Endocrine System: Hormones, Glands, and Mechanisms of Action

Endocrine System Overview

Comparison of Nervous and Endocrine Systems

The endocrine system and nervous system are the two main regulatory systems in the body, each with distinct mechanisms and effects.

• Nervous System: Rapid response, short duration, uses neurotransmitters, acts at specific locations, covers short distances, with action potentials.

• Endocrine System: Slow responses, long duration, uses hormones in blood, acts at diffuse locations, covers long distances, with hormones (chemicals).

Endocrine system influences metabolic activity by means of hormones, affecting reproduction, growth, development, maintenance of water/nutrient/electrolyte balance, cell metabolism, energy, and body defenses.

Endocrine Glands and Hormones

• Exocrine glands: Nonhormonal, have ducts (e.g., sweat, saliva).

• Endocrine glands: Hormonal, no ducts, hormones travel through blood or lymph (e.g., pituitary, thyroid, parathyroid, adrenal, pineal).

Hormones are long-distance chemical signals that travel in blood or lymph throughout the body.

• Paracrines: Short-distance chemical signals that act locally, affect cells other than those that secrete them.

• Autocrines: Short-distance chemical signals that act locally, affect the same cells that secrete them.

Hormone Classification and Mechanisms

Types of Hormones

• Amino acid-based hormones: Water-soluble, cannot cross plasma membrane, can travel without a carrier.

• Steroid hormones: Lipid-soluble, can cross plasma membrane, need a carrier to travel in blood.

• Eicosanoids: Mediate inflammation and allergic reactions, raise blood pressure, increase uterine contractions, blood clotting, pain, inflammation; highly localized effects.

Hormone Receptors and Target Cells

Hormones affect only target cells that have specific receptors for them. The response of a target cell depends on:

• Blood levels of the hormone

• Relative number of receptors on/in the target cell

• Affinity (strength) of the binding between hormone and receptor

Example: ACTH receptors are only found in the adrenal cortex, while thyroid hormone receptors are found on nearly all body cells.

Hormone Actions on Target Cells

• Alters plasma membrane permeability

• Stimulates synthesis of enzymes or proteins

• Activates or deactivates enzymes

• Induces secretory activity

• Stimulates mitosis

Water-Soluble vs. Lipid-Soluble Hormones

• Water-soluble hormones: Act on receptors in plasma membrane, usually via G proteins and second messengers.

• Lipid-soluble hormones: Act on intracellular receptors that directly activate genes.

Second Messengers

Common second messengers include:

• cAMP (cyclic adenosine monophosphate)

• PIP2-calcium

• cGMP (cyclic guanosine monophosphate)

Direct Gene Activation (Steroid Hormones)

1. Steroid hormone diffuses through the plasma membrane and binds an intracellular receptor.

2. Receptor-hormone complex enters the nucleus.

3. Receptor-hormone complex binds to a specific DNA region.

4. Binding initiates transcription of the gene to mRNA.

5. mRNA directs protein synthesis.

Hormone Regulation and Feedback

Negative Feedback Mechanisms

Most hormones are regulated by negative feedback, which maintains homeostasis by reducing hormone secretion when levels are adequate.

Types of Stimuli for Hormone Release

• Humoral stimuli: Hormones secreted in direct response to changing blood levels of certain critical ions or nutrients (e.g., parathyroid hormone released in response to low Ca2+ levels, insulin to blood glucose, aldosterone to Na+ or K+).

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic nervous system stimulates adrenal medulla to release norepinephrine and epinephrine).

• Hormonal stimuli: Hormones released in response to hormones from other endocrine glands (e.g., hypothalamus regulates anterior pituitary gland hormones).

Hormone Interactions

• Permissiveness: One hormone cannot exert its effects without another hormone (e.g., reproductive hormones require thyroid hormone).

• Synergism: More than one hormone produces the same effects at the target cell and amplifies effects (e.g., glucagon and epinephrine both cause the liver to release glucose; together, release is 150% of individual effects).

• Antagonism: One hormone opposes the action of another (e.g., insulin is antagonistic to glucagon).

Hormone Concentration and Receptor Regulation

• Hormones can circulate in the blood free or bound to a protein carrier.

• Hormone concentration reflects its rate of release and the speed at which it is inactivated and removed from the body.

• Prolonged low levels of a hormone cause up-regulation (more receptors are added).

• Prolonged high levels of a hormone cause down-regulation (cells respond less to hormone stimulation).

Pituitary Gland (Hypophysis)

Structure and Function

• Pituitary gland: Secretes at least 8 hormones and is connected to the hypothalamus.

• Posterior pituitary: Composed largely of neural tissue, releases neurohormones received from the hypothalamus. Functions as a hormone-storage area. Posterior lobe and infundibulum make up the neurohypophysis.

• Anterior pituitary: Composed of glandular tissue, releases a number of hormones.

Summary Table: Hormone Types and Properties

Hormone Type	Solubility	Transport	Receptor Location	Example
Amino acid-based	Water-soluble	Free in plasma	Cell membrane	Insulin, epinephrine
Steroid	Lipid-soluble	Bound to carrier protein	Intracellular	Cortisol, estrogen
Eicosanoid	Lipid-soluble	Local diffusion	Cell membrane	Prostaglandins

Key Equations

• Hormone-receptor binding (affinity): $[HR] = \frac{[H][R]}{K_d}$ where $[HR]$ is the concentration of hormone-receptor complex, $[H]$ is hormone concentration, $[R]$ is receptor concentration, and $K_d$ is the dissociation constant.

Additional info:

• Some details about the specific hormones released by the anterior and posterior pituitary are not included in the original notes but are important for further study (e.g., growth hormone, ACTH, TSH, etc.).

• Mechanisms of hormone inactivation and removal include enzymatic degradation, uptake by target cells, and excretion by the kidneys or liver.