Overview of Biochemical Endocrinology Hormone- "chemical substance secreted by a ductless gland into blood that is transported to a distant target organ whose activity it specifically affects".
Target Tissues- have receptors or specific binding proteins for each
hormone
Properties of Hormones: 1. They are biosynthesized. 2. They operate at vanishingly small concentrations in blood (10-12 to 10-8 M, nano-pico). 3. They have short half lives.
4. They often exert multipoint control and operate at a number of target organs; hormones awaken existing potential in target cells that are preprogrammed to respond. 5. They are feedback regulated by: (a) themselves, (b) the product(s) of their action, (c) the central nervous system.
Chemistry There are several chemically distinct classes of hormones: amine(epinephrine),
peptide (insulin, glucagon) and steroid (cortisol). Insulin, Glucagon, Epinephrine and Cortisol are the Hormones that Control Glucose Homeostasis: Hormone Receptors: Interaction between
hormone and receptor forms the "hormone receptor complex". Strength of binding is expressed as the dissociation constant Kd, the concentration at which the binding sites are half-saturated.
Classification of Hormones by Receptor Properties: Group I: Hormones that bind to intracellular receptors: Glucocorticoids, mineralocorticoids, estrogens, progestins, androgens, vitamin D, thyroid,
retinoic acid Group II: Hormones that bind to cell surface receptors: Group IIA: The second messenger is cAMP: vasopressin, glucagon, b-adrenergic catecholamines, somatostatin, opioids. Group IIB: The second messengers are IP3 (inositol trisphosphate) / Ca2+ and
diacylglycerol (DAG): oxytocin, angiotensin II, a-adrenergic catecholamines. Group IIC: The cell surface receptor posses tyrosine protein kinase activity (intracellular messenger unknown): insulin, growth factors. Group IIC': The cell surface receptor recruits soluble tyrosine kinases: growth hormone. Group IID: The second messenger is cGMP: atrial
natiuretic peptide (ANP). General Features of Hormone Classes, Group I and Group II:
| Feature
| Group I
| Group II
| | Solubility
| Lipophilic
| Hydrophilic
| | Plasma T1/2
| Long (Hours to Days)
| Short (Minutes)
| | Receptor Location
| Intracellular
| Plasma Membrane
| | Signal Mediator
| Receptor-Hormone Comp.
| cAMP,cGMP,Ca2+,IP3,DAG
|
Steroid / Thyroid / Retinoic Acid Hormone Actions: Group I- lipophilic, derived from cholesterol (except thyroid and retinoic acid) Passively diffuse through the plasma membrane of the target cell to associate with their receptors. The hormone-receptor complex is assumed to be the intracellular messenger
Effects of these hormones are quite specific.
Cell Surface Acting Hormone Action: Group II: peptide and amine hormones bind to membrane spanning receptors (7 transmembrane); communicate through second messengers, cAMP (Group IIA) or Ca2+ / IP3 / DAG (Group IIB).
G-Proteins
Review E = The Effector Protein, Adenylate Cyclase R = Receptor, 7-transmembrane cell surface receptor
Note: once the Effector protein is active the second messenger c-AMP is produced
activating Protein Kinase A and starting a "cascade" of events leading to transmission of message and physiological / biochemical responses. Hormones Functioning via IP3 / Ca2+ and DAG: PIP2 =
Phosphatidylinositol-4,5-bisphosphate IP3 = Inositol-1,4,5-triphosphate DAG = Diacylglycerol PLC = Phospholipase C PKC = Protein Kinase C CaM Kinase = Calmodulin Kinase
[1] hormones bind to receptor [2] receptor is coupled to phospholipase C by Gp [3] Phospholipase C splits PIP2 into IP3 and DAG [4] and [5] IP3 mobilizes intracellular (stored) Ca2+ from the ER for activation of CAM [6] and [7] DAG activates
protein kinase C which is further activated by Ca2+ [8]the concerted action of these two kinases elicit cellular responses
Clinical Correlate Bacterial Toxins: Cholera Toxin Symptoms: watery stools, vomiting, cyanotic, low blood pressure,
rapid / weak pulse Treatment: IV solution of elctrolytes, tetracycline Cause: cholera toxin inhibits GTPase preventing breakdown of GTP to GDP and locking the a subunit in the activated state producing a continuous oversupply of cAMP
Pertussis
Toxin Freezes Gi leaving Gs continuallt active, producing an oversupply of cAMP
© Dr. Noel Sturm 2019
Which of the following does not act as a second messenger of the hormone actions?
Sodium does not serve as the secondary messenger for any hormone.
What is the second messenger mechanism of hormone action?
Secondary messengers relay signals from receptors on the cell surface to the target molecules. The secondary messenger systems bind hormones to a receptor that causes a cascade of changes that leads to actions.
Which of the following is not a second messenger in signaling?
Therefore, it has been clear from the above discussion that Sodium is not a second messenger in hormone action.
Which of the following hormones act through a second messenger?
Some of the hormones that achieve their effects through cAMP as a second messenger: adrenaline. glucagon. luteinizing hormone (LH)
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