Endocrinology

What are the three broad classes of hormones?

Peptides, amines and steroids

Peptide hormones

Short chains of amino acids secreted by pituitary, parathyroid, heart, stomach, liver and kidneys.

Examples of peptide hormones

Oxytocin (and most other hormones)

Amine hormones

Derived from the amino acid tyrosine, secreted from the thyroid and adrenal medulla.

Examples of amine hormones

Melatonin and catecholamines

Steroid hormones

Lipids derived from cholesterol. Secreted by the gonads, adrenal cortex, and placenta.

Examples of steroid hormones

Testosterone is the male sex hormone. Oestradiol, similar in structure to testosterone, is responsible for many female sex characteristics.

Catecholamines

A subcategory of amine hormones including adrenaline and dopamine

What molecule are steroid hormones synthesized from?

Lipids

Which types of hormones occupy cell surface receptors?

Peptide, protein and catecholamines

Which types of hormone occupy intracellular (cytoplasmic and/or nuclear) receptors?

Steroids and thyroid hormone

Why can’t peptide hormones and catecholamines pass through the plasma membrane?

They are hydrophilic and cannot pass through the plasma membrane’s hydrophobic core

Which three molecules are hydrophobic hormones often derivatives of?

• cholesterol (as for the steroid hormones)

• tyrosine (as in the case of the thyroid hormones) or

• derivatives of the fat-soluble vitamins, A & D.

How do hydrophobic hormones travel across the plasma membrane?

Diffusion

True or false: hydrophilic hormones’ rate of secretion is directly proportional to their rate of synthesis?

False

True or false: hydrophobic hormones’ rate of secretion is directly proportional to their rate of synthesis?

True

Is cortisol hydrophobic or hydrophilic?

Hydrophobic

Is growth hormone hydrophobic or hydrophilic?

Hydrophilic

Is thyroid stimulating hormone hydrophobic or hydrophilic?

Hydrophilic

Where (in a target cell) might you find the receptor for a hydrophilic hormone?

Plasma membrane

Where (in a target cell) might you find the receptor for a hydrophobic hormone?

Cytosol, nucleus

In general, how do hydrophilic hormones affect cellular activity?

They activate transmembrane signal transduction pathways, causing the generation of second messengers. These pathways then cause the activation of protein kinases and/or phosphatases and result in the phosphorylation or dephosphorylation of enzymes. The enzymes then change cellular activity.

In general, how do hydrophobic hormones affect cellular activity?

They activate intracellular receptors which act as ligand-dependant transcription factors. The hormones/receptor complex binds to DNA and affects the level of expression of target genes including enzymes. The activated genes are transcribed and translated and then the protein output subsequently changes cellular activity.

The pituitary is connected to the hypothalamus by which structure

The infundibulum

What is the name of the circumventricular organ, lacking a blood-brain barrier, that allows secreted hormones to easily enter the capillaries from the pituitary?

The median eminence

What are nuclei (in regards to hypothalamic nuclei)?

Discrete groups of neurons that synthesize different hormones in response to physiological changes

Is cortisol secreted from the adrenal cortex or medulla?

Cortex

Which hormone from the hypothalamus, and which from the pituitary gland, are associated with a negative feedback loop with cortisol?

Corticotropin releasing hormone (CRH) and ACTH

Which hormone from the hypothalamus, and which from the anterior pituitary gland, are associated with a negative feedback loop with cortisol?

Thyroid releasing hormone (TRH) and thyroid stimulating hormone (TSH)

What does FSH stimulate in males?

Spermatogenesis

What does LH stimulate in males?

Testosterone production from testicular interstitial cells

Give two examples of endocrine axes involving positive feedback?

Oxytocin secretion in the Fergusson reflex to expel foetus, oxytocin secretion to stimulate contraction of milk glands in the milk letdown reflex

For hydrophobic hormones, rate of synthesis and secretion are directly proportional. What is an exception to this rule?

Thyroid hormones, which are stored long-term on a peptide backbone