Diabetes and Endocrinology Welcome Introduction & Case Launch Flashcards
Define the term ‘endocrine system’.
A system that integrates and controls organ function via the secretion of chemicals (hormones) from cells, tissues or glands which are then carried in the blood to target organs, distal from the site of hormone synthesis, where they influence the activity of that target organ.
Response may be fast (within seconds) e.g. increased heart rate in response to adrenalin, or slow (over days) e.g. increased protein synthesis in response to growth hormone.
Define the term ‘hormone’.
a
Define the term ‘neurohormone’.
a
Define the term ‘endocrinology’.
a
Describe the ‘life history’ of a typical hormone.
a
Classify hormones according to their chemistry and mechanisms of action upon target cells.
Peptide or protein hormones – composed of chains of amino acids (most common).
Amine hormones – all derived from one of two amino acids (tryptophan or tyrosine).
Steroid hormones – all derived from cholesterol.
State in logical sequence the factors that determine the availability of a hormone or neurohormone to its target cells.
a
List major categories of physiological function that are governed by endocrine pathways and mechanisms.
a
Describe the main routes and types of signal that determine the rates of hormone synthesis and secretion by a typical endocrine cell.
a
Endocrine hormones should not be confused with;
Paracrine chemicals – act local to the site of synthesis, do not travel to distant sites e.g. histamine.
Autocrine chemicals – act on/in the same cell that synthesises the hormone e.g. cytokines.
Exocrine chemicals – released from exocrine glands via ducts to the external environment including the GI tract e.g. saliva, sweat, bile.
Describe endocrine communication.
Hormones travel in the blood to their target organs/tissues.
Tissues detect hormones through the presence of specific receptors for that chemical on/in the cells. No receptor = no response.
Describe neural communication.
Neurotransmitters released from presynaptic neurons travel across the synaptic cleft to the postsynaptic cell to influence its activity.
Describe how endocrine and neural communication contrast one another.
Neurotransmitter is the chemical released by the neuron and acts locally within the synaptic cleft.
In contrast, hormones released by endocrine cells act distally.
Why do the endocrine and neural communication cooperate?
The endocrine and nervous systems co-operate intimately to provide further control, particularly for long-term phenomena, e.g. growth.
Describe neuroendocrine communication.
The endocrine and nervous systems combine. Nerves release hormones, which enter blood and travel to their target cells e.g. hypothalamic – posterior pituitary axis.
How is the response to a hormone highly specific?
Although all hormones circulate throughout the body in the blood, the response to any one hormone is highly specific because only target cells have receptors for the hormone.
What is the function of hormones (and neurotransmitters)?
The function of hormones (and NTs) is to bring about changes in the activity of their target cells and tissues (increase/decrease a particular activity).
In what systems is endocrine function involved?
Endocrine function is embedded in a number of physiological systems (reproductive, renal, gastrointestinal) as well as being a diffuse system of glands in it’s own right (thyroid gland, adrenal gland, pituitary gland, hypothalamus).
Is the endocrine system anatomically continuous?
No - the various glands form individual functional systems.
Describe the features of an endocrine hormone.
- Produced by a cell or group of cells
- Secreted from those cells into the blood
- Transported via the blood to distant targets
- Exert their effects at very low concentrations (act in the range 10^-9 -10^12 M)
- Act by binding to receptors on target tissues
- Have their action terminated, often via negative feedback loops
Describe the synthesis of a peptide hormone (and give examples).
The initial peptide hormone produced by ribosomes is large and inactive (preprohormone). Preprohormones contain one or more copies of the active hormone in their amino acid sequence.
Preprohormones are cleaved into smaller units in the endoplasmic reticulum to leave smaller but still inactive proteins called prohormones.
Prohormones are packaged into vesicles in the golgi apparatus, along with proteolytic enzymes, which break the prohormone down into active hormone and other fragments.
Hormones and fragments are stored in vesicles in the endocrine cells until release is triggered then all vesicle contents are released into plasma (co-secretion).
e.g. TRH, FSH, insulin.
Describe the clinical use of C-peptide in diabetes.
C-peptide is the inactive fragment cleaved from the insulin prohormone. Levels of C-peptide in plasma or urine are often measured to indicate endogenous insulin production from the pancreas (produced in equal amounts). However, because insulin is metabolised faster, levels of C-peptide are typically about 5x higher than endogenous insulin.
Describe the mechanism of action of peptide hormones.
Water soluble so dissolve easily in plasma making transport via the blood simple and easy. However, water solubility means they cannot cross cell membrane so they must bind to membrane bound receptors on the target cell.
Once bound, these receptors generally create relatively fast biological responses (seconds to minutes). Most peptide hormones work via modulating either the GPCR or tyrosine kinase linked signalling pathways. These pathways phosphorylate existing proteins in the cell and modify their function e.g. open or close ion channels, activate or inactivate enzymes.
State the two receptors through which peptide hormone is transducted.
G protein-coupled receptor - activates 2nd messenger system and/or ion channels leading to modification of existing proteins. Rapid response.
OR
Tyrosine kinase linked receptor: alters gene expression*. Slower, longer lasting activity.
*2nd messengers may also alter gene expression.
