Antihypertensive Actions of Moderate Hyperbilirubinemia: Role of Superoxide Inhibition

Abstract

Our lab has previously demonstrated that moderate (~ 2 fold) increases in plasma unconjugated bilirubin levels are able to attenuate the development of Angiotensin II (Ang II) dependent hypertension. In order to determine the specific role of decreases in superoxide production to the blood pressure lowering affects of moderate hyperbilirubinemia (MHyB), we performed the current study in which the NADPH oxidase inhibitor, apocynin, was given to Ang II infused mice in the presence and absence of moderate hyperbilirubinemia. Apocynin (14 mM) was administered in the drinking water prior to treatment with UGT1A1 antisense morpholino (ASM; 16 μg/kg) which was administered via intravenous injection every third day. Treatments were started before the implantation of Ang II containing minipumps (1μg/kg/min) and continued throughout the 7 day protocol. Ang II infusion increased blood pressure to 145 ± 2 mmHg. Apocynin treatment alone reduced blood pressure to 135 ± 5 while MHyB alone decreased blood pressure to 118 ± 5 in Ang II infused mice. Prior inhibition of NADPH oxidase with apocynin did not result in a further decrease in blood pressure in MHyB mice which averaged 117 ± 3 (n=6/group). In aortic preparations, apocynin treatment decreased Ang II mediated superoxide production from 2433 ± 120 to 1851 ± 126 RLU/min/mg (n=4/group) which was similar to levels observed in MHyB mice alone 1473 ± 132 or in combination with apocynin 1503 ± 115. Our results indicate that MHyB lowers blood pressure via a mechanism that is independent of the inhibition of superoxide production; however, we cannot exclude the possibility that other antioxidant properties of bilirubin may be important to the antihypertensive actions in Ang II dependent hypertension. This work was supported by grants from the National Heart, Lung and Blood Institute, HL088421 and PO1HL‐51971.