The results of the present study indicate that administration of the synthetic 20-HETE mimetic 5,14-HEDGE prevents the fall in blood pressure and loss of vascular responsiveness to norepinephrine in rats treated with endotoxin. The beneficial effects of 5,14-HEDGE were associated with a reduction in the levels of nitrite. Moreover, a competitive antagonist of vasoconstrictor effects of 20-HETE, 20-HEDE, reversed the beneficial effects of 5,14-HEDGE in endotoxin-treated rats.
Previous studies have indicated that the ω
-hydroxylation product of arachidonic acid, 20-HETE, formed by enzymes of the CYP4A pathway, is a potent vasconstrictor that contributes to the regulation of vascular tone and blood pressure (2
). NO inhibits the production of 20-HETE (4
), and NOS inhibitors increase the ω
-hydroxylase activity (6
), expression of CYP 4A protein (4
), and 20-HETE production in the kidney (4
). We have also previously demonstrated that the fall in MAP in rats treated with endotoxin is also associated with a decrease in the expression of CYP4A1/A3 protein in the kidney and increased levels of nitrite in serum, kidney, heart, thoracic aorta, and superior mesenteric artery (7
). These effects were prevented by blockade of iNOS. These observations led us to the present hypothesis that inhibition of CYP4A activity and decreased levels of 20-HETE may contribute to the fall in blood pressure and vascular reactivity in rats treated with endotoxin (7
). Supporting this view are the present findings that administration of the 20-HETE agonist 5,14-HEDGE prevented the fall in MAP and increase in HR as well as vascular hyporeactivity to norepinephrine and the increase in the levels of nitrite in serum, kidney, heart, thoracic aorta, and superior mesenteric artery in endotoxin-treated rats. Furthermore, the competitive antagonist of the vasoconstrictor effects 20-HETE, 20-HEDE (8
), reversed all of the beneficial effects of 5,14-HEDGE in endotoxin-treated rats, suggesting that the effects of this compound is caused by its 20-HETE agonist activity. These results suggest that besides maintaining the effects of 20-HETE on vascular tone, 5,14-HEDGE may also prevent the fall in MAP after endotoxin by diminishing the production or bioavailability of NO.
Although there is no evidence to date that 20-HETE has any direct effect on the expression or activity of NOS enzymes, it has been shown that increased production of 20-HETE in the vasculature is associated with endothelial dysfunction and increased vascular tone that contributes to the development of hypertension in animal models (13
). At least three different pathways have been suggested to play a role in these responses including increased vascular expression of subunits of reduced nicotinamide-adenine dinucleotide phosphate oxidase by 20-HETE, leading to production of superoxide (18
), decreased association of endothelial NOS with heat shock protein 90 by 20-HETE, leading to diminished formation of NO and increased formation of superoxide (19
), and increased formation of superoxide directly by 20-HETE in endothelial cells (21
). All of these changes in the vasculature diminish the bioavailability of NO, leading to endothelial dysfunction and hypertension (13
). Therefore, one possible mechanism by which 20-HEDE may increase the basal levels of nitrite in serum and tissues as well as the nitrite production in vascular tissue in rats treated with 5,14-HEDGE and endotoxin may be through antagonizing the stimulatory effect of 20-HETE on the formation of reactive oxygen species to reduce NO production and its bioavailability.
There are conflicting results concerning the effects of endotoxin on the endothelium-dependent and endothelium-independent relaxations in vessels isolated from endotoxemic rats (22
). In this study, 5,14-HEDGE did not prevent the endotoxin-induced decrease in endothelium-dependent relaxations in thoracic aorta and superior mesenteric artery, although it reversed the effects of endotoxin on the fall in blood pressure, vascular hyporeactivity, and overproduction of nitrite in these tissues. Moreover, the 20-HETE antagonist 20-HEDE also had no effect on the endothelial dysfunction in the endotoxemic rats treated with 5,4-HEDGE. In contrast to the findings on acetylcholine, 5,14-HEDGE prevented the endotoxin-induced decrease in endothelium-independent relaxations in thoracic aorta, and its effect was reversed by 20-HEDE. Endotoxin had no effect on the endothelium-independent relaxations in superior mesenteric artery; however, 20-HEDE caused a decrease in the relaxation responses in endotoxemic rats treated with 5,14-HEDGE. It has been shown that increased production of 20-HETE leading to diminished formation of NO and increased formation of superoxide in the vasculature is associated with endothelial dysfunction and increased vascular tone (13
). Therefore, increased production of superoxide in endothelial and/or vascular smooth muscle cells might contribute to the effects of 5,14-HEDGE on the endotoxin-induced decrease in endothelium-dependent and endothelium-independent relaxations in thoracic aorta and superior mesenteric artery.
In conclusion, the present study indicates that administration of the 20-HETE agonist 5,14-HEDGE prevents hypotension and vascular hyporeactivity associated with the changes in NO production in rats treated with endotoxin. Impairment of cardiovascular and renal function is critically involved in the pathophysiological sequela in inflammatory diseases such as septic shock finally resulting in multiorgan failure and death. Restoration of these impaired functions should have therapeutic benefit. These studies suggest that stable analogs of 20-HETE may have therapeutic potential for the treatment of devasting effects of organ hypoperfusion in septic shock.