Ch. 5: The Endocrine System

• The endocrine system consists of organs, known as glands, that secrete hormones.
• Hormones are signaling molecules that are secreted directly into the bloodstream to distant target tissues. At target tissues, ,hormones bind to the receptors, inducing a change in gene expression or cellular functioning
• Not all hormones have the same structure and function
• Hormones can be classifies by their chemical identities such as peptides, steroids, ,or amino acid derivatives
• Peptide hormones are made up of amino acids, ranging in size from quite small to relatively large as insulin
• Peptide hormones are derived from larger precursor polypeptides that are cleaved during posttranslational modification. These smaller units are transported to the Golgi apparatus for further modifications that activate the hormones and direct them to the correct locations in the cell. Such hormones are released by exocytosis after being packaged into vesicles.
• Bc peptide hormones are charged and cannot pass through the plasma membrane, they must bond to an extracellular receptor. The peptide hormone is considered the first messenger; it binds to the receptor and triggers the transmission of a second signal, ,known as seconds messenger.
• The connection btw the hormone at the surface and the the effect brought about by second messengers within the cell is known as signaling cascade.
• At each step of signaling cascade of peptide hormone, there would be a possibility of amplification. For example, one hormone molecule may bind t multiple receptors before it is degraded. Also, each receptor may bind to multiple enzymes, each of which will trigger the production of large quantities of second messengers. Thus, each step can result in an increase in signal intensity
• Some common second messengers are cyclic adenosine monophosphate (cAMP), inositol trisphosphate (IP3),and calcium
• For peptide hormone for example, the binding of a peptide hormone triggers the receptor to either activate or inhibit an enzyme called adenylate cyclase, raising or lowering the levels of cAMP accordingly. cAMP can bind to intracellular targets such as protein kinase A,, which phosphorylates transcription factors like cAMP response element-binding protein (CREB), an enzyme, to exert the hormone's ultimate effect
• Youtube for signaling cascade, When the peptide hormone binds to the extracellular receptor like G-protein it will activate subunits of alpha and (Gamma/Beta) in which it activates the (GTP-Alpha)which goes and binds to adenylate cyclase in which it would have ATP hydrolyze to cAMP in which the cAMP would activate cAMP dependent kinase A in which the cAMP dkA is able to activate or inhibit the intracellular enzyme by phosphorylating like CREB.
• The effects of peptide hormones are usually rapid but short-lived bc the hormones act through a second messenger cascades, which are transient. It is quicker to turn them on and off, compared to steroid hormones, but their effects do not last w/o relatively constant stimulus
• Bc peptides are hydrophilic, water-soluble, they can travel freely. in the bloodstream without any carriers in which the steroid homones are hydrophobic, lipid-soluble.
• Steroid hormones are derived from cholesterol and are produced primarily by the gonads and adrenal cortex
• bc the steroid hormones are derived from nonpolar molecules, they can easily cross the cell membrane. Hence, their receptors are usually intracellular ( in the cytosol), or intranuclear ( in the nucleus)
• Upon binding to the receptor, a steroid hormone-receptor complex undergoes a conformational change. The receptor can bind directly to DNA, resulting in either increase or decrease transcription of particular genes, depending on the identity of the hormone
• One common form of conformational change is dimerization
• Dimerization is the paring to of two receptor-hormone complexes
• The effect of steroid hormones are slower but longer-lived than peptides hormones bc steroid hormones participate in gene regulation, causing alterations in the amount of mRNA and proteins processed in a cell by direct action on DNA
• Steroid hormones are not water-soluble, so they must be carried by proteins in the bloodstream to travel around the body.
• Some of these proteins are very specific and carry only one hormone such as sex homrone-binding globulin), while other proteins are nonspecific such as albumin
• Hormones are generally inactive while attached to a carrier protein and must dissociate form the carrier to function.
• Amino acid derivative hormones are less common than steroid and peptide hormones, but include epinephrine, norepinephrine, triiodothyronine, and thyroxine. These hormones derived from one or two amino acids, usually with few additional modifications. For example, thyroid hormones are made from tyrosine modified by the addition of several iodine atoms
• Catecholamines ( epinephrine and norepinephrine) bind to G-protein- coupled receptors, while thyroid hormones bind intracellularly.
• Some hormones are known as direct hormones, are secreted and then act directly on target tissue. For example, insulin secreted by pancrease causes increased uptake of glucose by muscle
• Other hormones such as tropic hormone require an intermediary to act. For example, like how GnRH form the hypothalamus stimulus the. release of luteinizing hormone (LH) and follicle-stimulating hormone ( FSH). LH then acts on the gonads to stimulus testosterone production in males and estrogen in females
• GnRH and LH do not cause a direct change in the physiology of muscle, bone, and hair follicles; rather, they stimulate the production of another hormone by another endocrine gland that acts on these target tissues.
• Tropic hormones usually originate in the brain and anterior gland, allowing for the coordination of multiple processes within the body
• For amino acids derivatives the norepinephrine and epinephrine are fast but short lived like peptide hormones. However, triiodothyronine and thyroxine are slow acting and long lasting.
• Most peptide and amino acid derivative hormones have names that end in -in or ine
• Steroid hormones have names that end in -one, -ol, or -oid
• The hypothalamus, the pituitary, the thyroid, the parathyroid glands, the adrenal glands, the pancreas, the gonads ( testes and ovaries), and the pineal gland are all endocrine glands.
• Endocrine glands is capable of synthesizing and secreting one or more hormones. Some of the cells within organs like heart, kidney, thymus, etc
• Hypothalamus, the bridge btw nervous and endocrine systems.
• Hypothalamus regulates the pituitary gland via tropic hormones
• The hypothalamus us located in the forebrain, directly above the pituitary gland and below the thalamus
• Bc the hypothalamus and the pituitary are close to each other, the hypothalamus controls the pituitary through paracrine release of hormones into a portal system that directly connects the two organs
• The release of hormones by the hypothalamus is regulated by the negative feedback
• Negative feedback occurs when a hormone (or product) later in the pathway inhibits hormones ( or enzymes) earlier in the pathway