After pretreatment with 10 μM IBMX for 5 min, aortic segments were replaced in Ca2+-free Krebs’ (2.5 mM EGTA) buffer or treatment with inhibitors of l-NAME and methylene blue (MB) for 10 min and then sodium nitroprusside (SNP), acetylcholine (ACh) and PV were added for another 2 min. The aortic rings were immersed in liquid nitrogen to stop the reaction. cGMP formation were measured. Data are expressed as the mean ± standard error of the mean (S.E.M) of more than 5 individual experiments. –, not determined. * p < 0.05 indicates a significant difference from the control.