Complex sentence: Nitric oxide activates guanylate cyclase, which then increases cGMP (cyclic-guanosine monophosphate) synthesis, cGMP activates protein kinase G, which leads to increased calcium uptake and the opening of potassium channels (activated by calcium) that then leads to muscle relaxation.
Split into simpler sentences: Nitric oxide activates guanylate cyclase, which then increases cGMP (cyclic-guanosine monophosphate) synthesis. Then cGMP activates in protein kinase G leads to calcium uptake and opening potassium channels (activated by calcium) that then leads muscle relaxation.