L2-Posterior Pituitary Hormones

Posterior pituitary

neurohypophysis

posterior pituitary hormones

vasopressin (ADH) and oxytocin

oxytocin vs vasopressin

sharing nine amino acids (neuropeptides/nonapeptides) with differences only at the 3rd and 8th positions.

Vasopressin (ADH)

formed by supraoptic nuclei in the hypothalamus and are transported by neurophysin (carrier proteins) to be stored and secreted by the posterior pituitary gland.

vasopressin function

Regulates water retention, blood pressure, and urine concentration in the kidneys (blood osmolality decrease)

Vasopressin receptors

1. V1a Receptor (V1aR): Vascular smooth muscle cell, myocardium (heart); vasoconstriction.

2. V1b Receptor (V1bR / V3): anterior pituitary gland; secretion of adrenocorticotropic hormone (ACTH).

3. V2 Receptor (V2R): renal collecting ducts; promoting water reabsorption/retention, increases preload

Preload

The initial stretching of the cardiac muscle cells in the ventricles right before they contract (diastole)

Vasopressin and Vasoconstriction

causes vasoconstriction by binding to V1a receptors.

This causes the calcium levels to increase and causes vascular smooth muscle to contract, narrowing the blood vessels

Vasopressin and Renal Water Reabsorption

binding to V2 receptors on renal collecting ducts.

1. The binding activates adenylyl cyclase, raising the cAMP, activating the PKA (protein A kinase) **second messengers**

2. PKA phosphorylates aquaporin-2 (AQP2) signaling them to move toward and fuse to the membrane.

3. Water exits the cell across the membrane to the bloodstream by AQP3 and AQP4 channels.

This causes urine volume to decrease and become concentrated, while blood volume increases and osmolality normalizes.

Aquaporin Channels

Passageways on both sides of the cell which allow water to move easily

protein A kinase

The enzyme that transfers phosphate groups from ATP to protein.

diabetes insipidus

The ADH hormone is not secreted adequately, or the kidney is resistant to its effect.

So the urine concentration is decreased (diluted urine)

Vasopressin and Stress Response

Acting on V1b receptors located in the anterior pituitary gland, vasopressin stimulates the release of adrenocorticotropic hormone (ACTH).

ACTH travels through the bloodstream to stimulate the release of cortisol and maintain the blood pressure

Vasopressin and Platelet Aggregation

acts via V1 receptors on Vasular endothelial cells via a cAMP-PKA pathway.

to trigger the releases of vWF (von Willebrand factor).

vWF binds to exposed collagen, facilitating primary platelet adhesion and plug formation.

von Willebrand factor

plasma protein secreted by endothelial cells; helps with platelet aggregation

Regulation of Vasopressin

Activates:

-Hyperosmolality (rise in plasma osmolality)

-Hypovolemia (Low Blood Volume)

-Hypotension (Low Blood Pressure)

-Stressors

suppress:

-Hypo-osmolality (drop in plasma osmolality)

-Hypervolemia (High Blood Pressure)

-

Oxytocin (OT)

formed by paraventricular nuclei in the hypothalamus and are transported by neurophysin (carrier proteins) to be stored and secreted by the posterior pituitary gland.

Oxytocin function

uterine contractions during childbirth and milk ejection during breastfeeding

Oxytocin receptors

oxytocin receptor (OXTR) and cross-binds to V1aR.

Estradiol and OXTR

Estradiol levels rise as the pregnancy progresses; this causes the OXTR to be expressed in the uterus, making it more resposice to the Hormone.

Estradiol

estrogen secreted by the ovaries

Oxytocin during labor.

As the baby's head presses against the cervix, nerve signals trigger oxytocin release. This causes the muscles of the uterus to contract.

Cervical dilation during labor activates stretch receptors and stimulates a positive feedforward so more OT is released.

After birth, oxytocin helps uterine contracts the uterus to limit heavy bleeding

Oxytocin and milk ejection

An infant's suckling stimulates touch and pressure receptors in the nipple and areola.

Nerve impulses travel from the nipple via the spinal cord and brainstem up to the hypothalamus (supraoptic and paraventricular nuclei).

Oxytocin travels through the blood to the breast, where it binds to receptors on the myoepithelial cells (contains alveoli where milk is stored)

The myoepithelial cells contract, squeezing the alveoli and forcing the milk down into the milk ducts and out toward the nipple

areola

dark-pigmented area surrounding the breast nipple

oxytocin and Central Nervous System

regulate social behavior, emotional processing, and stress responses.

Hypotalamus: reduces stress by CRH-ACTH-cortisol axis. This triggers the maternal feeling.

Amygdala: suppresses the effects. (stress and aggression)

Basal Ganglia: stimulates it and causes the feeling of love and motivation rewards.