Hormones of the posteior pituitary gland

Vasopressin (Antidiuretic Hormone, ADH)

A hormone that increases water reabsorption in kidney collecting ducts to regulate body water balance.

water reabsorption

The process by which the kidneys take water back from the filtrate (urine) into the blood, helping the body conserve water and maintain proper fluid balance. It mostly happens in the nephron, especially in the collecting ducts under the influence of antidiuretic hormone (ADH).

Where is ADH produced?

In magnocellular neurons of the supraoptic (80%) and paraventricular (20%) nuclei of the anterior hypothalamus.

How does ADH reach the posterior pituitary?

It is transported down axons with carrier proteins to nerve endings in the posterior pituitary.

How is ADH released into the bloodstream?

By exocytosis of secretory granules in response to action potentials in the posterior pituitary nerve endings.

What triggers ADH secretion?

1. High osmolarity (salt concentration in the blood)

2. Low blood pressure (low blood volume)

How does high osmolarity trigger ADH release?

Water leaves osmoreceptor cells in the hypothalamus, causing them to shrink and fire signals to the supraoptic nuclei, which leads to ADH release from the posterior pituitary.

• osmoreceptors are very sensitive to osmolarity changes

What happens when blood volume drops by 10% or more?

ADH secretion increases significantly to conserve water and act as a vasoconstrictor to raise blood pressure.

How is ADH secretion eventually turned off?

Receptors in the heart detect high blood volume and send inhibitory signals to stop ADH release.

How does ADH affect water reabsorption in the kidneys?

It increases water reabsorption in the collecting ducts by inserting aquaporin channels into the cell membrane.

• so when ADH is high, many aquaporins are insertedinto the membrane, large amount of water is rebasored and cocnetarted urine is produced.

What happens in the kidney collecting ducts when ADH levels are low?

Few aquaporins are present; little water is reabsorbed, resulting in dilute urine with low salt concentration.

Aquaporins

Water channel proteins inserted into the cell membrane of collecting duct cells in response to ADH, allowing water reabsorption.

What non-osmotic factors increase ADH secretion?

Pain, nausea, surgical stress, strong emotions, morphine, nicotine, and large doses of barbiturates.

What common substance inhibits ADH release?

Alcohol

What happens at high concentrations of ADH?

ADH causes vasoconstriction of arterioles, which raises blood pressure.

How does ADH regulate the presence of aquaporins in the kidney tubule cell membranes?

ADH (antidiuretic hormone) binds to receptors on kidney tubule cells, triggering a signaling cascade that causes vesicles containing aquaporin-2 (AQP2) water channels to move to and fuse with the apical (urine-facing) plasma membrane. This increases water permeability, allowing more water to be reabsorbed from the urine. In the absence of ADH, the aquaporins are removed from the membrane by endocytosis and stored in intracellular vesicles, reducing water reabsorption.

Diabetes Insipidus

A condition caused by a deficiency of antidiuretic hormone (ADH), leading to excessive urine production and increased thirst due to the kidneys' inability to reabsorb water. Unlike diabetes mellitus, blood glucose levels remain normal and no glucose is found in the urine.

Diabetes Mellitus

A metabolic disorder caused by insufficient insulin production (Type 1) or reduced sensitivity to insulin (Type 2), leading to high blood glucose levels. Common symptoms include increased thirst, frequent urination, fatigue, and glucose present in the urine.