Endocrine System Overview

Introduction

• The function of the endocrine system is to provide a method of communication across the body

• The four structural components of the endocrine system are the endocrine glands, hormones, blood, and target cells

  • The effectors of the endocrine system are the target cells

• Endocrine glands: Ductless organs or groups of cells that secrete hormones directly intothe blood or other body fluids

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• Hormonal control systems help regulate homeostasis, stress responses, growth, and reproductive function

• True: T or F, A single gland can secrete more than one hormone

Structural Classes of Hormones

• The three major structural classes of hormones are amines, peptides/proteins, and steroids

• Amine Hormones

  • Amine: Hormones derived from the amino acid tyrosine

  • Amine hormones include thyroid, catecholamines, and dopamine

  • Thyroid hormones: Amine hormones from the thyroid gland

  • Catecholamines: Amine hormones form the adrenal medulla, such as epinephrine

  • Dopamine: An amine hormone secreted by the hypothalamus

• Peptide/Protein Hormones

  • Peptide and protein hormones are the most numerous class of all hormones

  • Peptide and protein hormones are primarily polar molecules

  • Synthesis

    • Polypeptides are packaged into preprohormones by the rough ER, then into prohormones by the Glogi apparatus, and then the hormone is secreted by exocytosis

• Steroid Hormones

  • Steroids: Hormones synthesized from cholesterol by gonads, adrenal cortex, and the placenta

  • Steroid hormones are secreted via diffusion through the plasma membrane once they are made

  • Steroid hormones circulate in the plasma and they are bound to plasma proteins (aka albumin)

Hormone Synthesis

• Adrenal Glands: Paired glands that sit atop the kidneys and are known for hormone synthesis

  • The adrenal glands have two separate areas, the adrenal medulla, and the adrenal cortex

  • Adrenal medulla: A modified sympathetic ganglion that releases catecholamines in resposne to sympathetic activation

    • The adrenal medulla releases about 20% norepinephrine and 80% epinephrine, which bind to adrenergic receptors

  • Adrenal cortex: Outer region of the adrenal gland that produces steroid hormones

    • Aldosterone: A mineralocorticoid that regulates sodium, potassium, and hydrogen ions to help with water balance; relelased by the adrenal cortex

    • Cortisol: A glucocorticoid that regulates metabolism of glucose and other nutrients to deal with stress response; released by the adrenal cortex

    • Adrostenedione: A less potent version of testosterone, used in sexual development; released by the adrenal cortex

• The gonads (the testes and ovaries)

  • Gonads: Produce steroids that are important for sexual development and reproductive function

  • Testes: Mainly secrete testosterone and small amounts of estrogens; often converted to estradiol in target tissues by way of aromatase

    • Aromatase helps convert testosterone into estradiol

  • Ovaries: Secrete estradiol (estrogen) and small amounts of testosterone, as well as progesterone

    • Progesterone can be secreted by the corpus lteum in ovulation or the adrenal cortex

Hormone Signaling & Action

Hormone Transport in Blood

• The structural components of the endocrine system are endocrine glands, hormones, blood, and the target cells (effectors)

• Catecholamines & protein hormones are water soluble and are easily transported by being dissolved in the plasma after being released during exocytosis

• Steroid hormones are nonpolar, so they attach to plasma proteins (in a hormone-protein complex) to travel through the blood after being secreted via diffusion

  • Most hormones are bound to carrier proteins, but a very small amount are not. This free hormone concentration is what ends up binding to receptors in target cells.

  • Some of the protein bound hormones in the blood dissacociate to reach receptor cells

• Plasma hormone concentration is affected by the rate of secretion into the blood and the rate of removal from the blood

  • Rate of Removal

    • Hormone removal from the blood is called hormone clearance

    • Hormones that bind to the receptor end up decreasing the free hormone concentration

    • Hormone clearance occurs in the liver and the kidneys

    • In the blood the rate of removal of free proteins happens by enzymatic breakdown or binding

      • Catecholamines take minutes to hours to break down (free hormones, so not as protected)

      • Steroids and thyroids can stay from hours to days (why? they’re bound to transporters so they’re protected)

    • Hormones can be activated by metabolism to increase binding to receptors

Hormone Action

• All tissues are exposed to circulating hormones, but only ones with the hormone receptor will respond

  • Non-polar chemical messengers can bind to intracellular receptors after diffusing through the cell membrane

  • Polar chemical messengers must have receptors built into the cell membrane

• Methods of regulating hormonal response

  • Number of receptors (if a gland oversecretes, the target cells would down-regulate the receptor building so there isn’t too much taken in)

  • Permissiveness: An up-regulation of the number of receptors for one hormone due to the presence of the second hormone, allowing for a larger response

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• Cellular effects of polar hormones

  • Catecholamines and peptide hormones cannot cross the cell membrane, so the receptor must be built into the cell membrane

  • Most polar hormones activate a second messenger system upon binding to a receptor

  • Polar hormones have fast, non-genomic effects (aka changing enzyme activity)

  • Non-genomic: Effects of a hormone that have short-term effects, and the components are already built within the cell

  • Less frequently, polar hormones can have longer-lasting genomic effects that do affect transcription

• Cellular effects of non-polar hormones

  • Steroid and thyroid hormones bind to intracellular receptors after diffusing through the cell membrane

  • Steroid and thyroid hormones form a hormone-receptor complex when the hormone binds inside of the cell

  • The hormone-receptor complex of non-polar hormones act as a transcription factor

  • Less frequently, non-polar hormones can bind to plasma membrane receptors to exert non-genomic effects

Control of Hormone Secretion

• Hormone secretion is regulated by changes in plasma concentration of subsances, neurotransmitters released by neurons synapsing upon endocrine cells, or other hormones

• Humoral Control: A modality of control for homone secretion where ion/nutrient concentrations within the blood act as the stimulus for hormone release

• Neural Control: A modality of control for homone secretionl where neurotransmitter release from the autonomic nervous system can influence hormone release from many endocrine glands

• Hormonal Control: A modality of control for homone secretion where one hormone can signal the release of a second hormone from a different endocrine gland

  • Tropic: The hormone that singals the release of a second hormone from a different endocrine gland

Hormone System Regulation & Endocrine Disorders

The Hypothalamus & Pituitary Gland

• The hypothalamus is part of the diencephalon & works with the pituitary gland

• Infundibulum: The pituitary gland and the hypothalamus are connected by this tissue.

  • The infundibulum includes axons and mood vessels to connect the hypothalamus and the pituitary gland

• The anterior pituaitary glands have more emphasis on blood vessels for hormone transport

• Hypothalamus controls the release of anterior pituitary hormones via release of hypophysiotropic hormones

  • Hypophysiotropic refers to hormones from the anterior pituitary gland that causes the release of another hormone

  • Portal system: This modality includes veins are in between two separate capillary beds are portal veins that make up the system that supports hypophysiotropic hormone transport

    • Pros of the portal system include that it’s faster & less diluted instead of travelling all the way through the heart and wait for a wrap around back to effectors

Three-Hormone System

• The first hormone in a three-hormone system is released from the hypothalamus and has a hypotropic effect on the anterior pituitary gland

• The second hormone in a three-hormone system is released from the anterior pituitary gland and effects the third gland

• The third hormone in a three-hormone system is released from a gland, that targets the effector

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• Hypophysiotropic hormone secretion is controlled by neural inputs & influenced by sensory info, biological rhythms, and developmental stages

• Long-Loop: The third hormone in a three-hormone system can have a negative feedback on the hypothalumus or the anterior pituitary, also called ______ negative feedback

  • Short-loop: The second hormones can exert negative feedback on the hypothalumus, called _________ negative feedback

Mechanisms of Endocrine Disorders

Hyposecretion issues

• Hyposecretion: There is a reduction in the amount of the hormone within the plasma, causing an impaired response

• Primary hyposecretion: The reduction of hormones in the plasma is caused by the hormone-producing gland (the third hormone in sequence), caused by destruction of the gland, enzyme deficiency, or dietary deficiency

• Secondary hyposecretion: There is too little stimulation by a tropic hormone, causing an unsafe reduction in hormone concentration; the second hormone is not doing enough so the final hormone cannot be released

• If there are high levels of the tropic hormone, the hyposecretion is probably primary

• If there are low levels of the tropic hormone, the hyposecretion is probably secondatry

Hypersececretion issues

• Hypersecretion: Increased circulating hormone concentrations

• Primary hypersecretion: The gland is producing too much hormone

• Secondary hypersecretion: Excessive stimulation by the tropic hormone on the final hormone producer

Hyporesponsiveness issues

• Hyporesponsiveness: The cell has a diminished response to hormonal inputs, despite a proper amount of hormone being present

• Hyporesponsiveness can be caused by not enough receptors, dietary issues, or the signal transduction mechanisms

Hyperresponsiveness issues

• Hyperresponsiveness: The cell has a very much increased response to hormonal inputs, although hormone levels are normal