Nitric  oxide  promotes  the
   relaxation   (dilation)   of  smooth  muscles  around  blood  vessels,
   increasing blood flow.
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   This  action  is  mediated  by  the  second  messenger  cGMP, which is
   converted to GMP by the enzyme PDE5.
<P>
   The  E.D.  drug Viagra inhibits PDE5, thus prolonging the dilation and
   erection.