The Endocrine System

Overview and Importance

The endocrine system is one of the body's two major control systems, working alongside the nervous system to coordinate and integrate the activity of most body cells. It uses hormones—chemical messengers transported in the blood—to influence metabolic activities. Endocrine responses are typically slower but longer-lasting than those of the nervous system. The study of hormones and endocrine organs is called endocrinology.

• Major processes controlled by the endocrine system:

  • Reproduction

  • Growth and development

  • Maintenance of electrolyte, water, and nutrient balance

  • Regulation of cellular metabolism and energy balance

  • Mobilization of body defenses

Comparison of Nervous and Endocrine Systems

Endocrine vs. Exocrine Glands

• Exocrine glands: Produce nonhormonal substances (e.g., sweat, saliva); have ducts to carry secretions to membrane surfaces.

• Endocrine glands: Produce hormones; ductless; secrete hormones directly into extracellular fluid. Major glands include the pituitary, thyroid, parathyroid, adrenal, and pineal glands. The hypothalamus is a neuroendocrine organ.

• Other organs with endocrine tissue: pancreas, gonads, placenta, stomach, intestine, heart, kidneys, skin, thymus, bone, adipose tissue.

Chemical Messengers

• Hormones: Long-distance chemical signals traveling in blood to target cells.

• Autocrines: Chemicals that exert effects on the same cells that secrete them (local action).

• Paracrines: Locally acting chemicals that affect neighboring cells (local action).

• Autocrines and paracrines are not considered part of the endocrine system.

Chemical Structure and Classification of Hormones

Hormone Classes

• Amino acid–based hormones: Includes amino acid derivatives, peptides, and proteins. Most are water-soluble (except thyroxine) and cannot cross the plasma membrane.

• Steroid hormones: Synthesized from cholesterol; lipid-soluble and can cross the plasma membrane. Includes gonadal and adrenocortical hormones.

• Eicosanoids: Sometimes considered hormones, but mostly act as paracrines and autocrines due to their localized effects.

Mechanisms of Hormone Action

Target Cells and Effects

Hormones circulate to virtually all tissues, but only cells with specific receptors (target cells) are affected. Hormones alter target cell activity by increasing or decreasing the rates of normal cellular processes.

• Possible effects include:

  • Altering plasma membrane permeability or membrane potential

  • Stimulating synthesis of enzymes or proteins

  • Activating or deactivating enzymes

  • Inducing secretory activity

  • Stimulating mitosis

Mechanisms Based on Hormone Solubility

• Water-soluble hormones (all amino acid–based except thyroid hormone): Act on plasma membrane receptors, usually via G proteins and second messengers; cannot enter the cell.

• Lipid-soluble hormones (steroid and thyroid hormones): Act on intracellular receptors that directly activate genes; can diffuse across the plasma membrane.

Second Messenger Systems

• Cyclic AMP (cAMP) pathway:

  1. Hormone (first messenger) binds to receptor.

  2. Receptor activates a G protein.

  3. G protein activates adenylate cyclase.

  4. Adenylate cyclase converts ATP to cAMP (second messenger).

  5. cAMP activates protein kinases, which phosphorylate other proteins.

  

• PIP2-Calcium pathway: Hormone-activated G protein activates phospholipase C, which splits a membrane phospholipid into two second messengers: diacylglycerol (DAG) and inositol trisphosphate (IP3). DAG activates protein kinases; IP3 causes Ca2+ release from intracellular stores, which can activate enzymes or bind to calmodulin to amplify the response.

• Other mechanisms: cGMP as a second messenger; some hormones (e.g., insulin) act via receptor tyrosine kinases.

Direct Gene Activation by Lipid-Soluble Hormones

• Lipid-soluble hormones diffuse into target cells and bind to intracellular receptors.

• The hormone-receptor complex enters the nucleus, binds to DNA, and initiates transcription of specific genes to produce mRNA, which is then translated into proteins.

Regulation of Hormone Release

Feedback Mechanisms

• Blood levels of hormones are controlled by negative feedback mechanisms.

• Hormone release is triggered by:

  • Endocrine gland stimuli (humoral, neural, hormonal)

  • Nervous system modulation

Types of Endocrine Gland Stimuli

• Humoral stimuli: Changing blood levels of ions/nutrients directly stimulate hormone release (e.g., low Ca2+ stimulates PTH release).

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic fibers stimulate adrenal medulla to secrete catecholamines).

• Hormonal stimuli: Hormones stimulate other endocrine organs to release their hormones (e.g., hypothalamic hormones regulate anterior pituitary hormones).

Target Cell Specificity and Regulation

• Target cells must have specific receptors for a hormone to respond.

• Degree of activation depends on:

  1. Blood levels of the hormone

  2. Number of receptors on/in the target cell

  3. Affinity (strength) of binding between hormone and receptor

• Up-regulation: Target cells form more receptors in response to low hormone levels.

• Down-regulation: Target cells lose receptors in response to high hormone levels.

Hormone Activity: Half-Life, Onset, and Duration

• Hormones circulate in blood either free or bound to plasma proteins (steroids and thyroid hormone are bound; others are free).

• Half-life: Time required for hormone level in blood to decrease by half.

• Onset and duration of hormone action vary by hormone type (water-soluble vs. lipid-soluble).

Hormone Interactions at Target Cells

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

• Synergism: More than one hormone produces the same effects, amplifying the response (e.g., glucagon and epinephrine).

• Antagonism: One or more hormones oppose the action of another (e.g., insulin and glucagon).

Summary Table: Lipid- vs. Water-Soluble Hormones

Location of Major Endocrine Organs

Key Terms and Definitions

• Hormone: Chemical messenger secreted by endocrine glands, transported by blood, and acting on distant target cells.

• Target cell: Cell with specific receptors for a hormone.

• Second messenger: Intracellular molecule (e.g., cAMP, Ca2+) that mediates hormone action inside the cell.

• Negative feedback: Mechanism that reduces or shuts off the original stimulus.

• Up-regulation/Down-regulation: Increase or decrease in receptor number in response to hormone levels.