| Summary: | In Dahl salt-sensitive (SS) rats, impaired vascular relaxation can be restored by: (<b>1</b>) minipump infusion of a low (sub-pressor) dose of angiotensin II (ANG II) to restore physiological levels of plasma ANG II, (<b>2</b>) inhibition of 20-HETE production, and (<b>3</b>) introgression of a normally functioning renin allele from the Brown Norway rat (SS-13<sup>BN</sup> consomic rat). Unlike SS rats, SS-13<sup>BN</sup> rats have normal levels of ANG II on a normal-salt diet and suppressed ANG II on a high-salt (HS) diet. This study tested whether chronically low ANG II levels in SS rats upregulate cytochrome P450-4A (CYP4A) increasing the production of the vasoconstrictor 20-HETE. Although salt-induced suppression of ANG II levels increased reactive oxygen species (ROS) in basilar arteries from SS-13<sup>BN</sup> rats in previous studies, this study showed no change in vascular 20-HETE levels in response to ANGII suppression. CYP4A inhibition significantly reduced vascular ROS levels and restored endothelium-dependent relaxation in response to acetylcholine in the middle cerebral artery (MCA) of SS rats and HS-fed SS-13<sup>BN</sup> rats. These data demonstrate that both the renin–angiotensin system and the CYP4A/20-HETE pathway play a direct role in the vascular dysfunction of the Dahl SS rat but are independent of each other, even though they may both contribute to vascular dysfunction through ROS production.
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