Can neurons act as endocrine cells?
Yes! They can do so by releasing their chemical signals into blood instead of the synapse
paracrine and autocrine signals
local signals that don’t travel through the blood
three classes of hormones
steroid, amine, and protein/peptide
steroid hormones
- derived from cholesterol
- all are HYDROPHOBIC
- includes the sex hormones, aldosterone, and cortisol
amine hormones
- derived from 1-2 amino acids
- catecholamines (dopamine, norepinephrine, and epinephrine) are HYDROPHILIC and have one tyrosine
- iodothyronines (thyroid hormones) are HYDROPHOBIC and have two tyrosines
protein/peptide hormones
- amino acid chains
- all are HYDROPHILIC
How are hydrophobic hormones transported through the bloodstream?
They must be carried with something else, such as a carrier protein
cell surface hormone receptor
hormone in extracellular fluid binds and activates response, usually a second messenger system
nuclear hormone receptor
hormone must enter cell or nucleus; receptor-hormone complex binds DNA and increases gene transcription
characteristics of peptides
- chains of amino acids
- hydrophilic
- dissolved in plasma, some bound to carrier proteins
- half-life of minutes
- cell surface receptors
- activate second messenger systems or alter membrane channels
- target cell changes activity of preexisting proteins, some of which may induce new protein synthesis
characteristics of steroids
- derived from cholesterol
- hydrophobic
- bound to carrier proteins
- half-life of hours
- receptors in cytoplast or nucleus, sometimes cell surface
- alter gene expression
- target cell synthesizes new proteins, sometimes changes activity of preexisting proteins
characteristics of catecholamines
- derived from tyrosine
- hydrophilic
- dissolved in plasma
- half-life of seconds to minutes
- cell surface receptors
- activate second messenger systems
- target cell changes activity of preexisting proteins
characteristics of thyroid hormones
- derived from tyrosine and iodine
- hydrophobic
- bound to carrier proteins
- half-life of days
- receptors in nucleus
- alters gene expression
- target cell synthesizes new proteins
synergism
two hormones working together have a greater effect than the sum of each one individually (1+1=4)
- thyroid hormone and epinephrine both cause release of fatty acids from the adipose tissue into the blood
permissiveness
some hormonal effects disappear if a second hormone is not also present; although that second hormone may have no effect alone, it permits the first to act
- epinephrine cannot cause constriction of blood vessels if there is no glucocorticoid present, even though glucocorticoid has no effect on blood vessel constriction
