Keywords: ترانس 4-هیدروکسی پرولین; ANP; atrial natriuretic peptide; Arg(Me); NÏ-methylarginine; cGMP; cyclic guanosine monophosphate; Cha; 3-cyclohexylalanine; GC; guanylate cyclase; hANP; human ANP; Hyp; trans-4-hydroxyproline; LC/MS/MS; liquid chromatography-tandem mass spectrometry;