (adrenaline) and norepinephrine. These are relatively simple 

 compounds, neither protein nor lipid, and are identical with the neurohumors 

 produced by the endings of the postganglionic fibers of sympathetic nerves. 

 The adrenal medulla, which is innervated by the sympathetic nerves, releases 

 its secretions in response to their discharge and is also very sensitive to 

 emotional stimuli. Epinephrine, like glucagon, activates phosphorylase, but 

 in muscle cells as well as in the liver. In muscle the glucose is available for 

 the energy requirements of contraction. More of the lactic acid produced in 

 muscle metabolism is removed by the blood in the presence of epinephrine. 

 Epinephrine mimics and thus adds to the effectiveness of sympathetic dis- 

 charge, one eflfect of which is to increase the rate of heart beat. It will be 

 noted that all these eflfects tend to make the muscular activity of the in- 

 dividual more efficient. Epinephrine also appears to be effective in bringing 

 about the release of the adrenocorticotropic hormone of the adenohypophysis, 

 another factor in the individual's defense in emergency or stressful circum- 

 stances, as will be pointed out later. Norepinephrine acts to constrict capil- 

 laries all over the body. 



A large number of complex steroid (lipid) compounds have been isolated 

 from extracts of the adrenal cortex. A number of the compounds crystallized 

 from the cortex are physiologically active. Their eff^ects fall into two main 

 categories: (1) regulation of salt and water balance (salt-active compounds) 

 and (2) regulation of carbohydrate and protein metabolism (sugar-active 

 compounds). Three cortical steroids have been crystallized from the blood 

 leaving the adrenal glands. Of these, aldosterone was crystallized for the 

 first time in 1953 and is by far the most active of any of the compounds ob- 

 tained from the cortex; its efTect is on metabolism of inorganic compounds. 

 Corticosterone and hydrocorticosterone are regulators of metabolism of organic 

 compounds. 



Adrenal cortical salt-active hormones control the amount of sodium, chlorine, 

 and potassium ions in the body. The importance of sodium and potassium 

 ions in nerve and muscle conduction has been discussed. Throughout the 

 body sodium (Na"*") and chlorine (CF) ions tend to remain outside the cell 

 membrane, and potassium (K^) and phosphate (HPO^"^) ions are localized 

 inside the cell. These ions play important, though not entirely understood, 

 roles in the permeability of cell membranes and in the osmotic environment 

 of cells. If the amount of salt-active cortical hormone is inadequate, too 

 many sodium and chlorine ions are excreted in the urine and their concentra- 

 tion in the blood and body fluids goes down; water is lost from the extra- 

 cellular spaces. Conversely, potassium ions are retained in the abnormal 

 amounts. These eflfects result from atypical behavior of the renal tubule. 



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