Veterinary Physiology-III

HORMONE TRANSPORT IN THE BLOOD

• Most water soluble hormone molecules circulate in the blood in the free from (not attached to a protein) Most lipid soluble hormones bind to transport proteins. Binding of hormones to proteins are loose and reversible.
• Transport proteins are albumin and globulins synthesized by the liver and have following functions
  • provide a circulating pool for the hormone
  • improve the transportability of the lipid-soluble hormones by making them temporarily water-soluble
  • limit the availability of the hormone on the target site
  • prolong the half-life of hormone and they protect circulating hormones from being broken down by enzymes in the blood plasma and liver - free hormones may be broken down or cleared from the blood in minutes, whereas bound hormones may circulate for hours or weeks.
  • provide a ready reserve of hormone, since already present in the bloodstream
• Bound hormones are hormones attached to a transport protein and unbounded (free) hormone is one that is not attached to a transport protein. Only free hormones (unbound/released from the binding protein) can leave a blood capillary to influence the target.
• Therefore only free hormones are physiologically active. There is a dynamic equilibrium between the quantities of free and bound forms of any hormone in circulation. As the free hormone leaves circulation to influence the targets, additional bound hormone is released from the binding proteins.
• Example of transport proteins
  • Thyroid hormone binds to three transport proteins
    • albumin
    • albumin-like protein called thyrectin
    • an alpha globulin named thyroxine binding globulin (TBG)
  • Transcortin is the corticosteriod‑binding globulin (CBG), which has high affinity for cortisol and corticosterone,
  • Sex hormone-binding globulin (SHBG) aids transport of estradiol, progesterone and testosterone.