PHA 329 - Endocrine System (Part 1)

Endocrine system

A system of ductless glands that secrete hormones into the bloodstream to regulate target tissues

Hormone

Chemical messenger released into the blood that binds receptors on target tissues to produce a physiological response

Endocrine gland

Group of epithelial-derived cells that secrete hormones directly into the bloodstream

Target tissue

Tissue that contains specific receptors for a hormone and responds when the hormone binds

Chemical classes of hormones

Peptide/protein hormones, steroid hormones, and amino acid–derived hormones

Peptide/protein hormones

Hormones made of amino acids; hydrophilic; stored in vesicles and released by exocytosis

Examples of peptide hormones

Insulin, growth hormone, TSH, oxytocin

Steroid hormones

Hormones synthesized from cholesterol; lipophilic; synthesized on demand and diffuse across membranes

Examples of steroid hormones

Cortisol, aldosterone, testosterone, estrogen, progesterone, vitamin D

Amino acid–derived hormones

Hormones derived from amino acids (usually tyrosine); can be hydrophilic or lipophilic

Examples of amino acid–derived hormones

Epinephrine, dopamine, thyroid hormone

Hydrophilic hormones

Hormones that are water-soluble, circulate freely in plasma, and bind cell membrane receptors

Lipophilic hormones

Hormones that are lipid-soluble, require carrier proteins in blood, and bind intracellular receptors

Protein hormone synthesis

Preprohormone → prohormone → active hormone with storage in secretory vesicles

Steroid hormone synthesis

Synthesized from cholesterol with pregnenolone as the rate-limiting step

On-demand synthesis

Hormone synthesis that occurs as needed rather than being stored, typical of steroid hormones

Hormone release via exocytosis

Method used by peptide and protein hormones to enter the bloodstream

Diffusion across membrane

Method used by lipophilic hormones to leave endocrine cells

Hormone transport

Movement of hormones through the bloodstream to target tissues

Carrier proteins

Plasma proteins that bind lipophilic hormones for transport in blood

Free hormone

Fraction of hormone not bound to carrier protein and biologically active

Cell membrane receptor

Receptor located on the plasma membrane used by hydrophilic hormones

Second messenger system

Intracellular signaling pathway activated by hydrophilic hormones after receptor binding

Intracellular receptor

Cytoplasmic or nuclear receptor used by lipophilic hormones

Gene transcription alteration

Process by which lipophilic hormone–receptor complexes change gene expression

Humoral stimulus

Hormone release triggered by changes in blood levels of ions or nutrients

Neural stimulus

Hormone release triggered by nervous system input

Hormonal stimulus

Hormone release triggered by another hormone (tropic hormone)

Negative feedback loop

Regulatory mechanism where the final hormone inhibits earlier hormone release to maintain homeostasis

Positive feedback loop

Regulatory mechanism where hormones amplify each other to produce a specific event

Homeostasis

Maintenance of a stable internal environment

Example of negative feedback

Thyroid hormone inhibiting TSH release

Example of positive feedback

Estrogen increasing LH and FSH during ovulation

Permissiveness

One hormone must be present for another hormone to exert its full effect

Example of permissiveness

Thyroid hormone required for normal reproductive development

Synergism

Two hormones produce the same effect and their combined effect is greater than additive

Example of synergism

Glucagon and epinephrine increasing blood glucose together

Antagonism

Two hormones have opposite physiological effects

Example of antagonism

Insulin lowers blood glucose while glucagon raises blood glucose

Up-regulation

Increase in number of receptors to enhance hormone signaling

Down-regulation

Decrease in number of receptors to reduce hormone signaling

Receptor desensitization

Decreased receptor responsiveness due to prolonged hormone exposure

Example of desensitization

Insulin resistance in type 2 diabetes