Endocrinology

Regulatory Mechanism Systems

·      Animals have 2 main systems of cell communication that use chemical signalling: the nervous system and the endocrine system.

·      The nervous system can send high speed chemical signals that are short term and very specific, via specialised nerve cells.

·      The endocrine system sends slower chemical signals, but signals are longer term and less specific as they can affect multiple cells.

-              The endocrine system acts through secretion of hormones through endocrine glands, these hormones effect target cells with appropriate receptors and this produces a response.

Chemical Signalling in Regulatory Mechanism Systems

·      Both the nervous system and endocrine system chemical signals have target cells.

-              These chemicals which send signals are neurotransmitters or hormones.

·      Target cells have specific proteins called receptors, these can bind to neurotransmitters or hormones.

-              Chemical (ligand) binding to a receptor causes a conformational change in cellular activity, activating a signal transduction pathway.

-              Receptors have a high affinity for chemicals that are specific to them

·      Many cells can be a target cell for multiple chemical messages, as they have multiple specific receptors.

·      Endocrine and nervous system are related:

-              Endocrine glands can contain neurosecretory cells that are related to neurons developmentally; they have similar properties.

-              Some chemicals have endocrine and nervous functions, for example, adrenaline (can be a neurotransmitter or hormone, hormone if found in circulatory system…).

-              Systems interplay to maintain homeostasis through negative and positive feedback

Basic concepts in Endocrinology

Endocrine System in Animals

·      Hormones that are secreted by endocrine glands can affect a few or many tissues.

·      Sex hormones effect most cells within the body.

·      Some hormones are known as tropic/trophic hormones, these effect other endocrine glands in the body, e.g. pituitary gland hormone release triggered by hypothalamus.

·      Tropic hormones are central to coordination of physiological processes.

Types and Structures of Hormones

Can be categorised into water-soluble (hydrophilic) or lipid-soluble (hydrophobic)

·      Polypeptides

-              Hydrophilic

-              E.g. insulin

·      Amines

-              Hydrophilic or hydrophobic (lipid soluble)

-              Amino acids

-              E.g. epinephrine (adrenaline) is hydrophilic, thyroxine is hydrophobic

·      Steroids

-              Hydrophobic (lipid soluble)

-              E.g. cortisol

·      Hormones differ in terms of:

-              Size

-              Chemical nature

-              Physical characteristics

Mechanisms of Hormone Chemical Signalling

·      Hydrophobic hormones usually bind to cell surface, plasma membrane receptors. This activates a signal transduction pathway, thereby changing cellular activity.

-              E.g. adrenaline.

·      Lipid-soluble (hydrophobic) hormones will usually bind to intracellular nuclear receptors, these alter gene expression and effect cellular activity this way.

-              E.g. steroid hormones.

·      Hormone signals can have different effects depending on the target cell type.

-              Different cell types can have different receptor classes that can respond to the same chemical.

-              Cells can have different signal transduction pathways for the same chemical.

-              These differences are due to differences in cell structure and proteins present within a cell.

Feedback in Endocrine Signalling

·      Hormones are regulated by feedback

·      In digestion, low pH in the duodenum stimulates secretin release, this signals the pancreas to release HCO3- which raises pH level.

-              Increased pH switches off secretin release. (feedsback)

Hypothalamus and Pituitary Gland

30.01.2025 / Lecture 2

The hypothalamus and the pituitary gland regulate the endocrine system.

·      The hypothalamus is a region of the lower brain, it has functions within the nervous system and endocrine system.

-              The hypothalamus coordinates endocrine system and receives neural information from the body.

-              Initiates appropriate endocrine responses based on internal and external environment.

·      The pituitary gland is regulated by neurosecretory cells from the hypothalamus

-              The pituitary gland has multiple endocrine functions and has 2 parts: anterior and posterior.

Pituitary Glands

·      Posterior Pituitary (PP) Gland

-              Also known as the neurohypophysis, the PP is an extension of the brain

-              Hypothalamic neurosecretory cells terminate within it

-              Hormones released from hypothalamus to posterior pituitary which are then released into the bloodstream in order to effect target cells.

-              Hormones travel from axons of neurosecretory cells in hypothalamus to capillaries of the PP before being released.

·      Posterior Pituitary hormones

-              Anti-diuretic hormone (ADH): regulates kidney function by increasing water retention in kidneys to regulate urine output. Alcohol resets ADH levels.

-              Oxytocin: regulates areas of female reproductive physiology, e.g. milk lactation.

Þ   Response amplifies signal: positive feedback control.

·      Anterior Pituitary Gland

-              Also known as the adenohypophysis

-              Contains endocrine cells that control hormones via inhibitory or releasing signals from the hypothalamus.

-              Neurosecretory cells in the hypothalamus release these hormones into capillaries that link to posterior pituitary capillaries: portal veins.

-              Anterior pituitary hormones released regulate many physiological processes.

Anterior Pituitary Hormones

·      Tropic hormones

-              FSH (follicle-stimulating) and LH: (luteinising): Glycoproteins that stimulate gonads (testes or ovaries)

-              TSH (thyroid-stimulating): stimulates thyroid hormone release

-              ASTH (adrenocorticotropic): regulates adrenal cortex (on top of kidneys), this stimulates production of steroids.

·      Non tropic hormones

-              Prolactin: stimulates mammary gland growth and lactation.

-              MSH: stimulates melanocytes (pigment cells), may be involved in regulation of lipid metabolism.

·      Tropic or non-tropic hormones

-              GH (growth hormone): 200 amino acid protein that stimulates cell growth mainly via stimulation of other protein growth factors

-              Endorphins (?, don’t know if tropic or non): 3 types a, b and g. They inhibit pain and create a sense of wellbeing. Released by exercise, sex, orgasm, music, chocolate. b Endorphin receptors are a target of strong analgesics as well as powerful narcotics like heroin.

·      ASTH, MSH and endorphins are made from a single protein (pro-opiomelanocortin, which target same receptors as opioids).

-              This protein is cleaved into these hormones and some others.

Anterior Pituitary Hormone Feedback Control

·      TRH releases when thyroid hormone drops, TRH causes anterior pituitary to secrete TSH.

·      TSH flows from capillaries to circulatory system, this causes release of thyroid hormones by stimulation of thyroid gland by TSH

-             TSH inhibits release of TRH

-             Thyroid hormones inhibit release of TSH and TRH

-             Negative feedback pathway

Thyroid Hormones

·      T3 (Triiodothyronine) and T4 (thyroxine)

-              Target cells convert T4 to T3 which is more active

·      Thyroid hormones are important in development

-              E.g. metamorphosis of tadpoles into frogs, flatfish development (amphibian metamorphosis)

-              Important for normal development of human brain

-              Regulates blood pressure, heart rate, metabolism, muscle tone, digestion and reproduction

-              Hyper and hypothyroidism have significant health consequences (goitre=hypo, proptosis=hyper).

Growth Hormone

·      Tropic and non-tropic hormone

·      Regulates growth via production of insulin like growth factor, produced by liver.

-             Important in regulating skeletal growth

-             Regulates metabolism directly and on skeletal muscle growth

·      Hyper/hypo secretion has health consequences (gigantism and hyposomatotropism)

Other Hormones and Their Roles

30.01.25 / Lecture 3

Regulation of physiological processes through hormones

·      Regulation of blood calcium

-              Parathyroid glands (4) found on surface of thyroid glands, secretes PTH that elavates blood calcium.

-              PTH stimulates calcium reuptake and activation of vitamin D in kidneys. Also, can stimulate calcium release from bone.

-              Calcitonin is a hormone that reduces blood calcium. It is produced in parafollicular cells in the thyroid gland.

-              PTH and calcitonin release oppose each other’s actions, therefore, these hormones are antagonistic.

·      Regulation of blood glucose

-              Pancreas is heavily involved which has endocrine and exocrine functions.

-              Pancreas endocrine gland secretes hormones insulin and glucagon which are antagonistic hormones.

-              Endocrine cell clusters b cell secretes insulin, asecretes glucagon.

-              Pancreas exocrine …

Blood Glucose Regulation

·      Regulation of blood glucose is highly important

-              3.5/

-              Important for respiration, brain function, nervous system function.

-              Low blood glucose: decreased brain function.

-              Hormones insulin and glucagon work to maintain homeostasis with glucose levels.

·      Insulin stimulates uptake of glucose into most cell types to lower blood glucose.

-              Secreted by beta cells of the pancreas when blood glucose rises

-              Stimulates conversion of glucose into glycogen (muscle and
liver) and triglycerides(adipocytes)

-              Also slows rate of glycogen breakdown

·      Glucagon stimulates breakdown of glycogen in the liver to raise blood glucose

-              Secreted by alpha cells of the pancreas when blood glucose lowers

-              Glucagon also switches off glycogen biosynthesis

·      Diabetes mellitus

-              Deficiency of insulin or lack of responsiveness to insulin

-              Type I: autoimmune, beta cells are destroyed by antibodies leading to no insulin production.

Þ   Treatment through insulin injection.

-              Type II: late onset, reduced cellular response to insulin

Þ   Treatment is diet and exercise control

Adrenal Glands and stress management

·      Adrenal glands are found on top of the kidneys

-              Their primary function is in dealing with short- and long-term stress responses.

-              Adrenal glands consist of cells derived from neural crest

·      Adrenal glands have 2 parts:

-              Medulla: secretes adrenaline and nor-adrenaline (catecholamines) to initiate ‘fight or flight’

-              Cortex: secretes corticosteroids that primarily regulate metabolism and mineralocorticoids.

·      Catecholamines

-              Secreted in response to stress

-              Controlled by nerves(sympathetic nervous system) that release acetylcholine.

-              Catecholamines increase BMR (to allow more energy availability) and glycogen breakdown, stimulate fat breakdown, increase cardiac output and gas exchange.

·      Corticosteroids

-              Produced in response to stress in adrenal cortex

-              Controlled by endocrine signalling from anterior pituitary, this releases ACTH (from hypothalamus signal).

Sex Hormones

·      Sex hormones are steroids, structurally related to corticosteroids.

·      Sex hormones are synthesised in the gonads

-              Testes produce testerorone

-              Ovaries produce oestrogen

Þ   Both drive biological sex characteristic formations

-              Ovaries also produce progestins which are involved in uterus preparation for conception

·      Sex hormones secretion is controlled by hypothalamus/anterior pituitary hormones

-              GnRH, FSH, LH

·      Sex determination

-              Derived from bipotential gonads in the embryo

-              XY = formation of testes

-              Presence or absence of SRY gene on the Y chromosome determines sex.

Þ   SRY induces expression of SOX9 which allows for other male cell formation