How Do Hormones Work?

A. Hormones

1. Overview of hormone functions

- Regulation of growth and development.
- Homestatic control (regulation of internal environment; parameters maintained w/in relatively narrow limits)
- Control of reproductive system processes (ovulation, menstruation, maintainence of pregnancy)
- Effects on Behavior (modification, modulation, initiation of specific patterns)

2. Endocrine glands, endocrine cells and some neurons secrete hormones into tissue fluid from which they diffuse into capillaries. Sources of hormones in humans = the endocrine system 

3. Target cells = cells with receptors for a given chemical messenger that can alter their activities in response to messenger.

- Hormones are transported throughout the body by the blood, but they only elicit responses in target cells.
- Receptors may be on cell surface (non-steroid hormones) or internal to cell in cytoplasm or nucleus (steroid hormones) 

B. Types of Hormones

There are two main classes (chemical groups) into which hormones fall:

1. steriods (in vertebrates sythesized from cholesterol by adrenal cortex, testis, ovary and placenta)

[examples: cortisol, estradiol]

2. non-steroid

- amines [epinephrine and norepinephrine]
- peptides [oxytocin, ADH]
- proteins [growth hormone, insulin]
- glycoproteins [FSH, TSH]

C. How hormones work in target tissues.

1. Steriod hormones and thyroid hormones (gene activation) :

a. Relatively small, lipid soluble. Therefore, diffuse freely into and out of cells.
b. Affect target cells that have receptor proteins in their cytoplasm (or nucleus)
c. Hormone-receptor complex moves into nucleus (if not already there). (Current evidence is that this is an active tranport process and that only target cells have transport mechanism)
d. Complex binds to DNA and stimulates transcription of specific genes (synth. of mRNA) --> protein synthesis.
e. Full effect of hormone manifested over minutes to days.

2. Peptide/protein hormones (second messenger mechanism) :

a. Do not diffuse into cells.
b. Target cells have specific receptor for hormone on cell surface.
c. Hormone binds to receptor and causes a second messenger to be released within the cell.

- cyclic nucleotide: c-AMP (rx. inside cell catalyzed by adenylate cyclase) --> many c-AMP molecules
- Ca ion; binding of hormone causes Ca channels in cell membrane to open --> influx of many Ca ions. Ca ions bind to protein = calmodulin.

e. c-AMP molecules or Ca-calmodulin complexes activate many enzymes (= cascade of reactions) --> rapid response to hormone (seconds to minutes).

D. Examples of hormone control

1. Secretion of ADH (antidiuretic hormone) by pitutitary. 

- dehydration (low blood pressure) --> secretion of ADH --> increased retention of water (more water reabsorbed by kidneys).
- cell bodies in region of brain called hypothalamus synthesize ADH. ADH moves down axons to posterior pitutitary; accumulates in axon terminals; released by action potentials. ADH enters bloodstream. = Neurohormone [What are the target cells for ADH?]
- What stops secretion of ADH by posterior pituitary? [As more water is retained by body, blood pressure rises and production of ADH falls off. = Negative feedback]

2. Stimulation of Cortisol Secretion by Adrenal Cortex

Low blood sugar --> Hypothalamus secretes CRH (corticotropin-releasing hormone) --> Anterior pituitary cells (certain cells only) secrete ACTH (adrenocorticotropic hormone) --> Adrenal cortex secretes cortisol (and other glucocorticoids)

[cortisol prevents sugar uptake by muscle cells]

- Negative Feedback control pathways:

- cortisol inhibits ACTH secretion by adrenal cortex
- cortisol also inhibits CRH secretion by hypothal.

3. Role of Pancreatic Islets 

- Islets of Langerhans = clusters of endocrine-secreting cells disbursed throughout pancreas. Alpha cells secrete glucagon; beta cells secrete insulin

a. Insulin lowers blood glucose level by stimulating cells (esp. skeletal muscle and fat cells as well as liver cells) to take up glucose; stimulates muscles and liver to store glucose as glycogen.
b. Glucagon raises blood glucose level by stimulating liver to convert glycogen to glucose and by stimulating the conversion of fatty acids and amino acids to glucose.
c. Secretion of insulin and glucagon is controlled directly by the concentration of glucose in the blood. [Normal "fasting level" = 90mg/100ml blood]
d. Insulin-glucagon system is fast-acting for keeping blood glucose level tightly regulated. (Brain cells cannot utilize other nutrients as fuel.)
e. Diabetes mellitus: