Renal vasoactive eicosanoids: interactions between cytochrome P450 and cyclooxygenase metabolites during salt depletion

Preglomerular microvessels (PGMVs) occupy the key position in governing the renal circulation. Characterization of mediators/modulators of PGMVs vasomotion is, therefore, essential to defining renal vascular mechanisms. 20-hydroxyeicosatetraenoic acid (HETE), a potent vasoconstrictor, is the principal eicosanoid produced by PGMVs (arcuate and interlobar arteries) via cytochrome P450 (CYP) monooxygenases and is released by angiotensin II (AII) via stimulation of AT2 receptors that activate phospholipase C. 20-HETE is metabolized by cyclooxygenase (COX) producing prostaglandin (PG) analogs (e.g., 20- OH- PGs), having biological effects that differ from unmetabolized 20-HETE. As low salt (LS) intake increases AII generation and COX-2 expression, we examined 20-HETE synthesis in renal arteries (PGMVs and interlobular arteries) from male Sprague-Dawley rats fed either normal (NS; 0.4% NaCl) or LS (0.05% NaCl) diets for 10 days. With NS intake, 20-HETE was higher (P<0.05) in PGMVs than in interlobar arteries (25.2 ± 3.6 ng vs. 15.3 ± 2.3 ng/mg protein/30 min, respectively). LS treatment, for 10 days, selectively increased interlobar artery 20-HETE (24.6 ± 3.9 ng/mg protein/30 min) and CYP4A levels whereas 20-HETE levels in PGMVs were diminished by 90%; these changes occurred concomitantly with expression of cortical COX- 2, only in LS rats. We have identified COX metabolites of 20-HETE using GC/MS criteria: renal cortical and medullary homogenates formed 19- hydroxy (OH) PGE2, 20- and 19-OH-PGF2α as well as PGE2 and PGF2α. Only LS cortex formed 20-OH-PGF2α. The vasoactivities of OH-PGs were compared to those of authentic standards in the rat isolated kidney, treated with phenylephrine (perfusion pressure (PP) ca 160 mmHg) and INDO (2.8 μM). The OH-PGs had similar dilator potencies to authentic PGs; the PGEs (0.01-1μg) were more active then PGFs, e.g., bolus injection (0.1 μg) of 20-OH-PGE2 and 20-OH-PGF2α vasodilated (decreasing PP by 32±6 mmHg, respectively) whereas 20-HETE (10 μg) constricted (increasing PP by 22±2 mmHg). Thus, LS intake induced 20-HETE formation and increased CYP-4A and COX-2 expression. Diminished levels of 20-HETE in PGMVs was associated with increased cortical COX-2 expression which serves as a metabolic pathway for 20-HETE, forming vasodilator prostaglandin analogs. Further, the cooperative interaction of ω- hydroxylase and COX- 2 is suggested to determinie glomerular hemodynamics in low salt states. We propose that the major adverse effects of NSAIDs result from the direct vasoconstrictor action of unmetabolized 20-HETE.