Impairment of relaxations to acetylcholine and nitric oxide by a phorbol ester in rat isolated aorta

1 This study compared the abilities of acetylcholine (ACh) (endothelium‐dependent) and nitric oxide (NO) (endothelium‐independent and which may be the active component of the endothelium‐derived relaxing factor) to relax rat isolated aortic rings contracted with equi‐effective concentrations of noradrenaline (NA) or phorbol 12‐myristate 13‐acetate (PMA). 2 ACh and NO induced concentration‐dependent relaxations of aortic rings contracted with NA (EC70 value: 0.2 μm). However, relaxations to both ACh and NO were markedly reduced in rings contracted with PMA (EC80 value: 0.5 μm). NO‐induced relaxations of tissues were not affected by removal of the endothelium, but ACh‐induced relaxations were confirmed to be endothelium‐dependent. 3 ACh (10 μm) induced a 10 fold increase in guanosine 3′:5′‐cyclic monophosphate (cyclic GMP) levels above control values in aortic rings contracted with NA (0.2 μm), but did not affect cyclic GMP levels in rings contracted with PMA (0.5 μm). 4 NO (3 μm) induced a 100 fold increase in cyclic GMP levels above control values in aortic rings contracted with NA (0.2 μm), but only an 11 fold increase in tissues contracted with PMA (0.5 μm). 5 It is concluded that the action (s) of EDRF (NO) are impaired in the presence of PMA by a mechanism that may involve the stimulation of protein kinase C in vascular smooth muscle cells.