6b

The Endocrine System: Endocrine Glands and Hormone Actions

Chapter Outline

  • 6.3 Hormone Actions at the Target Cell

  • 6.4 Abnormal Secretion of Hormones

  • 6.5 Hormone Interactions

Learning Outcomes

  • Tropic Hormones: Define the role of tropic hormones in regulating the release of other hormones, including the feedback loops involved in this process.

    • Tropic hormones are hormones whose primary function is to regulate the secretion of other hormones from target glands. They are critical in maintaining homeostasis and ensuring proper physiological functions.

  • Hormone Interactions: Describe the types of interactions between hormones acting on the same target cell, including:

    • Additive interactions

    • Synergistic interactions

    • Permissive interactions

6.3 Hormone Actions at the Target Cell

  • Control of Hormone Levels in Blood: Several factors regulate hormone levels in the blood, including:

    • The rate of hormone secretion: Hormone secretion can vary widely depending on various physiological stimuli.

    • The amount of hormone bound to plasma proteins: Free hormones (those not bound) can freely act on target cells.

    • The rate of hormone metabolism: Hormones are metabolized at different rates depending on their chemical nature and the target cells' activity.

Neural Control of Hormones

  • Direct Neural Regulation of Endocrine Glands: The hypothalamus is key in this process, influencing:

    • Posterior Pituitary: Secretion of Antidiuretic Hormone (ADH) and oxytocin.

    • Adrenal Medulla: Secretion of epinephrine (adrenaline).

Humeral Control of Hormones

  • Humeral Signals: These are blood-borne signals that influence the regulation of endocrine glands:

    • Hormonal signals: The hypothalamus stimulates the anterior pituitary, which in turn affects various endocrine glands through a hormone axis.

    • Ions: Regulation can also occur through the blood levels of ions (e.g., sodium Na+, potassium K+, calcium Ca++) which directly impact hormone secretion.

    • Metabolites: Blood glucose levels serve as a signal for the release of hormones as well (e.g., insulin).

Transport of Hormones

  • Hydrophilic/Lipophobic Hormones: Smooth functioning in the bloodstream:

    • Hormones: Such as peptides and catecholamines; they are dissolved in plasma and have a short half-life (time duration they remain active).

  • Hydrophobic/Lipophilic Hormones:

    • Hormones: Such as steroids and thyroid hormones; they are often bound to carrier proteins in the blood, and only free hormones can bind to receptors and be metabolized.

    • These hormones have longer half-lives due to their binding to carrier proteins.

Rate of Hormone Metabolism

  • Sites of Hormone Metabolism: Hormones can be metabolized at various sites, including:

    • Target Cells: At which they exert their functional actions.

    • Blood: Changes in metabolite levels can be assessed through blood tests.

    • Liver: Major organ for metabolizing hormones.

    • Kidneys: Metabolites are often excreted by the kidneys, observable in urine tests.

    • Adipose Tissue Storage: Some lipophilic hormones can be stored in adipose tissue.

6.4 Abnormal Secretion of Hormones

  • Pathologies:

    • Hyposecretion: Insufficient hormone production (e.g., Diabetes mellitus type 1).

    • Hypersecretion: Excess hormone production (e.g., Gigantism and acromegaly).

  • Abnormal Tissue Responsiveness: Hormones may be present at normal levels, yet the tissue fails to respond appropriately:

    • Hyperresponsive: An overactive response of tissues to normal hormone levels.

    • Hyporesponsive: A diminished response of tissues despite normal hormone levels.

Primary Versus Secondary Secretion Disorders

  • Primary Disorders: Caused by an abnormality within the endocrine organ that produces the hormone.

    • Comparison of blood levels of each hormone to the normal homeostatic range, for example:

    • Corticotropin-Releasing Hormone (CRH)

    • Adrenocorticotropic Hormone (ACTH)

  • Secondary Disorders: Caused by an abnormality in a tropic hormone that regulates another hormone.

    • Determination of blood hormone levels in relation to homeostatic range:

    • Comparative analysis of cortisol and CRH levels based on abnormal ACTH.

Abnormal Secretion of Cortisol: Diagram Description (Figures 6.13a and 6.13b)

  • Primary Hypersecretion of Cortisol:

    • An initial stimulus leading to a physiological response resulting from excess secretion by an abnormal adrenal gland.

    • Negative feedback mechanisms may be inadequate, resulting in continued hypersecretion.

  • Secondary Hypersecretion of Cortisol:

    • Initiated by excess ACTH from an abnormal anterior pituitary. The regulation may be inadequate, resulting in continued excessive secretion of cortisol from the adrenal gland.

    • Hypothalamus regulation may also be inadequate under these circumstances.

Hormone Interactions

  • Combined Actions of Hormones: These can occur in several ways:

    • Antagonism: The effects of two hormones oppose each other (e.g., glucagon versus insulin).

    • Additive Effect: The net effect equals the sum of individual hormone effects. This means the combined effect may not be greater than the sum of effects.

    • Synergism: The effects of two hormones favor each other, resulting in a net effect that exceeds the sum of their individual effects (e.g., glucagon, cortisol, and epinephrine on blood glucose levels).

    • Permissiveness: One hormone must be present for another hormone to exert its effects.

    • Example: Thyroid hormones lead to the expression of beta-adrenergic receptors in bronchiolar smooth muscle, allowing for the action of epinephrine.