. Explain Control of Secretion Rates of Hormones with Example


Best Answer

Hormones influence cellular functions by altering rates of many different biochemical processes. Many affect enzymatic activity and thus alter cellular metabolism, some change membrane per-meability, some regulate synthesis of cellular proteins, and some stimulate release of hormones from other endocrine glands. Because these are all dynamic processes that must adapt to changing metabolic demands, they must be regulated, not merely activated, by appropriate hormones. This regulation is achieved by precisely controlled release of a hormone into the blood. However, concentration of a hormone in body fluid depends on two factors: its rate of secretion and the rate at which it is inactivated and removed from the circulation. Conse¬quently, if secretion is to be correctly controlled, an endocrine gland requires information about the level of its own hor¬mones in the plasma.

Many hormones are controlled by negative feedback sys¬tems that operate between glands secreting hormones and target cells. A feedback pattern is one in which output is constantly compared with a set point, like a thermostat. For example, CRH (corticotropin-releasing hormone),-secreted by the hypothalamus, stimulates the pituitary (containing target cells) to release ACTH. ACTH stimulates the adrenal gland (con¬taining target cells) to secrete Cortisol. As the level of ACTH rises in the plasma, it acts on, or "feeds back" on, the hypothalamus to inhibit release of CRH. Similarly, as Cortisol levels rise.

Examples of feedback control on secretion of hormones

Regulation of thyroxine secretion from thyroid gland: Thermal changes (low body temperature) and increased food intake act as stimuli and stimulate the neurosecretory cells of hypothalamus to secrete a releaser hormone called Thyrotrophic releasing factor (TSH.RF) which, in turn, stimulates the thyrotropes of adenohypophysis to secrete a thyrotrophic hormone (TSH). TSH (molecular weight = 28,000) stimulates the acinal cells of thyroid follicles to secrete amine-hormone thyroxine in the blood by increased proteolysis of thyroglobulin, increased activity of iodide pump, increased synthesis of thyroxine, etc. When the thyroxine level of blood plasma reaches at required level, then thyroxine exerts a negative feedback (or feedback-inhibition) on the hypothalamus and anterior pituitary lobe to inhibit or decrease the secretion of TSH-RF and TSH respectively. When the rate of thyroxine secretion rises to about 1.75 times normal, the rate of TSH secretion falls essentially to zero. But when thyroxine level of blood plasma falls below the normal required level, it exerts positive feedback on the hypothalamus and anterior pituitary lobe which stimulate the thyroid gland to secrete more thyroxine and so on.

So it is evident that secretion of thyroxine from thyroid gland is controlled by the level of thyroxine hormone in blood through a feedback mechanism.

1. Feedback relationship between osmotic concentration and ADH secretion :

When the osmotic concentration of ECF increases due to deficiency of water in the body fluids, then osmoreceptors (modified neurons) of the hypothalamus are stimulated and appropriate signals are initiated which stimulate the hypothalamus to increase ADH secretion and increased release of ADH from the posterior pituitary. ADH is transported by the blood to the kidneys where it increases the permeability of the collecting tubules to water so most of water is reabsorbed from the nephric filtrate. It restores the normal osmotic concentration of blood. Reverse occurs when extracellular fluid becomes too dilute due to excess of water.

Therefore, a feedback control is available to control the osmotic concentration of the body fluids.

2. Other examples of feedback control

3. Feedback control of secretion of calcium. Secretion of Thyrocalcitonin from the parafollicular cells of thyroid gland is stimulated by increased plasma level of calcium (normal calcium level of blood is 8.5-10.5 mg/100 ml of blood) by the feedback mechanism. Similarly secretion of Parathormone (PTH) from the chief cells of parathyroids is stimulated by a decrease in calcium level in the blood from normal level.

4. Feedback control of secretion of Aldosterone. Secretion of Aldosterone (main mineralocorticoid) from the endocrine cells of zona glomerulosa of adrenal cortex is regulated by sodium level in the blood by feedback mechanism. Low level of sodium or high level of potassium increases the secretion of aldosterone.

 

 

 

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