Lecture 5

Hormonal Responses and Receptors

  • Cellular Responses to Hormones

    • Variability of response due to the presence or absence of receptors.

    • Example: “Fight-or-Flight” endocrine response influenced by types of receptors present on target cells.

Overview of Key Glands and Hormones

The endocrine system is composed of several glands that secrete hormones to regulate various functions in the body. Here’s a brief idea of the key glands and hormones:

Pineal Gland

  • Melatonin: Regulates daily rhythms (circadian cycles).

Thyroid Gland

  • Thyroid Hormones: Increases cell metabolism; essential for growth and neural development.

  • Calcitonin: Stimulates incorporation of calcium into bone.

Parathyroid Glands

  • Parathyroid Hormone (PTH): Stimulates release of calcium from bone, absorption by gut and kidney.

Adrenal Gland

  • Cortisol: Mediates metabolic responses to stress.

  • Aldosterone: Involved in salt and water balance.

  • Epinephrine and Norepinephrine: Stimulate immediate fight-or-flight reactions.

Gonads

  • Testosterone (Testes): Development and maintenance of male sexual characteristics.

  • Estrogens and Progesterone (Ovaries): Development and maintenance of female sexual characteristics, supports pregnancy.

Other Hormone-Producing Organs

  • Hormones produced by organs like adipose tissue, heart, kidney, stomach, intestine, liver, and skeletal muscle include leptin, atrial natriuretic peptide, erythropoietin, gastrin, ghrelin, secretin, and insulin-like growth factors.

Hypothalamus

  • Releases and inhibits neuro-hormones that control the anterior pituitary; transports ADH and oxytocin to posterior pituitary.

Endocrine System Regulation and Hormonal Actions

Interaction of Endocrine and Nervous Systems

  • Hypothalamus is composed of neurosecretory cells.

  • The pituitary is divided into:

    • Anterior Lobe (Adenohypophysis): Consists of endocrine cells.

    • Posterior Lobe (Neurohypophysis): Consists of neuroendocrine cells.

    • Neurohormones produced by posterior pituitary include:

    1. ADH (Vasopressin)

    2. Oxytocin

Actions of Antidiuretic Hormone (ADH)

  • ADH Present: Collecting duct is highly permeable to water, resulting in concentrated urine.

  • No ADH Present: Collecting duct is not permeable to water, resulting in dilute urine.

  • The effect of alcohol on urine output:

    • Alcohol Block: Blocks the release of ADH.

    • Alcohol Stimulation: Stimulates release of ADH (note: this point may need further clarity).

Hormonal Feedback and Regulation

Oxytocin

  • Involved in positive feedback mechanisms, especially during labor (refer to Chapter 38).

  • Considerations:

    • How might one induce labor in a late-term pregnancy?:

    • How could one stop premature labor contractions?:

Breastfeeding and Hormonal Interaction

  • Suckling Stimulus: Activates nerves in the nipple and areola that send signals to the hypothalamus.

    • Response: Hypothalamus releases oxytocin (posterior pituitary) and prolactin (anterior pituitary).

  • Effects:

    • Oxytocin: Stimulates milk let-down from mammary glands.

    • Prolactin: Promotes additional milk production.

Anatomical Relationships in Hormonal Regulation

  • Distinction between hypothalamic neurons and their relationship with the anterior and posterior pituitary.

  • Portal Blood Vessels: Carry hypothalamic releasing hormones (RHS) and release-inhibiting hormones (RIHS) to the anterior pituitary.

Hormonal Feedback Loops

  • Typically, negative feedback loops regulate hormone secretion with exceptions, such as posterior pituitary hormones (ADH and oxytocin).

  • Consideration: What might be the effect of steroid intake on this regulatory system?

Stress Response and Hormonal Release

  • Environmental Factors: Trigger responses involving epinephrine/norepinephrine, leading to glucocorticoid production (e.g., cortisol/corticosterone).

  • Effects of Glucocorticoids:

    • Promote protein catabolism (breakdown).

    • Promote lipid catabolism (breakdown).

  • Resulting Products:

    • Utilized by the liver to produce glucose (gluconeogenesis).

    • Inhibit immune and reproductive systems.

Thyroid Gland Anatomy and Hormone Synthesis

  • Components:

    • Calcitonin-Producing Cells, Follicle Cells, and Colloid.

  • Synthesis Steps:

    1. Dietary Requirement: Iodine necessary for hormone production.

    2. Uptake Mechanism: Follicle cells absorb iodide via Na+ cotransport.

    3. Thyroglobulin Production: Synthesized from tyrosine.

    4. Secretion Process: Thyroglobulin and iodide secreted into follicle lumen; peroxidase converts iodide to reactive iodine for iodination.

    5. Endocytosis: Iodinated thyroglobulin uptake and digestion lead to T3 and T4 production.

Thyroid Hormones and Function

  • T3 (Triiodothyronine): More biologically active compared to T4.

  • T4 (Thyroxine): More abundant, has minimal activity, and is converted to T3 in target tissues.

  • Characteristics of Thyroid Hormones:

    • Derivatives of tyrosine (amine hormones).

    • Lipid-soluble, acting through steroid-like receptors.

    • Metabolic rate increase leading to heat production; a role in thermoregulation.

Regulation of Thyroid Hormone Production

  • Involves the hypothalamus secreting Thyrotropin-Releasing Hormone (TRH) which stimulates the anterior pituitary to secrete Thyroid-Stimulating Hormone (TSH), promoting thyroid hormone production (T3 and T4).

Goiters

  • Can occur with either thyroxin underproduction (hypothyroidism) or overproduction (hyperthyroidism).

  • In regions with sufficient iodine, goiters associated with hyperthyroidism often arise due to autoimmune factors or excess stimulation.

  • Query: Consider potential reasons that may lead to the development of a goiter with hyperthyroid states.