Endocrinology Teaching: Glucose Regulation & Hormone Receptor Mechanisms

This page covers three tightly linked FCPS-exam themes: how the body maintains blood glucose between meals, which tissues need insulin to take up glucose (and which don't), and the second-messenger systems hormones use. Let's go through each concept properly rather than just memorizing the answers.

1. Maintaining glucose between meals

Key fact: Glucose between meals is maintained by hepatic glycogenolysis.

The liver stores glucose as glycogen after a meal (under insulin's influence) and breaks it back down (glycogenolysis) between meals to keep blood glucose stable during the fasting state.
Glycogenolysis is different from gluconeogenesis (making new glucose from lactate, amino acids, glycerol) - that becomes dominant only once liver glycogen stores run low (after ~12-24 hours of fasting).

Epinephrine and glucagon both raise blood glucose, but through a shared final step:

Hormone	Main receptor	Shared downstream effect
Glucagon	Gs-coupled receptor -> cAMP	Activates phosphorylase kinase -> activates glycogen phosphorylase -> glycogenolysis
Epinephrine	Beta-2 receptor -> cAMP (in liver)	Same - activates phosphorylase kinase

This is why the exam pearl says "the effect common to both glucagon and epinephrine is activation of phosphorylase kinase" - both converge on the same cAMP-PKA cascade that ultimately unlocks glycogen breakdown - Guyton and Hall Textbook of Medical Physiology.

2. Glucocorticoids and glucose metabolism

Cortisol is catabolic and diabetogenic. Its net effect is to raise blood glucose, but it does this in an interesting way - by making peripheral tissues insulin-resistant while pushing the liver to make more glucose:

Decreases peripheral glucose uptake/utilization - cortisol reduces the sensitivity of skeletal muscle and adipose tissue to insulin's stimulatory effect on glucose uptake - Guyton and Hall Textbook of Medical Physiology.
Increases hepatic gluconeogenesis and glycogen deposition - Schwartz's Principles of Surgery.
Net result: hyperglycemia, mild insulin resistance - this is exactly why chronic steroid use can precipitate "steroid-induced diabetes."
During prolonged exercise (compared to rest), skeletal muscle increasingly meets its calorie needs through release of free fatty acids from adipose tissue (lipolysis), sparing glucose for the brain and RBCs, which absolutely require it.

3. Insulin-dependent vs insulin-independent glucose uptake - the BRICK LIPS mnemonic

This is a classic, high-yield exam concept.

Insulin-DEPENDENT uptake happens via GLUT4 transporters, found almost exclusively in:

Skeletal muscle
Cardiac muscle
Adipose tissue

Insulin causes GLUT4 to translocate from intracellular vesicles to the plasma membrane, and also slows its removal from the membrane, increasing glucose entry up to 10-20 fold - Guyton and Hall Textbook of Medical Physiology; Medical Physiology (Boron & Boulpaep).

Insulin-INDEPENDENT uptake occurs via GLUT1, GLUT2, GLUT3 transporters (concentration-gradient driven, not insulin-triggered) in tissues that must never be starved of glucose regardless of insulin status. Mnemonic BRICK LIPS:

Letter	Tissue
B	Brain
R	RBCs
I	Intestine
C	Cornea
K	Kidney
L	Liver
I	Islet (beta) cells
P	Placenta
S	Spermatocytes

Important nuance the page tests: among insulin-independent tissues, exercising skeletal muscle actually takes up MORE glucose than the brain per the comparison given (Excursing skeletal muscle > Brain). This seems contradictory since skeletal muscle is normally GLUT4/insulin-dependent - but during vigorous exercise, muscle contraction itself triggers GLUT4 translocation to the membrane through an insulin-independent, AMPK-mediated pathway (via muscle contraction and calcium/AMPK signaling), separate from the insulin-PI3K-Akt pathway. That's the trick behind MCQ #249.

So skeletal muscle has two distinct triggers for GLUT4 translocation:

Insulin (at rest, after meals) - PI3K/Akt pathway
Muscle contraction/exercise - AMPK pathway (insulin-independent)

4. Hormone receptor mechanisms - the classic mnemonic table

This table (mechanism of hormone action) is one of the most frequently tested tables in endocrinology. Five mechanisms, five mnemonics:

Mechanism	Mnemonic	Hormones
cAMP	FLAT ChAMP	FSH, LH, ACTH, TSH, CRH, hCG, ADH (V2), MSH, PTH (also calcitonin, glucagon)
IP3/Ca2+ (phospholipase C)	GOAT HAG	GnRH, Oxytocin, ADH (V1), TRH, Histamine (V1), Angiotensin II, Gastrin
Steroid receptor (intracellular/nuclear)	PET CAT on TV	Progesterone, Estrogen, Testosterone, Cortisol, Aldosterone, Thyroid hormone, Vitamin D
Tyrosine kinase receptor	(Growth factors)	Insulin, IGF-1, Growth hormone, Prolactin (and other growth factor receptors like EGF, PDGF)
cGMP	—	ANP/BNP, Nitric oxide (sometimes remembered as "BAD GAS" or similar)

The physiologic logic behind this:

cAMP/IP3 hormones are peptide hormones that cannot cross the lipid membrane, so they bind surface G-protein-coupled receptors (Gs -> adenylyl cyclase -> cAMP, or Gq -> phospholipase C -> IP3/DAG) - Costanzo Physiology, 7th Ed; Guyton and Hall.
Steroid/thyroid/Vitamin D hormones are lipophilic, cross the membrane freely, and bind intracellular/nuclear receptors that act as transcription factors, so they take hours to produce effects (via new protein synthesis) rather than seconds.
Tyrosine kinase hormones (insulin, growth factors) bind receptors that have intrinsic enzymatic activity, autophosphorylating and triggering cascades (like the PI3K/Akt pathway responsible for GLUT4 translocation, tying directly back to point 3 above).

Quick self-test (matches the exam-style format on your page)

What maintains glucose between meals? → Hepatic glycogenolysis
What activates phosphorylase kinase? → Both glucagon and epinephrine
What does cortisol do to peripheral glucose uptake? → Decreases it
Insulin-independent glucose uptake mnemonic? → BRICK LIPS
Which tissue has higher insulin-independent uptake than the brain? → Exercising skeletal muscle
Receptor type for insulin-dependent glucose uptake? → GLUT4 (via tyrosine kinase receptor signaling)
Hormone that always acts via cAMP? → TSH (also a good example: it's on both the FLAT ChAMP list and doesn't have a competing pathway, which is why exams like this specific fact)

If you want, I can quiz you with more MCQs on this exact topic, or move to the next chapter section on this page (thyroid/adrenal receptor mechanisms) in the same style.

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