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How do Alpha receptors work?

  • Alpha receptors mainly function on the vascular smooth muscle wall and are mainly stimulated by norepinephrine.
  • Principally we are concerned with Alpha 1 receptors.
  • Alpha 2 receptors are presynaptic inhibiting norepinephrine release from the nerve terminal promoting vasodilation and postsynaptic promoting vasoconstriction.
  • Alpha 1 receptor activation on vascular smooth muscle activates a G protein which itself activates phospholipase C (PLC)PLC splits phosphatidylinositol into inositol triphosphate (IP3) and diacylglycerol (DAG)IP3 promotes the release of Ca from the SR secondary to Ca induced Ca release
  • The increase in cytosolic Ca binds to the protein calmodulin.
  • The Ca-calmodulin complex activates myosin light chain kinase, which enables the interaction of actin and myosin.
  • DAG activates a protein kinase C that promotes/augments contraction
  • In addition to activating the phosphatidylinositol system, Alpha 1 receptor activation directly opens the Ca channel to promote vasoconstriction.