Endocrine Physiology Lecture 2

Hormone Receptors

• Hormone receptors are located in or on cell membranes (peptides and catecholamines), cytoplasm (steroid hormones), and nucleus (thyroid hormones).
• They are large proteins with a variable number ranging from 2,000 to 100,000 per cell and can fluctuate over time.
• Hormonal sensitivity of target tissues is influenced by the number and sensitivity of these receptors.

Down-Regulation and Up-Regulation

• Down-regulation occurs when a hormone decreases the number or affinity of its receptors, exemplified by progesterone reducing its own receptors in the uterus.
• Up-regulation involves a hormone increasing receptor quantity or affinity, as seen with estrogen in the ovary enhancing its own and LH receptors.

Mechanism of Hormone Action

• Peptide hormones act through signaling molecules like cAMP, which is degraded to 5’-AMP by phosphodiesterase, an enzyme inhibited by caffeine.

Sensitivity of Hormones

• Sensitivity is defined as the hormone concentration producing 50% of the maximal response; increased sensitivity requires less hormone, while decreased sensitivity requires more.

Stimuli for Hormone Secretion

• Humoral stimuli involve changes in blood chemistry.
• Neural stimuli are driven by nerve signals.
• Hormonal stimuli occur when one hormone stimulates another hormone's release.

Control of Hormone Secretion

• Negative feedback prevents hormone overactivity and relies on the biological effects rather than hormone levels.
• Positive feedback enhances hormone secretion based on its biological actions, e.g., increased estrogen leading to higher LH secretion.
• Cyclical variations in hormone levels are influenced by development stages, circadian rhythms, and seasonal changes, impacting hormones like ACTH and glucocorticoids.
• Reflex release is triggered by stress and new experiences, affecting hormones like prolactin and components of the renin-angiotensin-aldosterone system.
• Feedforward control anticipates hormone needs, like GLP-1 from enteroendocrine cells promoting insulin secretion before glucose absorption.

Hormone Interactions

• Synergistic interactions occur when multiple hormones collaborate to produce a greater effect than individually, such as FSH and testosterone in spermatogenesis.
• Permissive actions enhance the target organ's response to a second hormone, e.g., cortisol enhancing catecholamine effects.
• Antagonistic actions happen when one hormone inhibits the effects of another, as seen with insulin and glucagon.

Endocrine Pathophysiology

• Underproduction can be primary (defect in the gland), secondary (defect in stimulating hormone), or apparent (receptor defects), exemplified by conditions like adrenal insufficiency or pseudohypoparathyroidism.
• Overproduction typically results from tumors, with primary being a direct tumor in the gland and secondary from excessive stimulation, such as a pituitary tumor leading to high cortisol.
• Apparent overproduction involves receptor mutations leading to continuous activation, like in Liddle's syndrome, which simulates excess aldosterone effects despite low levels of the hormone.

This quiz covers the fundamental concepts of hormone receptors, including their locations within and on cells, and the variability in receptor numbers. Dive into the details of how different types of hormones interact with their respective receptors to influence cellular sensitivity.