BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 15. INTEGRATION OF METABOLIC PATHWAYS. HORMONES

15.7. Vasopressin and Oxytocin

Vasopressin, also known as antidiuretic hormone (ADH), is synthesized in the Hypothalamus as a precursor, prepro-ADH, which enters the Golgi apparatus and is converted into pro-ADH. Packed into neurosecretory granules, pro-ADH is transported along the axons of hypothalamic Neurons to the neurohypophysis. Within the granules, pro-ADH is cleaved into mature ADH (a nonapeptide) and the protein neurophysin.

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Fig. 15.9. Structure of oxytocin and vasopressin

The antidiuretic effect of ADH is mediated by V₂-type receptors functionally coupled to the adenylate cyclase system, whereas its vasoconstrictor effect is mediated by V₁-type receptors coupled to the phosphoinositide system. The most critical function of the hormone is its antidiuretic action. V₂-type receptors are located on the Cells of the distal tubules and collecting ducts of the Kidneys. In the absence of ADH, the number of Water channels is minimal, making the epithelium virtually impermeable to water, which results in The excretion of hypotonic urine. Activation of adenylate cyclase stimulates the insertion of water channels (the protein aquaporin), thereby facilitating the passive reabsorption of water into the hypertonic renal medulla. Under physiological conditions, the primary factor regulating ADH secretion and thirst is plasma osmolality. Hypothalamic osmoreceptors are extremely sensitive to fluctuations in osmolality. A decrease in extracellular fluid volume coupled with an increase in serum osmolality triggers thirst and ADH secretion. The latter enhances water reabsorption and serum volume, which subsequently lowers serum osmolality and suppresses ADH secretion.

Under normal conditions, The Effect of ADH on peripheral arterioles and Blood pressure is negligible.

Absolute or relative deficiency of ADH leads to diabetes insipidus, a condition characterized by The production of large volumes of low-osmolality urine. This disorder may be caused by hypothalamic pathology, pituitary damage or tumors, or autoimmune processes.

Oxytocin is synthesized as a prohormone and increases the strength and frequency of myometrial contractions. Its action is enhanced by estrogens, particularly during The final stage of Pregnancy, which increases the density of receptors and the sensitivity of the Uterus to the hormone.