ADRENAL GLANDS

The adrenals are symmetric bilateral glandular organs located in the abdominal cavity on the anterior surface of the kidneys. The glands are divided into two parts (cortex and medulla) with different endocrine functions. The adrenal cortex is subdivided into three zones (a superficial layer called zona glomerulosa, a middle layer called zona fasciculata, and a inner layer called zona reticularis) in which steroid hormones (called corticoids) are produced (Greco and Stabenfeldt 2007; Martin and Crump 2003). Corticoids are hormones synthesized from cholesterol. The main steroid hormones produced in the adrenal cortex are mineralocorticoids and glucocorticoids. Other steroid hormones such as androgens and estrogens are also synthesized in the adrenal cortex, however, in normal physiological conditions the synthesis of these sex hormones in the adrenal glands is low (Greco and Stabenfeldt 2007).

Aldosterone is the main mineralocorticoid hormone produced in the zona glomerulosa. The synthesis and secretion of this hormone is regulated by the renin-angiotensin system and the level of sodium and potassium in the blood. The main action of this hormone is stimulation of sodium reabsorption and excretion of potassium which maintains proper sodium-potassium balance, volume of fluids and blood pressure (Greco and Stabenfeldt 2007; Martin and Crump 2003; Tan et al. 2004).

Cortisol is the main glucocorticoid hormone in dogs and cats produced in the zona fasciculata and zona reticularis (Greco and Stabenfeldt 2007). The synthesis and secretion of glucocorticoid hormones is regulated by the hypothalamic-pituitary-adrenal axis, in which adrenocorticotropin (ACTH) released from the pituitary gland leads to increased secretion of glucocorticoids. This group of steroid hormones has an influence on carbohydrate, protein and lipid metabolism, and proper functioning of various systems in the organism such as: endocrine system, musculoskeletal system, skin, cardiovascular system, renal system, and immune system (Martin and Crump 2003). Glucocorticoid hormones are involved in the maintenance of the proper level of blood glucose, stimulate protein catabolism, lipolysis, redistribution of the fat to the liver and abdomen, and water excretion (Greco and Stabenfeldt 2007; Martin and Crump 2003).

The adrenal medulla is the second part of the gland, and secretes hormones called catecholamines (Martin and Crump 2003). Among catecholamines, epinephrine is the major hormone secreted by the adrenal medulla. The other catecholamines (such as norepinephrine and dopamine) are precursors for epinephrine. Secretion of catecholamines is stimulated by hypoglycemia and acute stress. The tissue response depends on the kind of adrenergic receptors (α 1 , α 2 , β 1 , β 2 ). That is why catecholamines cause vasoconstriction in the skin and viscera, but dilation of blood vessels in skeletal muscles. Moreover, catecholamines increase cardiac output and contraction of muscles responsible for piloerection. These reactions to the catecholamines are associated with fight-or-flight responses. Catecholamines also stimulate glycogenolysis (release of glucose from glycogen, which is how glucose is stored in the animal) in the liver and skeletal muscles, which is associated with hypoglycemia (Greco and Stabenfeldt 2007; Martin and Crump 2003).

References

Greco D.S., Stabenfeldt G.H. Endocrine Glands and Their Function. In: Cunningham J.G. and Klein B.G. (eds.) Textbook of Veterinary Physiology. 4 th ed. Saunders Elsevier, St. Louis, 2007b, pp. 428-464.

Martin P.A., Crump M.H. The Adrenal Gland. In: Pineda M.H. and Dooley M.P. (eds.) McDonald’s Veterinary Endocrinology and Reproduction. 5 th ed. Blackwell Publishing, Ames, 2003b, pp. 165-200.

Tan L.-B., Schlosshan D., Barker D. Fiftieth anniversary of aldosterone: from discovery to cardiovascular therapy. International Journal of Cardiology, 2004, 96, 321-333.

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