Anatomy Week Ten

Continuing with the endocrine system and hormones, we learned how they are transported in the blood.  We also learned the mechanism of hormone action.  

The majority of the water-soluble hormone molecules circulate in the watery blood plasma and are in a sort of free state not bound to any other molecule or protein.  The lipid-soluble hormones in contrast have to travel in the bloodstream bound to what are called transport proteins.  These transport proteins are synthesized in the liver and have three main functions.  One function is that they provide a ready reserve of hormone in the bloodstream.  A second function of transport proteins is that they make the lipid- soluble hormones temporarily water-soluble increasing their solubility in the blood.  The last function of the transport protein is that they retard passage of small hormone molecules through the filtering mechanism in the kidneys, and they in turn slow the rate of the hormone that is lost in the urine.  If these hormones are not bound to transport hormones they are in a free state and thus diffuse out of capillaries and bind to receptors.  This triggers the hormonal responses ordered by the body.  What happens as these free hormones are released from the blood and are bound to the receptor cells, the transport hormones release new ones to replenish the free state.  

Lipid soluble hormones and water soluble hormones have different mechanisms of action.  The response to a hormone depends on not only the hormone but also the target cell.  Different target cells respond differently to the same hormone.  The synthesis of new molecules is not always the response to a hormone.  Changing the permeability of the plasma membrane, stimulating transport of a substance into or out of the target cells, altering the rate of specific metabolic reactions, or causing contraction of smooth muscle and cardiac muscle are all hormonal effects.  Because there are so many varied effects of hormones, it is possible a single hormone can set in motion several different cellular responses to that hormone.  

The first mechanism of a hormone is to announce its arrival to a target cell.  The hormone does this by binding to the receptors of this target cell.  The difference in the lipid soluble hormones and those of the water soluble hormones are that the receptors for the lipid soluble hormones are located inside the target cells and the receptors for the water soluble hormones are located in the plasma membrane of the target cells.  How these mechanisms of hormone action actually work are complex.