Regulation by nitric oxide and prostaglandins of endothelium-independent relaxation in two aortic segments

Segmental aortic vascular reactivity differences have previously been reported. Also, a negative feedback effect of nitric oxide (NO) on nitric oxide synthase (NOS) has been postulated. In order to establish if inhibition of endogenous NOS or cyclooxygenase (CO) modifies the relaxation induced by Na⁺ nitroprusside (NaNP), an exogenous source of NO, and if such modification is the same in two aortic segments, concentrationresponse curves to NaNP were done in the thoracic and abdominal segments of the rat aorta in the absence and presence of indomethacin, L-NAME or both. CO and NOS inhibition did not significantly modify the relaxation curves in the abdominal segment. However, indomethacin as much as L-NAME increased NaNP relaxation in the thoracic aorta. Nevertheless, simultaneous inhibition of CO and NOS decreased the relaxation in this segment. These results confirm that there are important differences in the contractile and relaxant mechanisms between the thoracic and abdominal segments of the main conductance vessel of the rat. Also they suggest a negative interaction between endogenous and exogenous nitric oxide and between prostaglandins and exogenous source of NO.