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ADH, Oxytocin and Growth Hormone

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Christopher Perumalla

Lecture 15 – ADH, Oxytocin and Growth Hormone ← The hypothalamus and the pituitary gland are located at the base of the brain. The two are connected by a tissue bridge called the infundibulum. The pituitary gland is divided into posterior and anterior lobes. The anterior pituitary is referred to as the adenohypophysis and the posterior pituitary as the neurohypophysis. ← ← ← The hypothalamus contains two clusters of endocrine cells that send their injections down into the posterior pituitary where the hormones they synthesized are released. These two clusters include the supraoptic nucleus and the paraventricular nucleus. Endocrine cells of the supraoptic nucleus synthesize and secrete the hormone oxytocin. Endocrine cells of the paraventricular nucleus synthesize and secrete ADH (anti diuretic hormone) or vasopressin. Because these endocrine cells are located in the brain, they are referred to as neurosecretory cells (endocrine cells). The projection of both endocrine cell clusters terminate in the posterior pituitary, adjacent to a rich blood supply, there is a capillary bed. When secreted by their endocrine cells, oxytocin and ADH can drain directly into the vein in the posterior pituitary. From there it enters the bloodstream, traveling towards their target cells. Therefore, the posterior pituitary is simply a release point for hormones. ← ← ← ← Functions of ADH ← ← An anti diuretic inhibits the formation of urine. Urine is water with added solutes. ADH regulates our total body water levels by its ability to stimulate the reabsorption of water from the urine in the kidneys. It regulates how much water is conserved in the blood versus the amount lost in the urine. ADH also conserves blood volume. Blood is mostly water, with proteins and a cellular component. ADH has the ability to stimulate water reabsorption which also conserves the blood volume. The conservation of blood volume also has the effect of conserving our blood pressure. ← ADH causes a direct increase in blood pressure by the stimulation of vasoconstriction at vasopressant sensitive receptors located on arteries. ← ← Signals ← ← ADH is secreted in response to decreased blood pressure and increased plasma osmolarity. A drop in blood pressure is immediately sensed by the barrel receptors. They signal an increase in activity of endocrine cells in the paraventricular nucleus which results in the secretion of ADH. ← Receptors in the paraventricular nucleus known as osmo receptors are sensitive to local changes in osmolarity and are monitoring the plasma continuously. If there is an increase in plasma osmolarity, the secretion of ADHD is triggered. ADHD lowers the plasma osmolarity to normal levels. ← ← ADH water reabsorption at the kidneys ← ← The target cell is called the principle cell. The principle cell is the epithelial cell which lines the kidney tubule. Each principle cell has an apical membrane in contact with the tubular fluid. This tubular fluid will be excreted as urine. On the opposite side of the principle cell, there is the basolateral membrane in contact with the peritubular fluid. The peritubular fluid is in close contact with the local blood supply. The peitubular fluid is a sample of what is in the bloodstream. In response to a signal such as elevated plasma osmolarity, ADH will be released. ADH then crosses the wall of the capillary and binds to its membrane bound G protein linked receptor which unleashes a chain of events. This eventually leads to the insertion in the opposite apical membrane of a hormone called aquaporin 2. Cylic AMP functions as a second messenger. ADH works to absorb water, regulating total body water levels. ← ← What happens when we experience dehydration? Dehydration is a deficiency in total body water. During dehydration, there is less water circulating in the body and therefore the solutes in our blood become more concentrated, there is an increase in plasma osmolarity. There is also a drop in blood volume since it is composed mostly of water. A drop in volume translates as a drop in pressure. These two act as signals for the release of ADH. ADH will conserve water by increasing the reabsorption of water in the kidneys, countering the increased plasma osmolarity. It also increases blood pressure by vasoconstriction. However, an increase in ADH will cause an increase in thirst, therefore signaling to consume water. The process lowers plasma osmolarity back to normal, increasing blood volume and pressure. ← ← At lower plasma osmolarity levels, there is little circulating ADH in the plasma but as plasma osmolarity begins to rise so do plasma levels of ADH and at a certain threshold, we begin to also experience thirst. The fluid intake which follows as well as the decrease in water excretion act to lower plasma osmolarity levels. ← ← ← ← The hypothalamus contains two sets of endocrine cells that are sent down to the posterior pituitary where the hormones they synthesize are released. These two clusters of endorine cells include those of the supraoptic nucleus. The endocrine cells of the supraoptic nucleus secrete oxytocin. Oxytocin is an extremely important hormone of the period immediately following birth. It is secreted in response
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