Anaphylaxis

Mechanism for How Epinephrine Relaxes Smooth Muscles

• Epinephrine helps control relaxation and contraction of smooth muscle cells. Contraction occurs through binding of calmodulin to Calcium ions. The Calcium-Calmodulin complex then activates the myosin light chain kinase (MLCK), which in turn phosphorylates the myosin light chains to cause contraction.

• When epinephrine binds to an epinephrine receptor it activates adenylyl cyclase, which produces cyclic AMP(cAMP) from ATP. Then cAMP activates a protein kinase, which phosphorylates Myosin Light Chain Kinase (MLCK). Phosphorylation inactivates MLCK so it has a lower affinity for the Calcium-Calmodulin complex.

• This stops the downstream signal for muscle contraction, and thus, relaxes the smooth muscle tissue. Epinephrine also decreases the release and membrane permeability of histamine to reduce the effects of histamine (Bunting, 2000).

Mechanism for How Epinephrine Raises Blood Pressure

• In the mechanism of raising blood pressure, epinephrine binds a beta-adrenergic receptor, which changes shape and activates a G-Protein, which activates Adenylyl Cyclase to convert ATP to cAMP. cAMP activates cAMP-dependent protein kinase (PKA).

• In cardiac muscle, PKA phosphorylates calcium channels in the plasma membrane and myosin heads. Phosphorylated calcium channels remain open longer and allow more calcium into the myocyte, which uncovers more myosin-binding sites on actin. This results in more cross-bridges formed, which creates a stronger force in myocyte contraction and a high blood pressure (Davidson College Biology Department, 2006).