Adrenal 101

The adrenal glands also known as the suprarenal glands. Supra meaning above, and renal meaning kidneys. So these glands are situated on top of the kidneys. These are endocrine glands that produce a variety of hormones, but most notable adrenaline, and the steroids aldosterone and cortisol. Each gland has an outer cortex which is divided into three different zones and an inner medulla. The three zones of the cortex are the zone glomerulosa, zone fasciculate, and zone reticularis.

This article will go briefly touch on the structure of the adrenal gland, including each zone of the cortex. Then it will dive into the function of the adrenal gland and the hormones it produces along with their specific cellular target. Finally the article will conclude with an overview of adrenal insufficiency and cortisol overproduction and diseases that illustrate those two conditions.

Structure

adrenal gland sections

As mentioned earlier, the gland is composed of an outer cortex, and an inner medulla. The outer cortex can be further divided into three zones that each have a specific function.

Zona Fasciculata

The zona fasciculata sits between the other two zones (zona glomerulosa, and zona reticularis) and consists of cells responsible for producing glucocorticoids such as cortisol. Its the largest of the three zones consisting of about 80% of the space in the cortex.

Zona Glomerulosa

The zona glomerulosa is the outermost zone of the adrenal cortex. The cells that are situated in this zone are responsible for the production of mineralocorticoids such as aldosterone. Aldosterone is an important regulator of blood pressure. Review the article covering the Renin-Aldosterone system.

Zona Reticularis

The zona reticular is the innermost cortical layer which is primarily responsible for producing androgens. Its main component synthesized is dehydroepiandrosterone (DHEA), and androstenedione, which is the precursor to testosterone.

Medulla

The medulla is in the centre of each adrenal gland with the cortex around the entire periphery. The chromatin cells within the medulla are the bodies main source of catecholamines. Catecholamines produced in the medulla are adrenaline (epinephrine), and noradrenaline (norepinephrine). Regulation of the synthesis of these catecholamines is driven by the sympathetic nervous system via the preganglionic nerve fibers stemming from the thoracic spinal cord (T5-T11) to the adrenal glands. When the medulla gets stimulated to produce these hormones it secretes them directly into the cardiovascular circulation system, which is unusual of sympathetic innervation as they usually have distinct synapses on specialized cells.

Mineralocorticoids

Mineralocorticoids such as aldosterone are named according to its function. They regulate minerals, such as salt and regulate blood volume (blood pressure). Aldosterone, the most prominent mineralocorticoid acts on the distal convoluted tubules and the collecting ducts by increasing the reabsorption of sodium and the excretion of both potassium and hydrogen ions. The amount of salt present in the body affects the extracellular volume, which influences the blood pressure.

Glucocorticoids

Glucocorticoids are also named due to its function. Cortisol is a prominent glucocorticoid that regulates the metabolism of proteins, fats and sugars (glucose). Cortisol increases the circulating level of glucose. They cause protein catabolism into amino acids and the synthesis of glucose from the amino acids in the liver. They also increase the concentration of fatty acids by increasing lipolysis (fat breakdown) which cells can use as an alternative energy source in situations of glucose absence. Glucocorticoids also play a role in suppression of the immune system. They induce a potent anti-inflammatory effect.

Cortisol

Cortisol is the prominent glucocorticoid produced by the adrenal gland. The adrenal gland secretes a basal level of cortisol depending on the time of day it is. Cortisol concentrations in the blood are highest in the early morning and lowest in the evening as part of the circadian rhythm of adrenalcorticotropic hormone (ACTH) secretion. The article on general endocrinology explains what ACTH is and how it affects the adrenal gland. Basically what happens is the hypothalamus secretes corticotropin releasing hormone that acts on the pituitary to produce ACTH that acts on the adrenal gland cortex to produce cortisol.

Androgens and Catecholamines

The primary androgen produced by the adrenal gland is DHEA, which is converted to more potent androgens such as testosterone, DHT, and estrogen in the gonads. DHEA acts as a precursor. Androgens drive sexual maturation.

Catecholamines are produced by the chromaffin cells from tyrosine. The enzyme tyrosine hydroxyls converts tyrosine to L-DOPA. L-DOPA is then converted to dopamine before it can be turned into norepinephrine. Norepinephrine is then converted to epinephrine by the enzyme phenylethanolamine N-methyltransferase (PNMT). Epinephrine and norepinephrine act as adrenoreceptors throughout the body, whose primary effect is to increase the blood pressure and cardiac output by way of vasoconstriction. Catecholamines play a huge role in the fight-or-flight response.

Corticosteroid Overproduction

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The normal function of the adrenal gland can be impaired from infections, tumors, autoimmune diseases, or from previous medical therapy such as radiation and chemotherapy. Cushing’s syndrome is the manifestation of glucocorticoid excess. Symptoms and sign are a direct result of chronic exposure to glucocorticoids. Diagnosis is difficult because the symptoms are often nonspecific and pathognomonic of the syndrome in isolation. Symptoms include proximal (distant) muscle weakness, wasting of the extremities, increased fat in the abdomen and face often leading to a moon face, bruising without trauma, and a buffalo hump. A buffalo hump is fat on the back of the neck and supraclavicular pads. In women, menstrual irregularities are common such as oligomenorrhea (infrequent menstrual periods), amenorrhea (absence of menstrual periods), and variable menses. Hyperpigmentation can occur by increased secretion of cortisol. Cortisol acts on the melanocyte-stimulating hormone receptors.

Glucose intolerance is common in Cushing’s syndrome. Primarily due to stimulation of gluconeogenesis by cortisol and insulin resistance caused by the obesity. This leads to hyperglycemia, which can exacerbate any diabetic patient.

Bone loss and osteoporosis is common in patients with Cushing’s syndrome because there is less intestinal calcium absorption. Calcium is vital to bone health and growth. The decrease in bone formation is coupled with an increased rate of bone reabsorption which can lead to more pathological fractures.

Adrenal Insufficiency

Addison’s disease is considered primary hypoadrenalism. There is an inherent deficiency of glucocorticoids and mineralocorticoids. Most commonly caused by an autoimmune condition. Autoimmune means that the body is attacking itself by production of antibodies against cells of the adrenal cortex. In cases of adrenal crisis due to autoimmune primary adrenal insufficiency clinical presentation is usually the patient presenting in a state of shock. Abdominal tenderness upon deep palpation is common. Patients present with hyperpigmentation due to chronic ACTH release by the pituitary. Proopiomelanocortin is overproduced which is a pro hormone that is cleaved into its biologically active hormones corticotropin and melanocyte-stimulating hormone (MSH). This causes increased melanin synthesis, causing the hyperpigmentation. Other non-specific symptoms such as lethargy, fatigue, weakness, confusion, anorexia, nausea, vomiting, or even coma can occur. One of the most commonly presented symptoms is fever and infection, which can be exaggerated by the hypocortisolemia.

Its important to take this article slowly. There a lot of different parts, but the aim was to look at the hormones themselves and how they physiologically act on the body, then take what was learned about those and apply them to two scenarios, hypo/hyperadrenalism and how it affects the body. Cushing’s syndrome is where there is hyperproduction of cortisol primarily leading to many disastrous effects on the body. Addisons disease is an autoimmune disease where the body produces antibodies against the cells of the adrenal cortex, causing destruction of the gland itself, again leading to detrimental effects on the body.

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Bilirubin Metabolism

Bilirubin is a metabolite of heme. It serves as a means to excrete unwanted heme, which is derived from various heme-containing proteins such as hemoglobin, myoglobin, and various P450 enzymes. Bilirubin is also notable for providing the color to bile, stool, and to a lesser extent the urine. Its produced by a two-stage reaction that occurs in cells of the RES (reticuloendothelial system). The RES includes the phagocytes, mainly being the macrophages, the Kupffer cells in the liver and the cells in the spleen and bone. Heme is taken up into these cells and acted on by the enzyme heme oxygenase, liberating the chelated iron from the heme structure and releasing carbon monoxide. The carbon monoxide is excreted via the lungs. The reaction yields a green pigment known as biliverdin. Biliverdin is then acted on by the enzyme biliverdin reductase which produces bilirubin. Bilirubin consists of a yellow pigment. Bilirubin is derived from two main sources. The majority, about 80% comes from heme which is released from senescent red blood cells. The other 20% originates from other heme-containing proteins found in the liver and muscles.

Synthesis

Bilirubin is toxic to tissues, therefore it is transported in the blood in its unconjugated form bound to albumin. For that reason, only a small amount of the free form is present in the blood. If the free fraction increases, bilirubin with invade and cause damage to the tissues. Excess unconjugated bilirubin can cross the blood-brain barrier and cause kernicterus in neonates. The unconjugated bilirubin is taken up by hepatocytes where the albumin bond is broken. Inside the hepatocyte, the bilirubin is bound to cytoplasmic proteins ligandins and Z proteins. The primary function of these proteins is too prevent the reflux of bilirubin back into the circulatory system. Unconjugated bilirubin is lipophilic. Its conjugation with glucuronic acid renders it hydrophilic, therefore it can be eliminated utilizing bile. Conjugated bilirubin synthesis occurs in a two step reaction. First glucuronic acid is synthesized from cytosolic glucose which then attaches to uridinediphosphate (UDP) via the enzyme UDP-glucose-dehydrogenase. This forms UDP-glucuronic acid. This compound has an affinity for bilirubin for which then the glucuronic acid is transferred to the bilirubin which is catalyzed by glucuronyl transferase. Conjugation of bilirubin takes place in the endoplasmic reticulum of the hepatocytes and the end result is an ester between the glurcuronic acid and one or both of the propionic side-chains of bilirubin.

Pathways in bilirubin metabolism

Metabolism

Once bilirubin is conjugated it is excreted with bile acid into the small intestine. The bile acid is reabsorbed in the terminal ileum for enterohepatic circulation, the conjugated bilirubin is not absorbed and instead passes into the colon. In the colon, the bacteria metabolize the bilirubin into urobilinogen, which can be oxidized to form urobilin, and stercobilin. Urobilin is excreted by the kidneys to give urine its yellow color and stercobilin is excreted in the feces giving stool its characteristic brown color. There can be traces levels of urobilinogen present in the blood.

Toxicity

Unconjugated hyperbilirubinemia in a neonate can lead to an accumulation of unconjugated bilirubin in the brain tissue. The neurological disorder is called kernicterus. The blood-brain barrier is not yet fully developed and bilirubin can freely pass into the brain interstitium. In cases of liver impairment, biliary drainage is blocked, and some of the conjugated bilirubin leaks into the urine, turning it a dark amber color. In cases of hemolytic anemia, there is increased hemolysis of red cells causing an increase in unconjugated bilirubin in the blood. In these cases, there is no problem with the livers mechanism to conjugate the bilirubin, and there will be an increase in urobilinogen in the urine. This is the difference between an increased urine bilirubin, and an increased urine urobilinogen.

Ketosis and Ketoacidosis

Ketosis is a metabolic state in which the bodies energy supply comes from ketone bodies in the blood in contrast to a state of glycolysis in which there is adequate glucose breakdown. In most cases, ketosis results from a high metabolism of fatty acids which are converted to ketone bodies. Ketone bodies are formed from ketogenesis when liver glycogen stores are depleted. Most cells in the body utilize both ketone bodies and glucose for energy, and while in ketosis the body works to maintain normal metobolism so it ramps up gluconeogenesis. Gluconeogenesis is glucose synthesis used to go through glycolysis.

Most of the time ketosis is a short interval of time, although long-term ketosis may be a result of fasting or a dietary insufficiency of carbohydrates. In glycolysis, high levels of insulin are released which promotes storage of fat and delayed release of fat from adipose tissue. In ketosis, fat reserves are readily available and are consumed. For this reason, ketosis has become one of the more recent diet fads as a way to burn fat quickly and lose weight.

Ketoacidosis

Although similar, ketosis is not ketoacidosis. Ketoacidosis is a physiological life-threatening situation due to insulin deficiency. Ketone bodies are acidic, and acid-base homeostasis in the blood is normally maintained through bicarbonate buffering, respiratory compensation, and renal compensation. Prolonged excess of ketone bodies can overwhelm the normal compensatory mechanisms and cause a state of acidosis when the blood pH falls below 7.35.

There are multiple precipitating factors that leads to ketoacidosis, which is most prevalent in patients with type 1 diabetes. Ketoacidosis in the case of a patient with type 1 diabetes is deemed diabetic ketoacidosis (DKA). In established type 1 diabetes, patients often forget to take insulin, with non-compliance being the bigger issue. This does not rule out other causes of ketoacidosis, as those are still prevalent. Acute major illnesses such as a myocardial infarction, cerebrovascular accident, sepsis, or pancreatitis. Certain drugs that affect carbohydrate metabolism such as glucocorticoids, diuretics, or anti-psychotic agents can cause ketoacidosis. General malnutrition associated with physiological problems can also lead to ketoacidosis. Such disorders lead to psychological starvation, which leads to ketone production and if prolonged ketoacidosis.

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Clinical Presentation

The clinical presentation of DKA is a two headed monster. The earliest symptoms of marked hyperglycemia is polyuria, polydipsia, and unexplained weight loss. As the duration of hyperglycemia continues neurological symptoms, including lethargy, focal signs and obtundation develop. Further progression can lead to a coma. The other head is the extent of the metabolic acidosis due to the excess ketone bodies. As the acidemia worsens accompanied with it is abdominal pain which can sometimes be severe. The electrolyte imbalance and metabolic acidosis causes delayed gastric emptying and an ileus (obstruction of the bowel). Vomiting and nausea are common.

Diagnostic Evaluation

The initial laboratory evaluation of patients with suspected DKA should include a serum glucose to establish whether or not the patient is hyperglycemic or not. Its helpful to measure the serum electrolytes and calculate the anion gap, BUN, plasma creatinine, and a plasma osmolality. This gives a broad picture of the metabolic state of the patient. Urinalysis is commonly performed along with urine ketones measured by dipstick method. Serum ketones are also measured to assess whether or not the patient is undergoing ketogenesis. An arterial or venous blood gas can be helpful to determine whether the serum bicarbonate is substantially reduced, which presumptively leads to metabolic acidosis. This also aids in determining hypoxia if it is present.

Findings

Hyperglycemia and hyperosmolality are the two primary laboratory findings in patients with DKA. Patients with DKA have a high anion gap metabolic acidosis. Serum glucose often times exceeds 350-500 mg/dL. Three ketone bodies are produced and accumulate in DKA; acetoacetic acid, beta-hydroxybutyric, and acetone. Acetoacetic acid is the only true ketoacid. A serum ketone measurement gives levels of beta-hydroxybutyric, while a urine dipstick measures the presence of acetoacetic acid using the nitroprusside method.

Other findings that may or may not be present are leukocytosis, and lipidemia. The majority of patients with hyperglycemic emergencies present with leukocytosis, which is proportional to the degree of ketonemia. Patients with DKA also present with marked hyperlipidemia. Lipolysis, primarily caused from insulin deficiency, and to a lesser extent elevated levels of lipolytic hormones including catecholamines, GH, ACTH, and glucagon. Lipolysis releases glycerol and free fatty acids into circulation which causes insulin resistance and serves as the substrate for ketoacid generation in the hepatocyte mitochondria.

To recap, ketosis is a dietary manipulation that if done right can lead to results. Ketoacidosis is a life-threatening metabolic state that requires immediate medical care.

This discussion will be continued with the next article focusing on ketoacid generation.

The Pseudohyponatremia Debacle

Measurement of electrolytes in a sample of plasma is probably one of the most common tests that is performed. Every patient who gets blood drawn will also get a Chem 7 or CP13 which will include an analysis of electrolytes. There are two different technological approaches to measuring electrolytes, depending on what kind of chemistry analyzer is employed. Some use an indirect ISE and some are centered on direct ISE.

When a whole blood sample is used it is typically centrifuged beforehand to separate the sample into its plasma and red cell layers. Measurement of the electrolytes will involve using the plasma portion of the sample. Plasma consists of 93% water and approximately 7% of solid constituents, mainly proteins and lipids. The electrolytes being measured are somewhere in the water portion the plasma.

The two different methods; indirect and direct ISE differ in that direct ISE is able to respond to the electrolyte concentration within the plasma water while the indirect measures the electrolyte concentration by volume of total plasma. Total plasma including the 93% water content and 7% protein and lipid portion. That is an important distinction because the distribution of the water and protein content of plasma is important. The ratio of water/protein content is not always going to be 93/7. Depending on the patient that ratio can be skewed and inaccurate results can be reported.

So lets recap, the direct ISE measures the electrolytes using the water content of plasma, while the indirect ISE measures the electrolytes using the ratio of water/protein in whole plasma.

Indirect ISE

Indirect ISE measures electrolytes using a total plasma sample that has been diluted with diluent. This requires that the whole blood sample has been centrifuged and separated. The method measures the mean concentration in the entire plasma; water and protein content of plasma. The concentration is then multiplied by the dilution factor.

Pseudohyponatremia

Variation of the content of proteins and lipids from a normal with cause an error in the reported electrolyte results when using an indirect ISE. In particular, when measuring sodium (Na+). Dilution, as needed in the indirect ISE involves taking a pre-determined volume of the “total” sample, not only the water content of the plasma, and adding to a diluent. The total number of measurable ions in the sample is expressed as the average concentration in the total volume of the original sample, because of this the reported values depend on the protein and lipid content of the samples. A patient with hyperlipidemia will skew the 93/7 ratio which will cause falsely decreased levels of electrolytes, most often sodium. If the low sodium level is due to lipids, its possible to centrifuge to clarify the sample to get a more accurate result. Sometime the best thing to do is to rerun the sample using direct ISE. If its possible to get a whole blood sample then use point-of-care analyzers, like an i-STAT, or some blood gas analyzers (ABG) which can give a more accurate result.

Direct ISE

Direct ISE measures electrolytes using non-diluted whole blood or a plasma sample. The actual measurement that is gained is based on the water content of the plasma. It measures the electrolyte activity in the plasma water. The electrochemical activity of the ions in the water is converted to the readout concentration by a fixed multiplier that is ion-specific. The use of the fixed factor reflects the actual, clinically significant result, irrespective of the level of proteins or lipids within the solid phase of plasma.