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The Endocrine Glands in the Dog: From the Cell to HormoneHelena Vala1, 2, Joao Rodrigo Mesquita2, Fernando Esteves2, Carla Santos2, Rita Cruz2, Cristina Mega2 and Carmen Nobrega2[1] Center for Studies in Education, and Health Technologies.
Endocrine glands are ductless glands that produce and release hormones to the blood through diffusion. Endocrine glands may be strictly endocrine, such as the pituitary, thyroid, parathyroid, adrenal, pineal and thymus; or they may be organs that have hormone production as one of many functions, such as the pancreas, gonads, hypothalamus, and others. Hormones are long-distance chemical signals that are secreted by the cells to the extracellular fluid and regulate the metabolic functions of other cells.
Most hormones are amino acid based, but gonadal and adrenocortical hormones are steroids, derived from cholesterol.
Water-soluble hormones (all amino acid-based hormones except thyroid hormone) exert their effects through an intracellular second messenger that is activated when a hormone binds to a membrane receptor.
Lipid-soluble hormones (steroids and thyroid hormone) diffuse into the cell, where they bind to intracellular receptors, migrate to the nucleus, and activate specific target sequences of DNA. Target cell response depends on three factors: blood levels of the hormone, relative numbers of target cell receptors, and affinity of the receptor for the hormone. The concentration of a hormone reflects its rate of release, and the rate of inactivation and removal from the body. The half-life of a hormone is the duration of time a hormone remains in the blood, and is shortest for water-soluble hormones. Permissiveness occurs when one hormone cannot exert its full effect without another hormone being present (reproductive hormones need thyroxine to properly stimulate development of reproductive organs). Synergism occurs when more than one hormone produces the same effects in a target cell, and their combined effects are amplified (glucagon + epinephrine together stimulate more glucose release from the liver than when each acts alone).
Antagonism occurs when one hormone opposes the action of another hormone (glucagon antagonizes insulin). Nervous system modulation allows hormone secretion to be modified by the nervous stimulation in response to changing body needs. The pituitary gland is connected to the hypothalamus via a stalk, the infundibulum, and consists of two lobes: the anterior pituitary, or adenohypophysis, and the posterior pituitary, or neurohypophysis. Two neurohormones are synthesized by the hypothalamus and secreted by the posterior pituitary.
Growth hormone (GH) indirectly (through insulin-like growth factors, IGFs) stimulates body cells to increase in size and divide. Direct effects are insulin-sparing: mobilization of fatty acids for fuel, inhibition of insulin activity, release of glucose from liver to blood, and stimulation of amino acid uptake by cells.
The thyroid gland consists of hollow follicles with follicle cells that produce thyroglobulin, and parafollicular cells that produce calcitonin. Thyroid hormone consists of two amine hormones: thyroxine (T4) and triiodothyronine (T3), that act on all body cells to increase basal metabolic rate and body heat production. The parathyroid glands contain chief cells that secrete parathyroid hormone, or parathormone.
The adrenal glands, or suprarenal glands, consist of two regions: an inner adrenal medulla and an outer adrenal cortex. The adrenal cortex produces corticosteroids from three distinct regions: the zona glomerulosa, the zona fasciculata, and the zona reticularis. The adrenal medulla contains chromaffin cells that synthesize epinephrine and norepinephrine (stimulus is acetylcholine released by preganglionic sympathetic fibers).
Insulin is an anabolic hormone and will stimulate not only glucose uptake but also storage in the form of glycogen (glycogenesis), fat (lipogenesis) and amino acid incorporation into proteins (inhibits amino acid breakdown by liver to form new glucose molecules - gluconeogenesis).
Stimuli for insulin release are primarily high blood glucose levels but insulin release is also potentiated by rising blood levels of amino acids and fatty acids and release of acetylcholine by parasympathetic neurons (all of these things happen after a meal). Glucagon is released by the pancreas in response to low blood glucose levels (primarily) and raises blood glucose levels back to within normal range by stimulating glycogenolysis, gluconeogenesis, and release of glucose to the blood by the liver.
Indirectly receives input from the visual pathways in order to determine the timing of day and night.
Adipose tissue produces leptin, which acts on the CNS to produce a feeling of satiety; secretion is proportional to fat stores.
Adipocytes also produce adiponectin, which enhances insulin activity, and resistin, an insulin antagonist. Osteoblasts in bone produce osteocalcin, which stimulates pancreatic beta cells to divide and secrete more insulin. Adiponectin levels are low in type II diabetes, suggesting higher levels may help reverse the insulin resistance characteristic of type II diabetes. Endocrine glands derived from mesoderm produce steroid hormones; those derived from ectoderm or endoderm produce amines, peptides, or protein hormones.
Environmental pollutants have been demonstrated to have effects on sex hormones, thyroid hormone, and glucocorticoids.
The walls of all blood vessels except the smallest consist of three layers: the tunica intima, tunica media, and tunica externa. Elastic, or conducting, arteries contain large amounts of elastin, which enables these vessels to withstand and smooth out pressure fluctuations due to heart action. Muscular, or distributing, arteries deliver blood to specific body organs, and have the greatest proportion of tunica media of all vessels, making them more active in vasoconstriction. Arterioles are the smallest arteries and regulate blood flow into capillary beds through vasoconstriction and vasodilation. Capillaries are the smallest vessels and allow for exchange of substances between the blood and interstitial fluid.
Fenestrated capillaries are more permeable to fluids and solutes than continuous capillaries. Sinusoidal capillaries are leaky capillaries that allow large molecules to pass between the blood and surrounding tissues.
Capillary beds are microcirculatory networks consisting of a vascular shunt and true capillaries, which function as the exchange vessels.
A cuff of smooth muscle, called a precapillary sphincter, surrounds each capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary. Venules are formed where capillaries converge and allow fluid and white blood cells to move easily between the blood and tissues. Venules join to form veins, which are relatively thin-walled vessels with large lumens containing about 65% of the total blood volume. If blood pressure increases, blood flow increases; if peripheral resistance increases, blood flow decreases. The pumping action of the heart generates blood flow; pressure results when blood flow is opposed by resistance.
Systemic blood pressure is highest in the aorta, and declines throughout the pathway until it reaches 0 mm Hg in the right atrium. Arterial blood pressure reflects how much the arteries close to the heart can be stretched (compliance, or distensibility), and the volume forced into them at a given time. When the left ventricle contracts, blood is forced into the aorta, producing a peak in pressure called systolic pressure (120 mm Hg). Diastolic pressure occurs when blood is prevented from flowing back into the ventricles by the closed semilunar valve, and the aorta recoils (70–80 mm Hg).
The mean arterial pressure (MAP) represents the pressure that propels blood to the tissues. Capillary blood pressure is low, ranging from 40–20 mm Hg, which protects the capillaries from rupture, but is still adequate to ensure exchange between blood and tissues. Short-term mechanisms include both (1) neural and (2) hormonal controls, which alter blood pressure by changing peripheral resistance and CO. Most neural controls work through reflex arcs that send information on stretch to effectors (muscle) that respond accordingly. The cortex and hypothalamus can modify arterial pressure by signaling the medullary centers.
Atrial natriuretic peptide acts as a vasodilator and an antagonist to aldosterone, resulting in a drop in blood volume. Antidiuretic hormone promotes vasoconstriction and water conservation by the kidneys, resulting in an increase in blood volume. Angiotensin II acts as a vasoconstrictor, as well as promoting the release of aldosterone and antidiuretic hormone. Endothelium-derived factors promote vasoconstriction, and are released in response to low blood flow.
Nitric oxide is produced in response to high blood flow or other signaling molecules, and promotes systemic and localized vasodilation. Inflammatory chemicals, such as histamine, prostacyclin, and kinins, are potent vasodilators.
Alcohol inhibits antidiuretic hormone release and the vasomotor center, resulting in vasodilation. Monitoring circulatory efficiency is accomplished by measuring pulse and blood pressure; these values together with respiratory rate and body temperature are called vital signs. A pulse is generated by the alternating stretch and recoil of elastic arteries during each cardiac cycle. Systemic blood pressure is measured indirectly using the ascultatory method, which relies on the use of a blood pressure cuff to alternately stop and reopen blood flow into the brachial artery of the arm. Alterations in blood pressure may result in hypotension (low blood pressure) or transient or persistent hypertension (high blood pressure). Metabolic controls of autoregulation are most strongly stimulated by a shortage of oxygen at the tissues.
NO - Nitric oxide, released from endothelial cells locally or from hemoglobin can stimulate relaxation of smooth cells and inhibit production of endothelin synthesis by endothelial cells.
Long-term autoregulation develops over weeks or months, and involves an increase in the size of existing blood vessels and an increase in the number of vessels in a specific area, a process called angiogenesis.
Muscular autoregulation occurs almost entirely in response to decreased oxygen concentrations.

Cerebral blood flow is tightly regulated to meet neuronal needs, since neurons cannot tolerate periods of ischemia, and increased blood carbon dioxide causes marked vasodilation. In the skin, local autoregulatory events control oxygen and nutrient delivery to the cells, while neural mechanisms control the body temperature regulation function.
Autoregulatory controls of blood flow to the lungs are the opposite of what happens in most tissues: low pulmonary oxygen causes vasoconstriction, while higher oxygen causes vasodilation. Movement of blood through the coronary circulation of the heart is influenced by aortic pressure and the pumping of the ventricles. Vasomotion, the slow, intermittent flow of blood through the capillaries, reflects the action of the precapillary sphincters in response to local autoregulatory controls. Capillary exchange of nutrients, gases, and metabolic wastes occurs between the blood and interstitial space through diffusion. Colloid osmotic pressure (OP), the force opposing hydrostatic pressure, is created by the presence of large, nondiffusible molecules that are prevented from moving through the capillary membrane.
Fluids will leave the capillaries if net HP exceeds net OP, but fluids will enter the capillaries if net OP exceeds net HP. Circulatory shock is any condition in which blood volume is inadequate and cannot circulate normally, resulting in blood flow that cannot meet the needs of a tissue. Hypovolemic shock results from a large-scale loss of blood, and may be characterized by an elevated heart rate and intense vasoconstriction.
Vascular shock is characterized by a normal blood volume, but extreme vasodilation, often related to a loss of vasomotor tone, resulting in poor circulation and a rapid drop in blood pressure.
Septic shock - bacterial toxins, some increase vascular permeability and lower blood volume and others stimulate vasodilation. Transient vascular shock is due to prolonged exposure to heat, such as while sunbathing, resulting in vasodilation of cutaneous blood vessels. Cardiogenic shock occurs when the heart is too inefficient to sustain normal blood flow, and is usually related to myocardial damage, such as repeated myocardial infarcts. There is one terminal systemic artery, the aorta, but two terminal systemic veins: the superior and inferior vena cava. Arteries run deep and are well protected, but veins are both deep, running parallel to the arteries, and superficial, running just beneath the skin. Arterial pathways tend to be clear, but there are often many interconnections in venous pathways, making them difficult to follow. There are at least two areas where venous drainage does not parallel the arterial supply: the dural sinuses draining the brain, and the hepatic portal system draining from the digestive organs to the liver before entering the main systemic circulation. Four paired arteries supply the head and neck (common carotid arteries and three branches from the subclavian arteries; the vertebral arteries, the thyrocervical trunks, and the costocervical trunks.
The internal iliac arteries serve mostly the pelvic region; the external iliacs supply blood to the lower limb and abdominal wall.
Blood drained from the head and neck is collected by three pairs of veins (internal jugular veins, external jugular veins, and the vertebral veins). Blood draining from the abdominopelvic viscera and abdominal walls is returned to the heart by the inferior vena cava. The vascular endothelium is formed by mesodermal cells that collect throughout the embryo in blood islands, which give rise to extensions that form rudimentary vascular tubes. By the fourth week of development, the rudimentary heart and vessels are circulating blood. Fetal vascular modifications include shunts to bypass fetal lungs (the foramen ovale and ductus arteriosus), the ductus venosus that bypasses the liver, and the umbilical arteries and veins, which carry blood to and from the placenta. Congenital vascular problems are rare, but the incidence of vascular disease increases with age, leading to varicose veins, tingling in fingers and toes, and muscle cramping. ThyroidCanine thyroid gland is located at cervical ventral region, lateral and ventral to the 5th-8th tracheal rings (Figure 7), being composed by two separate lobes, occasionally connected by an isthmus [17, 18]. Adrenal glandsThe adrenal glands are paired organs located against the roof of the abdomen near the thoracolumbar junction, in a position immediately prior to the kidneys and close to their cranial poles (Figure 16).
Clinical significance Endocrine diseases, associated to altered functions of endocrine glands, are frequently seen in veterinary practice.
Molecular and genetic study of the role of hormones receptors, and enzymes in regulation of reproduction, lipid metabolism, and other human physiological functions. Follicle-Stimulating Hormone and Luteinizing Hormone Mediate the Androgenic Pathway in Leydig Cells of an Evolutionary Advanced Teleost. G proteins and autocrine signaling differentially regulate gonadotropin subunit expression in the pituitary gonadotrope.
Thyroxine-induced expression of pyroglutamyl peptidase II and inhibition of TSH release precedes suppression of TRH mRNA and requires type 2 deiodinase. Secretory cells of the supraoptic nucleus have central as well as neurohypophysial projections.
Physiology of the pancreatic ?-cell and glucagon secretion: role in glucose homeostasis and diabetes. The stimulus for GHRH release is low blood levels of GH as well as hypoglycemia, low blood levels of fatty acids, and high blood levels of amino acids.
Hypersecretion of GH in childhood results in gigantism; in adulthood hypersecretion of GH causes acromegaly (increase in size of flat bones after epiphyseal plates of long bones have sealed).
Thryroid releasing hormone (TRH) from the hypothalamus stimulates TSH release; Thyroid hormone (Thyroxine) exerts negative feedback control of both TRH and TSH. Excretion of ketoacids (with their negative charge) by the kidney is accompanied by loss of cations, particularly K+ and Na+.
Secretion of resistin is proportional to fat stores; secretion of adiponectin is inversely proportional to fat stores. The carotid sinus reflex protects blood flow to the brain, while the aortic reflex maintains blood pressure throughout the systemic circuit. Agrarian School of Viseu, Polytechnic Institute of Viseu, Viseu, Portugal[2] Agrarian Superior School of Viseu, Polytechnic Institute of Viseu. In the dog there are four parathyroid glands, one external and one internal per each thyroid gland [17, 19, 20].External parathyroid glands are capsulated and may be found in varied positions, according to the species, which means that they can be placed between a cranial location to the thyroid gland and the entrance of the chest. The endocrine glands described may be targeted by several conditions summarized in table 7.
Two histologic forms of primary hypothyroidism predominate in the dog: lymphocytic thyroiditis and idiopathic atrophy. IntroductionThe animal body represents one of the more complex and perfect systems of nature.
They may be absent in some species or may be found included within the thyroid or close to them. The slender right lobe runs within the mesoduodenum and the thicker but shorter left lobe extends over the caudal surface of the stomach towards the spleen, within the greater omentum (Figure 13) [4, 17]. The cortex is yellowish and radially striated, while the medulla is more uniform and darker [17].
Doberman Pinschers and Golden Retrievers are the two breeds most frequently described [42]. Despite its complexity and its functionality, which is incredibly effective, the control of its basic functions is performed by only two systems: the nervous system and endocrine system. Bar = 100 µm.The parenchyma of the pars distalis is composed by chromophilic cells, acidophilic or basophilic, arranged in cords, nests or follicles (Figure 4) and by small, round endocrine cells, without evident granules - chromophobe cells which means that they have low tinctorial affinity, also known as principal cells, reserve cells or C cells [4]. Bar = 25 µm.The follicle is, therefore, the structural and secretory unit of the thyroid gland, whose center is filled with a colloidal secretion (Figure 9), comprising the hormones triiodothyronine (T3) and thyroxine (T4) [4].
In the dog, internal parathyroid glands are within the thyroid capsule, in the caudal and medial aspects of the thyroid [19, 20].The parenchyma of parathyroid glands is composed of secretory cells, named chief or principal cells, arranged in cords, clusters, chains or rosettes (Figure 11, Figure 12). Both regions, cortex and medulla, correspond to areas specialized in producing different hormones [4, 32, 34]:The cortex consists of polyhedral secretory cells, arranged into two layered thick cords, which originate radially from the medullar zone [4, 33]. Clinical signs associated with hypothyroidism are varied, and behavioural changes are associated with a reduced metabolic rate, dermatological signs, cardiovascular and neuromuscular abnormalities. The nervous system is associated with electrical and chemical signals that are transmitted at high speed, resulting in rapid organic activities. Chromophilic cells secrete somatotropic hormone (also known as somatotropin - STH or growth hormone - GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH) (which is also called interstitial cell-stimulating hormone or ICSH in males), thyrotropin or thyroid-stimulating hormone (TSH), melanocyte-stimulating hormone (MSH) and lipotropines [4]. In between the follicles, in a parafollicular position, cells with a pale cytoplasm, and a basal nucleus can be found - parafollicular cells or C cells which produce calcitonin [4, 16, 18]. In some species, such as cattle and humans, it is also composed by oxyphilic cells, organized in small clusters which have yet no known functions [4]. The later is produced in the islets of Langerhans, which are randomly scattered in the exocrine parenchyma (Figure 14) [10, 17, 24]. The endocrine system acts through the synthesis and release of chemical messengers and is responsible for several functions of the organism, in a slower, but more durable way.Endocrinology is the science that studies the internal secretions produced by endocrine glands.
Somatotropin promotes epiphyseal growth [6] and protein production, whereas prolactin leads to mammary gland development and milk secretion. In dogs, this type of cells were only found in senile animals [21] and were also described in canine parathyroid adenoma [22].
Pancreatic islets are aggregates of endocrine cells required for blood glucose control and diabetes prevention after birth, consisting of polygonal shaped cells, with pale eosinophilic cytoplasm and coarse heterochromatin. The outer zone is the zona glomerulosa, a thin region that in dogs and cats is composed by cells arranged in an arched or arcuate pattern, therefore also named as zona arcuata (Figure 17) [32, 35, 36].The zona glomerulosa produces mineralocorticoids, particularly the steroid hormone aldosterone [32, 37], which is responsible for increasing sodium retention and water reabsortion in the distal tubules of the kidneys and sodium reabsorption in intestine. Endocrine glands are distributed throughout the body and secrete chemical messengers – hormones, in response to an internal or external stimulus. ACTH acts on the adrenal gland cortex, resulting in an increased release of adrenocortical hormones [7]. Cells are arranged in cords or clusters separated by sinusoids and are subdivided in the following subtypes (Figure 15) [4, 24, 25]:? cells, located peripherally and representing 10-20% of islet cells, secrete glucagon, a hyperglycaemic hormone, as well as cholecystokinin, gastric inhibitory protein and ACTH-endorphin [4, 10, 24, 26].
Aldosterone also maintains blood concentration and stimulates potassium excretion by the kidneys, thereby, indirectly regulating extracellular fluid volume. Seborrhoea, bilateral and symmetrical alopecia, and pyoderma are also common dermatological symptoms. These hormones are released directly into the bloodstream – endocrine mechanism, in contrast to exocrine glands, which use a ductal system to release their secretions in locations that lead, ultimately, to the exterior of the body – exocrine mechanism.

? cells, located in the centre of the islets, are more abundant, representing 60-80% of the islets of Langerhans, and secrete insulin, a hypoglycaemic hormone [4, 10, 24].
A decrease of mineralocorticoids, by loss of this zone or its functional ability, gives rise to water outlet of the blood to the tissues and may result in death, due to retention of high levels of potassium with excess loss of sodium, chloride and water [32].
These signs can be accompanied by bradycardia, low voltage ECG complexes and weakness [40, 41, 43, 44].Diabetes mellitusDiabetes mellitus is the result of any situation that affects insulin production, insulin transport or the sensitivity of target tissues to insulin [45]. Hormones are transported through the bloodstream to target organs, where they will exert a physiological control, even in low concentrations, coordinating a multiplicity of organic functions and maintaining homeostasis.
Many factors are known to contribute to the development of diabetes and its complications [46, 47]. The main endocrine glands in the animal body include pituitary gland, thyroid, parathyroid, pancreas, adrenal (Figure 1), and gonads (ovaries and testes).
This hormone is also capable of reducing intestinal motility and secretion of digestive juices. These include genetics, diet, sedentary lifestyle, perinatal factors, age, obesity and inflammatory causes [47, 48]. These cells represent about 5-40% of the islets [10, 24].Besides the above-mentioned cell types, the endocrine pancreas also contain some minor types of cells, that represent around 5% of the total pancreatic cells. Canine diabetes mellitus is generally classified as insulin dependent or non-insulin dependent based on the need for insulin treatment [40, 48]. Gonadotropes or gonadotropic cells which secrete gonadotropins LH and FSH (basophilic) [13].
This hormone, together with calcitonin and vitamin D, is involved in the regulation of calcium homeostasis. Bar = 50 µm.The zona fasciculata is the middle and thickest zone (corresponding to more than 70% of the cortex) and is composed of parallel columns of secretory cells, one to two cells thick, separated by prominent capillaries. Most diabetic dogs are thought to have a disease like human type I diabetes mellitus and are insulin dependent [40, 49, 50].
Hypothalamic-pituitary axisThe hypothalamus, located at the base of the ventral diencephalon is limited rostrally by the optic chiasm, caudally by the mammillary processes, laterally by the temporal lobes and dorsally by the thalamus.
It is released in response to low blood calcium level [23] due to it’s ability to enhance the mobilization of calcium ions from the small intestine and from bone resorption, and also by increasing the simultaneous absorption of calcium and excretion of phosphate from distal convoluted tubules of kidneys [10] (Table 4).
The cells are cuboidal or polyhedral, containing vesicular nucleus, frequently binucleated) and foamy cytoplasm (intracellular lipid droplets) (Figure 18). Unlike human type I diabetes mellitus, which occurs mainly in young human patients, in dogs it is more likely to occur later in life [40, 45, 48].
The chromophobe cells are also arranged in clusters or cords and their low tinctorial affinity probably indicates that these cells may correspond to depleted cells of any of the above types described or a state of partial degranulation.
Dogs do not appear to progress from obesity-induced insulin resistance to type 2 diabetes mellitus, probably because pancreatic beta cells in dogs are either not sensitive to toxicity due mild hyperglycemia or lack other components of the pathophysiology of beta cell failure in type 2 diabetes mellitus [51]. Although not considered a real endocrine gland, the hypothalamus coordinates all pituitary activity by the release of a number of peptides and amines that control the secretion of hormones in the pituitary gland (also named hypophysis). However, some of the chromophobe cells may also be undifferentiated, nonsecretory cells [4, 15]. Glucocorticoids have several functions, including protein catabolism and stimulation of the hepatic gluconeogenesis from amino acids [10, 37]. Certain breeds, including Australian Terrier, Keeshounds, Alaskan Malamutes, Finnish Spitzes, Standard and Miniature Schnauzers, Miniature Poodles, and English Springer Spaniels, seem to be at increased risk to develop diabetes.
Bar = 25 µm.Insulin and glucagon are pancreatic hormones that play pivotal roles in regulating glucose homeostasis and metabolism, which have opposite effects on glycaemia as well as on the metabolism of nutrients [24, 26, 28]. Others, such as Boxers, German Shepherd dogs, Cocker Spaniels and Collies seemed to be at decreased risk [40, 52]. So, the actions of the two hormones combined contribute to the control of blood glucose.Glucagon secretion by ?-cells is highly regulated by multiple factors, being the most important the glucose and insulin levels. Exercise intolerance or decreased activity, ketonic breath, recurrent infections (urinary tract, conjunctivitis), cataracts and hepatomegaly can also be present [45]. Bar = 25 µm.The pars tuberalis is composed by cuboidal weakly basophilic cells, arranged in cords, nests or follicles and its function is not yet well established [4].
Low glucose levels activate specific channels in the brain and in pancreatic ?-cells to generate action potentials of sodium and calcium currents, leading to glucagon secretion. Bar = 25 µm.The zona reticularis is also composed of polyhedral cells, whose arrangement consists in freely anastomosing cords (Figure 19) [32]. Also, somatostatin inhibits glucagon secretion by inhibiting adenylate cyclase and cAMP production [26]. These cells contain less lipid content but have densely granular cytoplasm for which they are called “compact cells” [34]. Spontaneous hyperadrenocorticism is associated with inappropriate secretion of ACTH by the pituitary (pituitary-dependent hyperadrenocorticism) or with a primary adrenal disorder (adrenal-dependent hyperadrenocorticism) [40, 53, 54]. Insulin has several physiologic actions that include stimulation of cellular glucose and potassium uptake [29]. Over 80% of dogs with spontaneous hyperdarenocorticism suffer from pituitary dependent hypercorticism, resulting in an over secretion of ACTH [53, 54]. Glucose-induced insulin secretion from pancreatic ?-cells depends on mitochondrial activation, amongst other factors, like ATP, glutamate and others [30]. These hormones are produced in the hypothalamus, transported through the hypothalamic-pituitary axis and stored in the neurohypophysis, until a stimulus induces their release [4, 16] (Table 2).ADH has several effects on the body, particularly in terms of water saving and in increasing blood pressure. Incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon like peptide-1 (GLP-1), secreted by cells of the gastrointestinal tract in response to meal ingestion, exert important glucorregulatory effects, including the glucose-dependent potentiation of insulin secretion by pancreatic ?-cells [31].Table 5 summarizes hormones of the pancreas and their functions. Oxytocin stimulates contraction of the myoepithelial cells of the mammary gland, causing the ejection of milk.
Adrenal-dependent hyperadrenocorticism represent the remaining 20% of spontaneous hyperadrenocorticism and is generally associated with unilateral (or occasionally bilateral) adrenal tumours. It also binds to the smooth muscle cells of the uterus, promoting uterine contractions during parturition [10] (Table 2). Dogs showing pituitary-dependent hyperadrenocorticism exhibit a mean age of 7-9 years old, and those with adrenal-dependent hyperadrenocorticism, a mean age of 11-12 years old. There are some breeds that are most frequently associated with hyperadrenocorticism, but any breed can develop it. The medulla consists of large columnar or polyhedral secretory cells, randomly distributed, with rich blood supply (Figure 20). The cells have large, vesicular nucleus, basophilic cytoplasm with fine positive chromaffin granules, due to the presence of catecholamines such as epinephrine and norepinephrine, which after exposure to oxidizing agents, such as chromate, yield a brown reaction by the formation of colored polymers [4, 31, 33]. This can arise from a destruction of more than 90% of both adrenal cortices, causing inability to produce corticosteroids (Primary hypoadrenocorticism or Addison’s disease), or from a deficiency in ACTH production by the pituitary (Secondary hypoadrenocorticism) [56, 57]. Besides different pathophysiology, primary and secondary hypoadrenocorticism also exhibit different clinical signs. In adults, there are 3 types of adrenal medullary cells (1): epinephrine cells (66–75%), norepinephrine cells (25–33%) and small granule-containing cells (SGC, 1-4%). It is often associated with idiopathic adrenocortical atrophy, or with some therapeutic or surgical procedures, like mitotane treatment for hyperadrenocorticism or bilateral adrenalectomy.
The adrenal SGC cells of dogs vary from cells with a few granules and a high nucleo-cytoplasmic ratio to cells filled with many granules and a large mass of cytoplasm. This condition generally affects young and middle age dogs with a median age of 4 years [57].
Most of the chromaffin granules of these cells are small, ranging from 100 to 200 nm in diameter [39]. Great Danes, Portuguese Water Dogs, Rottweilers, Standard Poodles, West Highland White Terrier and Soft-coated Wheaten Terriers are the breeds in greater risk to develop hypoadrenocorticism [56].
The medullary cells produce other peptides in addition to epinephrine and norepinephrine, such as met-enkephalin, substance P, neurotensin, neuropeptide Y, and chromogranin A. The adrenal medulla contains also presynaptic sympathetic ganglion cells, randomly scattered [4, 32].
The first step in the synthesis of epinephrine is the enzymatic conversion of tyrosine to dihydroxyphenylalanine (DOPA) by tyrosine hydroxylase. The animal can be in a hypovolaemic shock, and generally is found in a state of collapse, or collapses when stressed. Weak pulse, bradycardia, abdominal pain, vomiting, diarrhoea, dehydration and hypothermia can also be present. Norepinephrine then leaves the granule to be converted into epinephrine in the cytosol by phenylethanolamine-N-methyltransferase (PNMT), and epinephrine re-enters the granule for storage in the cell. Animals with chronic hypoadrecocorticism may present anorexia, vomiting, lethargy, depression and weakness [57].8.
The activity of PNMT is induced by the high local concentration of glucocorticoids in sinusoidal blood from the adrenal cortex [10, 32]. ConclusionsMalfunction of endocrine glands leads to severe multivariate syndromes, requiring a specialized medical approach and appropriate nursing care. Acute stress, hypoglycemia or other similar situations, result both in catecholamine secretion and in transsynaptic induction of catecholamine biosynthetic enzymes, including tyrosine hydroxylase. For this reason, professionals engaged in clinical veterinary practice need to know animal organic structures and how they function as a whole, including the role of the endocrine system, its glands and their hormones. Other environmental influences, including growth factors, extracellular matrix, and a variety of hormonal signals that generate cyclic AMP, also may regulate the function of chromaffin cells [32].
This systematic approach to endocrine disorders promotes not only a trained professional but a professional with technical and scientific knowledge, enabled with the exact notion of the involved etiopathogenic mechanisms and capable of making a diagnostic and therapeutical difference. In table 6, we can see a summary of some of the hormones produced by adrenal glands and their functions.
This concerted attitude would certainly contribute to improve veterinary medical care to a level of excellence in the care of animal patients.NaN.
AcknowledgementsThis work was supported by Portuguese Foundation for Science and Technology and Center for Studies in Education, Technologies and Health.

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