Endocrine System: Blood Levels & Pituitary Anatomy (Part 2)

Endocrine System - Chapter 16 Part II

• The following endocrine topics are covered:
  • Blood levels (transport, duration, removal)
  • Target cell interactions
  • Release control
  • Pituitary gland anatomy
  • Anterior pituitary hormones

Blood Hormone Levels

• Hormones circulate in the blood in two forms: bound and free (unbound)
• Steroid hormones attach to blood carrier proteins and are considered bound
• Most peptide hormones lack carrier proteins and circulate freely
• An exception is thyroid hormone, which is carried by thyroxine-binding globulin (TBG)
• Hormone blood concentration reflects the rate of hormone release and the speed of inactivation or body removal
• Hormone half-life refers to the length of time required for a hormone's blood level to decrease by 50%
• Water-soluble hormones have a short half-life (seconds), while lipid-soluble hormones have a longer half-life (hours to days)
• Hormones can be removed by target cell enzyme degradation or kidney/liver action, or urine excretion
• Very low hormone blood concentrations are sufficient for their effects
• A narrow desirable range of hormone concentration exists in the blood

Target Cell Interaction

• Hormones interact with each other in a variety of ways at target tissues, which have different receptors for each target tissue
• Permissiveness describes when one hormone cannot exert its full effects without another hormone being present
  • Reproductive hormones and thyroid hormones must be present for timely sexual development
• Synergism describes when one hormone's effects amplify another hormone's effect
  • Glucagon + epinephrine + cortisol = large increase in blood glucose, having a long-acting effect
• Antagonism is when one or more hormones opposes the action of another hormone
  • Glucagon increases blood glucose level, while insulin decreases it

Hormone Release Control

• Several stimuli trigger hormone secretion or release of hormones
• Stimulus types include humoral, neural, and hormonal
• The hormone is secreted in response to changing ion or nutrient blood levels, which is a concentration effect
  • Low blood Calcium causes parathyroid hormone (PTH) release
• Neural stimuli describes when direct nerve fiber innervation stimulates hormone release
  • Sympathetic fibers stimulate the adrenal medulla to release epinephrine (adrenaline)
• Hormonal describes when a hormone released by one gland causes another gland to release hormones
  • TRH from the hypothalamus causes the pituitary to release TSH which causes the thyroid to release thyroid hormone (TH)
• Most synthesis/release is controlled by a negative feedback system
• Very few utilize a positive feedback

Pituitary (Hypophysis)

• The unusual bi-lobed pituitary gland composed of two different tissues, secretes 8 major hormones
• It is connected to the hypothalamus by a stalk-like infundibulum
• The lobes include the neurohypophysis (posterior lobe) and the adenohypophysis (anterior lobe)
• The neurohypophysis (posterior lobe) is composed of neural tissue originating in the hypothalamus
  • It stores/releases hypothalamic-produced peptide hormones
  • It does not synthesize any hormones
• The adenohypophysis (anterior lobe) is composed of glandular tissue not originating in the hypothalamus
  • It produces/secretes several peptide hormones

Pituitary-Hypothalamic Relationships: Overview

• There are different relationships between the pituitary and the hypothalamus depending on whether you are looking at the posterior or anterior pituitary

Pituitary-Hypothalamic Relationships: Posterior

• Neurohypophysis (Posterior Pituitary):
  • Formed by downward growth of brain hypothalamic neural tissue
  • Stores/releases two hypothalamic peptide neurohormones: oxytocin and antidiuretic hormone (ADH)
• Hypothalamic-Hypophyseal Tract:
  • Neural transport pathway for the delivery of oxytocin/ADH to the posterior pituitary
  • Hypothalamic nuclei fibers extend down the infundibulum into posterior pituitary
  • Hypothalamic nuclei directly causes the release of stored ADH and oxytocin hormones

Pituitary-Hypothalamic Relationships: Anterior

• Adenohypophysis (Anterior Pituitary)
  • Upward growth of oral mucosa with no direct hypothalamic contact
  • Has 6 different cells types that synthesize/release 6 peptide hormones: GH, TSH, ACTH, FSH, LH, and PRL
• Hypophyseal Portal System:
  • Local blood capillary system linking the hypothalamus with the anterior pituitary
  • Hypothalamic hormones use capillaries to reach and stimulate the anterior pituitary cells
  • Hypothalamic hormones indirectly cause production/release of anterior pituitary hormones

Anterior Pituitary: Hormonal Regulation

• Hypothalamus sends its peptide "signaling" hormones to anterior pituitary via hypophyseal portal system
• They include releasing hormones which stimulates hormone synthesis/release
  • An example is growth hormone releasing hormone (GHRH)
• Inhibiting hormones are also used that shut off that hormone synthesis/release
  • An example is growth hormone inhibiting hormone (GHIH)
• Several anterior pituitary hormones regulate activity of other endocrine glands
• Hypothalamic releasing hormones don't always have a corresponding hypothalamic inhibiting hormone

Anterior Pituitary: Stimulation of Other Glands

• The anterior pituitary releases tropic hormones that stimulate other endocrine glands produce hormones
  • Thyroid-stimulating hormone (TSH) stimulates the thyroid to release thyroid hormone
  • Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to release corticosteroids
  • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) act on gonads to release estrogen/testosterone
• All anterior pituitary hormones are amino-acid based peptides
• Tropic means "turning towards" so the anterior pituitary hormone TSH "turns towards" the thyroid gland

Growth Hormone (GH, Somatotropin)

• Characteristics include:
  • Anabolic hormones produced by anterior pituitary somatotropic cells, which mobilizes stored fat while conserving glucose
  • Feedback is regulated by antagonistic hypothalamic hormones
  • Hypothalamic GHRH causes anterior pituitary GH production/release
  • Hypothalamic GHIH (somatostatin) shuts off pituitary GH production
• Functions
  • Increases body size/cell division (most cells)
  • Promotes bones/skeletal muscles
  • Increases the synthesis of growth-promoting insulin-like growth factors (IGF) or somatomedins to promote skeletal growth

Thyroid Stimulating Hormone (Thyrotropin)

• TSH characteristics include:
  • TSH is produced by anterior pituitary thyrotrope cells, stimulates normal thyroid development/secretory activity
  • Hypothalamic thyrotropin-releasing hormone (TRH) triggers TSH release by anterior pituitary
  • TSH relies on two negative feedback mechanisms that do not include an inhibiting hormone
    • Excess thyroid hormone (TH) in blood acts on both pituitary & hypothalamus glands blocking further TSH & TRH release, respectively
    • GHIH reinforces inhibition of TRH release
• TSH function:
  • Causes thyroid gland to produce/release thyroid hormones (TH)