Lecture
Outline #2: The Vertebrate Endocrine
System
I. MORE INTRODUCTORY COMMENTS ABOUT ENDOCRINOLOGY
1. Roger Guillemin has redefined a hormone as: "any substance released by a cell and which acts on another cell near or far, regardless of the singularity or ubiquity of the source and regardless of the means of conveyance, blood stream, axoplasmic flow, or immediate intracellular space". (see the bottom of page 21)
2. While one can still make distinctions between typical "endocrine glands" and other body tissues, we now know that hormones are produced by all cells, including neurons.
3. One can no longer separate the endocrine and nervous systems into unrelated physiological entities. (The "endocrine hypothalamus".)
4. Now, the term hormone includes neurohormones (or neuropeptides, e.g., GnRH), neurotransmitters (fast-acting), and neuromodulators (slower, more sustained action, such as opioid peptides).
5. These agents of neuronal origin may be: (a) secreted into the blood (neuroendocrine), or they may (b) influence the membrane potentials of local adjacent nerve cells (i.e., ‘neuroparacrine’).
6. Hormone classification is often based on their source and their manner of transmission (Fig 2.6).
a. Endocrine = when the hormone travels through the blood to the target cells.
b. Neuroendocrine = when a neurohormone travels thru the blood to target.
c. Paracrine = when the hormone travels locally thru interstitial fluids to the target cells.
d. Neurocrine
= when a neurotransmitter travels across a synapse to stimulate postsynaptic
cell.
e. Autocrine = when the hormone travels locally and self-stimulates.
7. Some hormones cause rapid responses (e.g., oxytocin and epinephrine), while others initiate slower responses (e.g., steroids and thyroxine).
II. A SURVEY OF COMMON VERTEBRATE HORMONES (Tables 2.1 & 2.2)
These tables give an overview of the diverse nature of hormones.
III. GENERAL CLASSES OF HORMONES (i.e., chemical messengers) (pages 21-25)
A. PEPTIDE/PROTEIN HORMONES (receptors in plasma membrane, except T3 & T4).
1. Amines and other small hormones: (mostly neurotransmitters)
(a) tyrosine derivatives include epinephrine and norepinephrine (Fig. 14.1)
(b) other amines include acetylcholine, dopamine, and serotonin (Fig. 2.3)
2. Peptides: (these are smaller, proteinaceous compounds)
(a) thyrotropin releasing hormone (TRH) (Fig. 6.5)
(b) endorphin & enkephalins (look at the figure on this website)
(c) oxytocin & vasopressin (Table 7.1)
3. Proteins:
(a) insulin (Fig. 11.6)
(b) growth factors, such as IGF, NGF, EGF, FGF (Fig. 12.2)
B. LIPID SOLUABLE HORMONES (receptors in nucleus, or cytoplasm)
1. Steroids (Fig. 2.1 & Fig. 2.2) (also, see cholesterol)
2. Thyroid hormones (which are also derived from the amino acid tyrosine (Fig. 13.1)
3. Eicosanoids (e.g., prostaglandins and other derivatives of arachidonic acid) (Fig. 2.4)
IV. OTHER
BASIC CHARACTERISTICS OF HORMONES
A. PROTEIN HORMONE SECRETION (i.e., exocytosis)
1. Hormones are packaged within secretory vesicles, except for thyroid and steroid hormones.
2. Stimulation of hormone secretion usually involves an exocytotic process in which the vesicles fuse with the plasma membrane and are actively extruded (involving ATP and Ca++ ions) from the cell.
3. Exocytosis is often initiated when tropic hormones interact with target cell membrane receptors and promote membrane depolarization (or, hyperpolarization usually reduces exocytosis).
4. Examples of patterns of hormone secretion following receptor activation of secretory cells:
a. hormones stimulating hormone secretion: releasing hormones from the hypothalamus stimulate cells in the anterior pituitary to secrete trophic hormones such as FSH/LH, TSH, and ACTH, which stimulate hormone secretion from the gonads, thyroids, and adrenal cortex, respectively.
b. neurons stimulating hormone secretion: neurons in the central nervous system stimulate hormone secretions from the neurohypophysis, adrenal medulla, and pineal gland.
c. blood components stimulating hormone secretion: important components of the blood such as glucose, Ca++, amino acids, and free fatty acids (FFAs) can stimulate hormone secretion by selected cells.
5. Keep in mind that peptide/protein hormones are oftentimes secreted as prohormones (Fig. 2.7).
6. Also, keep in mind that steroid/thyroid hormones diffuse freely across the phospholipid bilayer of the plasma membrane, and therefore they do not require an elaborate exocytotic mechanism.
B. HORMONE DELIVERY (see section I.6 above, or see Fig. 2.6)
C. HORMONE METABOLISM (accumulation of anything becomes deleterious)
1. Amines such as catecholamines are inactivated in the liver and at synapses.
2. Peptides are mainly inactivated by blood, liver, and kidney proteases. (Fig. 2.8)
3. Steroids (usually protected by plasma proteins) are metabolized in the liver. (Fig. 2.9)
4. Thyroxines are deiodinated in many different tissues.
5. (There is evidence of internalization of some hormones and membrane receptors.)
D. GENERAL MECHANISMS OF HORMONE ACTION
1. All peptide hormones and neurotransmitters act on membrane receptors.
a. this may lead to depolarization, or to hyperpolarization.
b. this appears to always lead to phosphorylation reactions (kinase).
c. this appears to always lead to gene expression.
2. All steroid hormones and thyroxines act on intracellular receptors. (This usually leads to gene expression.)
V. PHYSIOLOGICAL ROLES OF HORMONES
(Students are encouraged to look at the list of roles on page 33 of the text, but it is not necessary to memorize this list.)