Up-Regulation and Down-Regulation

Mechanisms of Hormone Sensitivity

Cells can adjust their sensitivity to hormones by changing the number of receptors present on their surface. This process is crucial for maintaining homeostasis and appropriate cellular responses to hormonal signals.

• Up-regulation: An increase in the number of hormone receptors on a target cell. This enhances the cell's sensitivity to a hormone, often in response to low hormone concentrations.

• Down-regulation: A decrease in the number of hormone receptors on a target cell. This reduces the cell's sensitivity to a hormone, typically in response to high hormone concentrations.

Pemissiveness, Synergism, and Antagonism

Interactions Between Hormones

Hormones can interact in various ways to modulate physiological effects. Understanding these interactions is essential for grasping endocrine system complexity.

• Permissiveness: One hormone is required for another to exert its full effect. For example, thyroid hormone must be present for reproductive hormones to function correctly.

• Synergism: The combined effect of two or more hormones is greater than the sum of their individual effects. For instance, glucagon and epinephrine both increase blood glucose, but together, the increase is amplified.

• Antagonism: One hormone opposes the action of another. For example, insulin and glucagon have antagonistic effects on blood glucose levels.

Regulation of Plasma Calcium by Negative Feedback

Homeostatic Control of Blood Calcium

Blood calcium levels are tightly regulated by negative feedback mechanisms involving the parathyroid hormone (PTH).

• Trigger: A drop in blood calcium levels.

• Response: The parathyroid glands secrete parathyroid hormone (PTH).

• Effect of PTH:

  • Increases calcium release from bones (bone resorption).

  • Increases calcium reabsorption by the kidneys.

  • Increases intestinal absorption of calcium by activating vitamin D.

Hormone Receptors

Types and Locations of Hormone Receptors

Hormone receptors are proteins that bind specific hormones to initiate cellular responses. Their location and type determine the mechanism of hormone action.

• Definition: Protein molecules on or within a target cell that bind to a specific hormone to initiate a cellular response.

• Location:

  • Cell membrane: For water-soluble, non-steroid hormones.

  • Intracellular (cytoplasm or nucleus): For lipid-soluble, steroid, and thyroid hormones.

• Types:

  • G protein-coupled receptors: Membrane-bound receptors for many peptide and protein hormones.

  • Intracellular receptors: Cytoplasmic or nuclear receptors for steroid and thyroid hormones.

Tropic Hormone

Definition and Examples

Tropic hormones are produced by one endocrine gland and stimulate another endocrine gland to produce and secrete its own hormones.

• Examples: TSH, ACTH, FSH, and LH, all secreted by the anterior pituitary.

Hypothalamus, Posterior Pituitary, and Anterior Pituitary

Structure and Function

The hypothalamus and pituitary glands are central to endocrine regulation, controlling hormone release throughout the body.

• Hypothalamus and Posterior Pituitary: Neurons originating in the hypothalamus produce and transport oxytocin and ADH down their axons. These hormones are stored and released from the posterior pituitary in response to nerve signals from the hypothalamus.

• Inhibiting hormones: These hormones travel through the hypothalamo-pituitary portal vein to the anterior pituitary, where they regulate the release of anterior pituitary hormones.

Hypothalamo-Pituitary Portal Vein

Transport System for Releasing Hormones

This system of blood vessels connects the hypothalamus and the anterior pituitary gland at the base of the brain.

• Function: To transport hypothalamic releasing and inhibiting hormones directly to the anterior pituitary, ensuring rapid and targeted regulation of anterior pituitary hormone secretion.

Major Hormones of the Anterior Pituitary

Functions and Examples

The anterior pituitary secretes several key hormones that regulate growth, reproduction, and metabolism.

• Growth Hormone (GH): Stimulates growth, cell reproduction, and regeneration.

• Prolactin (PRL): Stimulates milk production.

• Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to produce corticosteroid hormones.

• Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones.

• Follicle-Stimulating Hormone (FSH): Regulates reproductive processes.

• Luteinizing Hormone (LH): Regulates reproductive processes.

• Pro-hormone (POMC): A precursor molecule cleaved to produce ACTH and other hormones.

Roles of LH and FSH

Functions in Males and Females

LH and FSH are essential for reproductive function in both sexes.

• In Males:

  • LH: Stimulates testosterone production by the testes.

• In Females:

  • LH: Triggers ovulation and stimulates the formation of the corpus luteum.

  • FSH: Stimulates the growth of ovarian follicles and the production of estrogen.

Stimulation of Anterior Pituitary Hormones

Regulation by Hypothalamic Releasing Hormones

The secretion of anterior pituitary hormones is stimulated by releasing hormones from the hypothalamus, which are transported via the hypothalamo-pituitary portal system.

GH and IGF Relationship, Control, and Effects

Growth Hormone and Insulin-like Growth Factor

GH stimulates the liver and other tissues to produce insulin-like growth factor (IGF-1), which mediates most of the growth-promoting effects of GH.

• Control: GH secretion is regulated by GHRH (stimulatory) and somatostatin (inhibitory) from the hypothalamus.

• Effects:

  • Increases lipolysis: Breaks down fats.

  • Decreases glucose uptake: Reduces glucose uptake by most cells.

Location of TRH, CRH, and GnRH Production

Releasing Hormones of the Hypothalamus

TRH (Thyrotropin-releasing hormone), CRH (Corticotropin-releasing hormone), and GnRH (Gonadotropin-releasing hormone) are produced and released in the hypothalamus.

Oxytocin and ADH Release, Function, and Location

Posterior Pituitary Hormones

Oxytocin and ADH (antidiuretic hormone, also called vasopressin) are produced in the hypothalamus and released from the posterior pituitary.

• Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding. Also stimulates smooth muscle of the uterus and mammary glands.

• ADH: Regulates water balance by increasing water reabsorption in the kidneys.

• Hormone release location: Both oxytocin and ADH are released from the posterior pituitary.

• Cell bodies location: The cell bodies that produce these hormones are in the hypothalamus (supraoptic and paraventricular nuclei).

The Neurohypophysis

Posterior Pituitary Structure

The neurohypophysis is another name for the posterior pituitary gland. It does not synthesize its own hormones but rather stores and releases hormones (oxytocin and ADH) produced by the hypothalamus.

Thyroid Hormones and Related Molecules

Types and Functions

The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), which regulate metabolism, growth, and development.

• MIT: Monoiodotyrosine (a tyrosine with one iodine atom).

• DIT: Diiodotyrosine (a tyrosine with two iodine atoms).

• T3: Triiodothyronine, formed from MIT and DIT.

• T4: Thyroxine, formed from two DIT molecules.

• Thyroxine function: Increases the basal metabolic rate, regulates growth and development.

• Thyroxine binding globulin (TBG): A protein that binds to and transports T3 and T4 in the blood.

Effects of Calcitonin

Regulation of Blood Calcium

Calcitonin is a hormone that lowers blood calcium levels.

• Increased calcitonin secretion: Lowers blood calcium levels.

• Produced by: The parafollicular cells (C-cells) of the thyroid.

Atrial Natriuretic Peptide (ANP)

Role in Fluid and Blood Pressure Regulation

ANP is a hormone involved in regulating blood pressure and fluid balance.

• Production location: Produced by cells in the atria of the heart in response to atrial stretch.

• Action location: Acts on the kidneys to increase sodium and water excretion, thereby decreasing blood volume and pressure.

Parathyroid Hormone (PTH)

Regulation of Blood Calcium

PTH is secreted by the parathyroid glands and increases blood calcium levels by stimulating bone resorption, increasing renal reabsorption of calcium, and promoting intestinal absorption.

Hormone Inhibiting Osteoclast Activity

Role of Calcitonin

Calcitonin inhibits osteoclast activity, preventing the breakdown of bone and release of calcium into the bloodstream.

Adrenal Cortex Hormones and Layers

Structure and Hormone Production

The adrenal cortex consists of three layers, each producing different types of hormones.

• Zona glomerulosa: Mineralocorticoids (e.g., aldosterone).

• Zona fasciculata: Glucocorticoids (e.g., cortisol).

• Zona reticularis: Androgens.

• Example mineralocorticoid: Aldosterone, which regulates salt and water balance.

• Glucocorticoid and function: Cortisol, which regulates metabolism, suppresses inflammation, and helps the body respond to stress.

Adrenal Medulla Hormones

Functions of Epinephrine and Norepinephrine

The adrenal medulla produces epinephrine and norepinephrine, which are involved in the "fight-or-flight" response.

• Function: These hormones increase heart rate, blood pressure, and glucose release from the liver.

Insulin and Glucagon

Regulation of Blood Glucose

Insulin and glucagon are hormones produced by the pancreas that regulate blood glucose levels.

• Insulin: Decreases blood glucose levels by promoting glucose uptake into cells and stimulating the liver to store glucose as glycogen.

  • Secretion location: Secreted by beta cells of the pancreatic islets.

• Glucagon: Increases blood glucose levels by stimulating the liver to convert glycogen to glucose (glycogenolysis) and synthesize glucose (gluconeogenesis).

  • Secretion location: Secreted by alpha cells of the pancreatic islets.