Alpha Receptors

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.