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  1. acts at smooth muscle cells of the blood vessel walls
  2. activation of muscarinic receptors in the endothelial cell membrane increases intracellular inositol triphophate (IP3) which releases Ca from internal cytoplasmic stores; increased cytosolic Ca concentration activates nitric oxide synthetase and catalyzes the release of the gaseous free radical nitric oxide from arginine; nitric oxide diffuses through the membrane of the endothelial cells into adjacent smooth muscle cells where it activates soluble guanylyl cyclase; this results in increases in cGMP concentration stimulating cGMP dependent protein kinases which phophorylates muscle proteins leading to muscle relaxation; thus nitric oxide acts as a second messenger – this same mechanism is also seen in other tissues such cerebellar granular cells and Purkinje cells
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