Cyclooxygenase-2 and the renal vascular effects of arachidonic acid in the diabetic rat Quilley, J. and Chen, Y-J. Department of Pharmacology, New York Medical College, New York, USA.

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Arachidonic acid (AA) elicits cyclooxygenase (COX)-dependent vasoconstriction in the rat isolated perfused kidney that is mediated by endoperoxides (Quilley et al., 1989). We also reported enhanced renal vasoconstrictor responses to AA in the diabetic rat that were associated with increased release of prostaglandins (Quilley and McGiff, 1990), suggesting increased COX activity. However, these studies were conducted before the recognition of multiple COX isoforms. As renal COX-2 expression is reportedly increased in the diabetic rat (Komers et al., 2001), we examined the role of COX-2 in renal vasoconstrictor responses to AA in diabetic and control rats by contrasting the effects of a selective COX-2 inhibitor, nimesulide, and a non-selective inhibitor, indomethacin.

Diabetes was induced with streptozotocin (70mg/kg iv) and control rats were given the vehicle, citrate buffer. 4-8 weeks later, kidneys from diabetic and age-matched control rats were perfused with oxygenated Krebs' buffer (37^oC) at constant flow to obtain perfusion pressures (PP) of 60-90 mmHg. Vasoconstrictor responses to AA were determined under control conditions or following treatment with nimesulide (5µM) or indomethacin (10µM). As an index of AA conversion, release of 6-ketoPGF₁was determined by EIA in 1 min perfusate samples collected immediately before and after the administration of 1µg AA. In some of the samples we also determined levels of 20-HETE by GC-MS as vasoconstrictor responses to this eicosanoid in the rat kidney also exhibit COX-dependency (Askari et al., 1997).

One, 3 and 10µg AA increased PP by 85±37, 186±6 and 161±29mmHg, respectively, in diabetic rat kidneys (n=5) compared to 3±1, 17±8 and 74±18mmHg, respectively, in the control rat kidneys (n=7). The increased vasoconstrictor response to AA in diabetic rat kidneys was associated with greater release of 6-ketoPGF₁ which increased by 3.37±0.59ng/min after 1µg AA compared to 1.46±0.50ng/min for the control group. Inhibition of COX with indomethacin virtually abolished the vasoconstrictor effect of AA in both groups (n=3) and prevented the associated increase in the release of 6-ketoPGF₁. In contrast, the COX-2 inhibitor, nimesulide, was without effect on the renal vasoconstrictor effect of AA in control rats (n=7) but reduced the effect of 1mg and 3µg AA by 77% and 42%, respectively, in the diabetic rat (n=8). Nimesulide did not affect basal release of 6-ketoPGF₁ in either group but reduced that stimulated by 1µg AA to 0.64±0.15ng/min and 1.46±0.50ng/min in the control and diabetic groups, respectively. These results indicate that COX-2-derived prostanoids contribute to the vasoconstrictor response to AA in the diabetic rat kidney and are consistent with Western blot analysis of renal cortical homogenates that showed a 3-fold increase in COX-2 protein expression in the diabetic rat (n=5) versus the control rat (n=4). In contrast to 6-ketoPGF₁, release of 20-HETE from the diabetic rat (n=5) kidney was greatly reduced (0.34±0.06ng/min) compared to that from control rat (n=6) kidneys (2.00±0.66ng/min) and was not increased in response to AA.

In summary, renal COX-2 expression is increased in the diabetic rat and contributes to the vasoconstrictor effect of AA in the diabetic, but not the control, rat kidney. Measurements of 20-HETE do not support a contribution of this eicosanoid to the enhanced vasoconstrictor effect of AA in the diabetic rat.

Askari et al. (1997) J. Pharmacol. Exp. Ther. 282:101-107.
Komers et al. (2001) J. Clin. Invest. 107: 889-897.
Quilley et al. (1989) Br. J. Pharmacol. 96:111-116.
Quilley and McGiff (1990) Br. J. Pharmacol. 100:336-340.