Exam 2 (Lecture 1) - Intro to Endocrine Physiology Flashcards
Explain Berthold’s experiment (observations/conclusion).
Took place in 1894 with chickens
Observation #1: Removal of testes (before puberty) leads to caponization (female-like) phenotype
Observation #2: Transplanted testes supported the development of male phenotype. This effect was not mediated by nerves, because he had cut the nerves before implantation
Conclusion: Postulated existence of a substance that travels through the bloodstream to target organs = (hormones); discovered presence of testosterone, even though it wasn’t known yet
What do you need to be able to do in order to define what a hormone is?
Answer these 3 questions:
1) What is it?
2) What it its chemical nature?
3) What does it do at the target tissue?
So…
What is it?
A signaling molecule released by a cell and conveyed by the blood stream, by neural axons, or by local diffusion to cells in target tissues.
What is its chemical nature?
Protein, peptide, catecholamine, steroid or iodinated tyrosine derivative.
What does it do at the target tissue?
Regulates existing metabolic pathways (through second messengers) OR
Regulates synthesis of enzymes and other proteins at the DNA level.
**In this way, it regulates the rates of specific reactions without itself contributing energy or initiating the process (ie hormones DO NOT start the process; they REGULATE it)
Hormones
Products of cells (signaling molecules)
Endocrine secretion
Hormone-releasing cells secrete hormones into the internal environment
Endocrine cells
Cells that release hormones into the environment in which they live (interstitial fluid); can be found in endocrine glands or scattered throughout tissues
Endocrine glands
Specialized glands made up of endocrine cells; hormones leave here and can diffuse into blood, for example, and travel to target tissues
Endocrine system
The collection of endocrine glands and other endocrine cells
Endocrinology
Sub-discipline of physiology that studies the endocrine system
Exocrine glands
Release secretions outside the body; typically in ducts first and then out to the outside. (Example is acinar glands in pancreas (main part of pancreas) that dumps hormones into duodenum)
What is the function of the endocrine system?
To regulate:
1) Metabolism
2) Fluid status
3) Growth
4) Sexual development
5) Reproduction
*The endocrine and nervous systems work together to maintain homeostasis
What is an autocrine cell?
A cell that has receptors for the hormone it releases; hormone acts on the cell that released it
What is a paracrine cell?
Hormone acts on adjacent cells without entering the blood stream (based on diffusion)
VERY common type
What is an endocrine cell?
Before reaching the target cells, hormones first enter the blood stream (via exocytosis); then diffuse out of the blood to act on target cell(s)
What is a neurocrine (neuroendocrine) cell?
Hormone secreted by neuron; interaction between neurons and endocrine cells
What are the 5 categories of hormones?
1) Peptide, protein, and glycoprotein hormones
2) Catecholamine hormones
3) Thyroid hormones
4) Steroid hormones
5) Non-traditional hormones (cytokines, prostaglandins)
What are characteristics of peptide and protein (and glycoprotein) hormones?
1) Synthesis: DNA –> mRNA –> preprohormone –> pro hormone –> hormone (ie larger molecule cleaved to smaller molecule)
2) Storage: in secretory granules (vesicles) originating from Golgi complex
3) Secretion: Exocytosis
*Water or lipid-soluble
What are examples of peptide and protein hormones?
Peptides: vasopressin, oxytocin, glucagon
Proteins: insulin, growth hormones, prolactin
What are characteristics of catecholamine hormones?
1) Synthesis: From amino acids and tyrosine
2) Storage: In secretory granules (vesicles)
3) Release: By exocytosis
*Soluble in water; determines how quickly it can diffuse (ie interstitial fluid is water-based so would diffuse well/quickly)
What are examples of catecholamine hormones?
Epinephrine, norepinephrine, dopamine
What are characteristics of thyroid hormones?
1) Synthesis: From tyrosine and iodide (most synthesis is extracellular)
2) Storage: Extracellularly in follicles of thyroid gland
3) Secretion: Retrieval from the follicle and enzymatic release from the storage protein
*Lipophilic (hydrophobic) - transported in plasma where they are bound to carrier proteins; they are charged and thus CANNOT cross membranes freely; their uptake is mediated by transporters
What are examples of thyroid hormones?
Thyroxine (T4) and triiodothyronine (T3)
What are characteristics of steroid hormones?
1) Synthesis: From cholesterol
2) Storage: Not stored in the gland of origin or elsewhere - the increase of secretion is achieved by mobilizing the synthesis from cholesterol
3) Release: Transported in plasma where they are bound to carrier proteins
*Lipophilic, but CAN cross cell membranes easily
What mechanism controls hormone secretion?
Negative feedback.
It’s a closed loop; the outcome of the process has affect on the beginning (inhibition is the effect)
Common, stable, and CRITICAL for the maintenance of homeostasis
Positive Feedback
Rare; controls surges of hormones
The outcome of the process has affect on the beginning to produce more of the same outcome (produce more hormone)
Rare, but sometimes necessary
Unstable
Used when a surge of hormone is required, such as luteinizing hormone surge before ovulation
What happens to hormones after they are released to the extracellular fluid?
After release into the ECF, hormones circulate either free or bound to other plasma constituents
Eventually, hormones are taken up be cells and are metabolically degraded, excreted in urine, or excreted in bile
What is the hormone half-life time?
The time during which the hormone loses 50% of its biological activity
t1/2 varies between hormones. Peptide/proteins range from minutes to tens of minutes; vitamin D is ~ 15 days
