Mechanism of Hormone Action & Hypothalamic-Pituitary Axis

These flashcards cover receptor regulation, water- vs lipid-soluble hormone mechanisms, first and second messengers, steroid hormone actions, and key components and hormones of the hypothalamic-pituitary axis.

What is receptor up-regulation and how does it affect a target cell’s sensitivity to a hormone?

An increase in the number of hormone receptors on the target-cell membrane, making the cell more sensitive to the hormone.

What is receptor down-regulation?

A decrease in the number or responsiveness of hormone receptors, reducing the cell’s sensitivity to the hormone.

Where can hormone receptors be located in target cells?

Either in the plasma membrane (surface) or inside the cell in the cytosol/nucleus.

Why can’t water-soluble hormones diffuse through the plasma membrane?

They have high molecular weight and are hydrophilic, so they cannot cross the lipid bilayer without a receptor.

How do lipid-soluble hormones enter their target cells?

They are small, lipophilic molecules that cross the plasma membrane by simple diffusion and bind intracellular receptors.

In hormone signaling, what is the “first messenger”?

The hormone itself, which binds to a membrane receptor and starts signal transduction.

Define a “second messenger” in hormone action.

An intracellular molecule that relays the signal from the activated receptor to the cytoplasm/nucleus, producing the hormone’s effect.

List four common second-messenger molecules.

cAMP, cGMP, Ca²⁺/IP₃, and tyrosine kinase (Trk) pathways.

Which second messenger is most closely associated with activating protein kinase A?

Cyclic adenosine monophosphate (cAMP).

How quickly do water-soluble hormones usually act compared with lipid-soluble hormones?

Water-soluble hormones often produce rapid effects (seconds to minutes); classic genomic actions of lipid-soluble hormones take hours to days.

Where are receptors for lipid-soluble hormones located and what do they modulate?

In the cytosol or nucleus; they directly modulate gene expression.

Describe the classic genomic mechanism of steroid hormones.

Steroid detaches from carrier → diffuses into cell & nucleus → binds receptor → hormone-receptor complex binds DNA → transcription → mRNA → new protein synthesis → cellular effect.

How can steroid hormones produce rapid (non-genomic) effects?

By binding to specific receptors on the plasma membrane and triggering second-messenger cascades.

From which precursor are most steroid hormones synthesized?

Cholesterol (thyroid hormones are the main exception).

What is the hypothalamic-pituitary axis (HPA)?

The central neuroendocrine system integrating hypothalamic and pituitary function to regulate thyroid, adrenal, reproductive, and other activities.

How is the hypothalamus connected to the anterior pituitary?

Through the hypophyseal portal blood vessels (hypothalamic-hypophyseal portal system).

How is the hypothalamus connected to the posterior pituitary?

By the hypothalamo-hypophyseal nerve tract.

Which hypothalamic hormone stimulates release of thyroid-stimulating hormone (TSH)?

Thyrotropin-releasing hormone (TRH).

Which hypothalamic hormone stimulates secretion of FSH and LH?

Gonadotropin-releasing hormone (GnRH).

What is the action of somatostatin on the anterior pituitary?

It inhibits release of growth hormone (GH) and TSH.

What does growth hormone–releasing hormone (GHRH) do?

Stimulates release of growth hormone (GH).

What is the primary function of corticotropin-releasing hormone (CRH)?

Stimulates release of adrenocorticotropic hormone (ACTH) and β-endorphin.

State two principal effects of Substance P in hypothalamic regulation.

Inhibits synthesis and release of ACTH; stimulates secretion of GH, FSH, LH, and prolactin.

Which hypothalamic factor inhibits prolactin secretion?

Prolactin-inhibiting factor (PIF), also known as dopamine.

What is the main action of antidiuretic hormone (ADH)?

Increases water reabsorption in renal collecting ducts, lowering plasma osmolarity.

What is the primary role of oxytocin in the body?

Stimulates uterine contractions during labor and triggers milk ejection in lactating women.