Hormone Synthesis and Function

Synthesis and Secretion of Amine Hormones

• Epinephrine and norepinephrine are synthesized in the adrenal medulla and stored in preformed vesicles until secreted.
• These catecholamines are released from adrenal medullary cells by exocytosis.

Characteristics of Hormone Functions

• Hormones have their own characteristic onset and duration of action, e.g., epinephrine and norepinephrine have rapid onset and duration, while thyroxine may take months to have full effect.
• The concentrations of hormones in the circulating blood and hormonal secretion rates are small (range from 1 picogram to a few micrograms).
• Hormonal secretion is controlled by feedback mechanisms, including:
• Negative feedback: The hormone or its product has a negative effect to prevent over-secretion or over-activity.
• Positive feedback: The biological action of the hormone causes additional secretion of the hormone, leading to a surge.
• Cyclical variations in hormone release: Periodic variations in hormone release influenced by development, aging, diurnal cycle, and sleep (e.g., growth hormone and cortisol).

Transport of Hormones in the Blood

• Water-soluble hormones are dissolved in plasma and diffuse into interstitial spaces to target cells.
• Lipid-soluble hormones (steroid and thyroid hormones):
• Circulate in the blood bound to plasma proteins.
• Must dissociate from the carrier and diffuse across capillaries to access target cells.
• Can act as hormone reservoirs.
• Binding of hormone to plasma proteins slows their clearance from the plasma.