Arginine Vasopressin Is Critical in the Control of Renal Water Output and Plasma Osmolality

Arginine vasopressin (AVP), also known as antidiuretic hormone (ADH), is a nonapeptide synthesized in the body of nerve cells located in the supraoptic and paraventricular nuclei of the anterior hypothalamus (Fig. 24.3) (see Chapter 32). The hormone travels by axoplasmic flow down the hypothalamic-neurohypophyseal tract and is stored in vesicles in nerve terminals in the median eminence and, mostly, the posterior pituitary. When the cells are brought to threshold, they rapidly fire action potentials, Ca2+ enters the nerve terminals, the AVP-containing vesicles release their contents into the interstitial fluid surrounding the nerve terminals, and AVP diffuses into nearby capillaries. The hormone is carried by the blood stream to its target tissue, the collecting ducts of the kidneys, where it increases water reabsorption (see Chapter 23).

Factors Affecting AVP Release. Many factors influence the release of AVP, including pain, trauma, emotional stress, nausea, fainting, most anesthetics, nicotine, morphine, and angiotensin II. These conditions or agents produce a decline in urine output and more concentrated urine. Ethanol and atrial natriuretic peptide inhibit AVP release, leading to the excretion of a large volume of dilute urine.

The main factor controlling AVP release under ordinary circumstances is a change in plasma osmolality. Figure 24.4 shows how plasma AVP concentrations vary as a function of plasma osmolality. When plasma osmolality rises, neurons called osmoreceptor cells, located in the anterior hypothalamus, shrink. This stimulates the nearby neurons in

Osmoreceptors The Hypothalamus
to a lesser extent in the paraventricular nuclei in the anterior hypothalamus. It is then transported down the hypothalamic neurohy-pophyseal tract and stored in vesicles in the median eminence and posterior pituitary, where it can be released into the blood.

Daily Water Balance in an Average

70-kg Man

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  • Karoliina
    What stimulates the hypothalamus?
    8 years ago
  • diana
    Where is water output controlled?
    8 years ago

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