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Introduction:
a general overview of the
endocrine system
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The Endocrine System
Hormone =
Types:
• peptide or protein = at least 3 amino acids
• steroid = derived from cholesterol
• amine = derived from single amino acids (tryptophan, tyrosine)
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Hormones and Target Cells
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Peptide Hormones
Synthesis/transport/half-life =
Storage?
Multiple processing patterns for protein hormones
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Because peptides are impermeable, they must use membrane receptors and second messenger signal transduction mechanisms to produce the desired effects.
Most use g-protein coupled receptors, but some use tyrosine kinase type receptors (i.e. insulin)
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Steroid Hormones
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Steroid hormone synthesis/storage/half-life
Mechanism of cellular activation?
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The Endocrine Pancreas
The gross anatomy of the pancreas
A pancreatic islet surrounded by exocrine-
secreting cells
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The Pancreas Secretes Insulin And Glucagon
• The endocrine cells of the pancreas are contained in the pancreatic islets or islets of Langerhans.
• These make up only 1-2% of the mass of the pancreas.
• They are scattered throughout the pancreas.
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Pancreatic Islets
• Alpha cells produce glucagon (a 51 amino acid peptide hormone)
• Beta cells produce insulin (a 29 amino acid peptide hormone)
• Islets heavily invested with capillaries
• Islets innervated by both sympathetic and parasympathetic neurons
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The Insulin-glucagon Ratio Regulates Metabolism
• Insulin and glucagon act in an antagonistic fashion to keep plasma glucose concentrations within an acceptable range (70-110 mg/dL).
• Both are present in the blood most of the time.
• The ratio of the two hormones determines which hormone dominates.
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Insulin Is The Dominant Hormone Of The Fed State
Secretion of insulin is affected by the following factors:
1. Increased glucose concentrations
2. Increased amino acid concentrations
3. Feedforward effects of GI hormones
4. Parasympathetic activity
5. Sympathetic activity
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Insulin Promotes Anabolism
Insulin lowers plasma glucose by:
1. Increasing glucose transport into most insulin sensitive cells
2. Enhancing cellular utilization and storage of glucose
3. Enhancing utilization of amino acids
4. Promoting fat synthesis
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Glucagon Is Dominant In The Fasted State
• Glucagon prevents hypoglycemia.• Glucagon is secreted when plasma
glucose levels fall below 100 mg/dL.• The liver is the primary target of glucagon.• Glucagon stimulates glycogenolysis and
gluconeogenesis to increase glucose output by the liver.
• Glucagon release is also stimulated by plasma amino acids.
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Tissues can be targeted by multiple hormones
Hormones can act synergistically, permissively, or antagonistically
Synergistic effects of hormones on blood glucose concentration