Negative Feedback Regulation & β2-Adrenergic Signal Transduction

Thirty question-and-answer flashcards reviewing negative feedback control of thyroid hormones and the signal transduction steps of the β2-adrenergic receptor pathway in hepatocytes.

What type of feedback mechanism regulates the hypothalamus and pituitary by serum thyroid hormones?

Negative feedback.

Which hormone is released by the hypothalamus to stimulate the anterior pituitary in the thyroid axis?

Thyrotropin-releasing hormone (TRH).

TRH acts on which gland to stimulate thyroid-stimulating hormone release?

The anterior pituitary gland.

Which hormone does the anterior pituitary secrete to activate the thyroid gland?

Thyroid-stimulating hormone (TSH).

Which endocrine gland is the direct target of TSH?

The thyroid gland.

What two major hormones are secreted by the thyroid gland in response to TSH?

Thyroxine (T4) and triiodothyronine (T3).

What is the primary metabolic effect of T4 and T3 on body cells?

They stimulate overall metabolism.

How do elevated serum levels of T4/T3 affect TRH and TSH secretion?

They inhibit TRH and TSH release (negative feedback).

What happens to serum T4/T3 levels as tissues use these hormones?

Levels fall, reducing negative feedback on the hypothalamus and pituitary.

Give one physiological condition that can increase hypothalamic TRH release besides low T4/T3 levels.

Stress, exercise, malnutrition, changes in serum glucose, or sleep can stimulate TRH release (any one).

Which hormone initiates the hepatic signal cascade that releases glucose during stress?

Epinephrine (adrenaline).

To which specific receptor on hepatocytes does epinephrine bind?

The β2-adrenergic receptor.

The β2-adrenergic receptor belongs to which broad receptor family?

G-protein-coupled receptors (GPCRs).

How many subunits compose the heterotrimeric G-protein linked to GPCRs?

Three: α, β, and γ.

Upon receptor activation, which G-protein subunit activates adenylyl cyclase?

The activated α subunit.

Which enzyme converts ATP to cAMP in the β2-adrenergic pathway?

Adenylyl cyclase.

What second messenger is generated by adenylyl cyclase?

Cyclic AMP (cAMP).

Which kinase becomes active when cAMP binds to it?

Protein kinase A (PKA).

PKA phosphorylates and activates which enzyme that liberates glucose from glycogen?

Glycogen phosphorylase.

What reaction is catalyzed by glycogen phosphorylase?

Hydrolysis of glycogen into glucose monomers.

Through what mechanism does glucose leave the hepatocyte after glycogen breakdown?

Via specialized glucose transporter proteins in the cell membrane.

What is the net physiological effect of the β2-adrenergic pathway in the liver?

Elevation of blood glucose levels.

Which acute stress response relies on the rapid glucose release produced by this pathway?

The fight-or-flight response.

Name one additional bodily change that accompanies sympathetic activation besides glucose release.

Examples: increased heart rate, increased breathing rate, pupil dilation, or decreased digestion (any one).

In classroom explanations, the linear sequence of messengers in signal transduction is compared to what?

A biochemical relay race or a game of ‘telephone.’

In signal transduction terminology, cAMP is classified as what?

A second messenger.

What chemical modification does PKA apply to its target enzymes?

Addition of a phosphate group (phosphorylation).

What does the abbreviation GPCR stand for?

G-protein-coupled receptor.

During G-protein activation, what happens to the β and γ subunits?

They detach from the α subunit and diffuse away.

Which high-energy molecule is the direct substrate for adenylyl cyclase when forming cAMP?

ATP (adenosine triphosphate).