<prompt> The primary action of Nitric oxide NO in the gastrointestinal tract is:VasodilatationVasoconstrictionGastrointestinal smooth muscle relaxationGastrointestinal slow smooth muscle contraction  <response>: C i.e. Gastrointestinal smooth muscle relaxation - Nitric oxide NO is synthesized from arginine 02, NADPH by NO synthase requiring FMN, FAD, herne BH4 in nerves and vascular endothelial cells hence called EDRF. It relaxes smooth muscles by increasing cyclic GMPQ. - Nitric oxide NO and atrial natriuretic factor ANF are hormones that act through c-GMPQ. - NO along with VIP and PACAP are inhibitory neurotransmitter in enteric gastrointestinal neurons that primarily mediate gastrointestinal smooth muscle relaxation c-GMP pathwayQ. 