Ch 49.  Hormones and Endocrine Systems 

Chemical signaling is an important, fundamental biological process.   Examples of chemical signaling range from neurotransmitters that carry signals across synapses to pheromones (chemicals released by one individual that have an effect on another individual).  Hormones are another example of chemical signaling.  Hormones are proteins or steroids that are produced and released in one part of the body and then travel to another part of the body where they have an effect.  Hormones operate by a process called signal transduction. 

Signal Transduction – a chemical signal attaches to a membrane protein (or enters cytoplasm and attaches to a cytoplasmic protein).  The proteins to which hormones attach are called receptor proteins.  The match between hormone and receptor protein is best understood as a “lock-and-key” model.  The hormone is the key and the receptor protein is the lock.  The attachment of the hormone to the receptor protein transfigures molecular structure bringing about a cellular response.  How proteins induce a cellular response when prompted by chemical signals is beyond the scope of our coverage.  Page 883 in Mader (see fig. 49.2) does state that steroid hormones regulate gene expression (protein production) and that protein hormones activate particular chemical reaction series (an example is the protein hormone glucagon that activates the chemical reactions converting glycogen to glucose).  Steroid hormones actually enter the cytoplasm before reaching target (receptor) proteins while protein hormones bind to receptor proteins on the surface of plasma membranes. 

Most hormones are secreted from endocrine glands.  An endocrine gland is a ductless gland in which glandular cells secrete hormones directly into the bloodstream and interstitial fluid. 

Pancreas - is both and endocrine and exocrine gland.  Exocrine glands are not part of the endocrine system.  The secretions of exocrine glands flow into a duct (tube) that carries the chemically laden secretions directly to the tissues lining body parts such as sweat glands secrete sweat upon the skin; salivary glands secrete saliva into the mouth, mucous glands secrete mucous onto mucous membranes.  As and exocrine gland the pancreas secretes digestive enzymes into the duodenum.  As an endocrine gland the pancreas possesses special cells that secrete hormones into body fluids. 

Within the pancreas, alpha cells secrete the hormone glucagon and beta cells secrete the hormone insulin.  

Type I Diabetes Mellitus – beta cells have been destroyed, thus insulin is not produced

Type II Diabetes Mellitus – target cells bearing insulin receptor proteins are not responsive to insulin (i.e. signal transduction fails to occur).

 

Negative Feedback and Antagonistic Hormones.

We will use the pancreatic hormones to illustrate the above physiological concepts:  

Negative feedback.  output eventually inhibits output.  The pancreas's release of insulin  illustrates the concept.  First, it is necessary to understand the pancreas does not release insulin when blood sugar level is low.  Insulin is released only when sugar level is high.  Once released, insulin lowers blood sugar, therefore inhibiting further insulin release from the pancreas.  Insulin release therefore results in negative feedback to inhibit insulin release. 

Antagonistic Hormones - oppose each other's actions.  Ex. glucagon and insulin are antagonistic hormones.  Glucagon promotes the release of glucose to the bloodstream from stored glycogen by a process called signal transduction.  Insulin promotes the removal of glucose from the bloodstream for storage as glycogen by a process called signal transduction. 

Another Antagonistic Pair of Hormones:

Calcitonin & Parathyroid Hormone

These hormones regulate Ca++ levels in blood.  Calcitonin is produced and released by the thyroid gland located in the neck.  Parathroid Hormone is produced and released by the parathroid glands embedded in the thyroid gland. 

Blood delivers calcium to tissues where it is required.  Calcium is important in:

  • presynaptic membrane of neurons; here calcium is involved in transferring nerve impulses (action potentials) to the release of neurotransmitters from synaptic vesicles.
  • muscle cell contractions;  here calcium is involved in signal transduction between neurotransmitters released by motor neurons and the action of the filamentous contractile proteins actin and myosin that bring about muscle contractions.  See fig. 48.13.
  • blood clotting
  • bone (especially during bone formation)

As in blood sugar levels, calcium levels in blood are hormonally regulated to insure that blood maintains a proper level of calcium.  Calcitonin is released from the thyroid gland when blood calcium levels rise.  Calcitonin promotes (via signal transduction!) the uptake of calcium by bone and the secretion of calcium by the kidney, both effects lower blood calcium levels.  Parathyroid hormone is released from the parathyroid glands when blood calcium levels fall.  Parathyroid hormone promotes (also via signal transduction!) the release of Ca++ from bone and greater absorption of Ca++ by the intestines.

 

Other Thyroid Hormones

The thyroid hormones listed below increase the rate of general metabolism in the body, that is, the presence of these hormones in body fluids reaches receptor proteins found on all active, living cells.  Each cell in contact with these hormones respond via signal transduction by speeding up the rate of all chemical reactions that occur within their cytoplasm.

  • Thyroxine or T4 indicating the hormone molecule contains four atoms of iodine.
  • Triiodothyronine or T3 indicating the hormone molecule contains three atoms of iodine

Goiter – an enlarged thyroid gland (see fig. 49.7) that may be due to inadequate iodine in the diet. 

Adrenal Gland

As a final example of a hormonal response, have you ever wondered about your body’s chemical response to stress?  How your body delivers the rush of adrenaline (epinephrine) we’ve all experienced as increased heart rate and energized muscles?    In response to conscious perception of danger or thrill (short term stress, - & +), the brain (hypothalamus precisely) initiates a nerve impulse that is sent to the middle of the adrenal gland (adrenal medulla) which in turn releases adrenaline. Through the circulation of body fluids adrenaline from the adrenal medulla is delivered to receptor proteins in heart and skeletal muscles thus empowering “flight or fight.”  The adrenal glands sit atop each kidney (see fig. 49.3).  The outer layer of each adrenal gland is called the adrenal cortex and it is responsible for producing and releasing steroidal hormones that allow us to cope with long term stress such as starvation.  The corticosteroids from the adrenal cortex promote the breakdown of muscle and also suppress the immune system.  Long term personal anxiety may bring about the release of corticosteriods and thus reduce physical health.