Kidneys in hypertensive rats show reduced response to nitric oxide synthase inhibition as evaluated by BOLD MRI.

THE KIDNEY IS BELIEVED TO PLAY A ROLE in the pathogenesis of essential hypertension (1,2). In particular, reduced renal medullary blood flow is thought to be one of the important factors in the development of the disease (3–8). Animal studies have shown that medullary blood flow is decreased in hypertension and, more importantly, that reduced medullary blood flow is sufficient to produce hypertension (9). While significant advances have been made in understanding the causes of essential hypertension through the use of such animal studies, it has proven difficult to extend the research to humans because of the lack of suitable monitoring techniques. In animals, medullary blood flow is typically measured using invasive laser Doppler probes (1) and to date there is no noninvasive alternative for use in humans. The motivation behind the present work was to determine whether blood oxygen level dependent magnetic resonance imaging (BOLD MRI) could provide such an alternative. While BOLD MRI does not measure blood flow directly, a large body of evidence now exists, derived primarily from functional MRI studies in the brain (10–12), suggesting a strong correlation between BOLD measurements and blood flow. The BOLD MRI technique exploits the fact that the magnetic properties of hemoglobin vary depending on whether it is in the oxygenated or deoxygenated form. This affects the T2* relaxation time of the neighboring water molecules and in turn influences the MRI signal on T2*-weighted images. Because the ratio of oxyhemoglobin to deoxyhemoglobin is related to the pO2 of blood, and since the pO2 of capillary blood is thought to be in equilibrium with the surrounding tissue, changes estimated by BOLD MRI can be interpreted as changes in tissue pO2 (13). Tissue pO2 is in turn correlated with blood flow since an increase in blood flow, unless it is compensated by a corresponding increase in metabolic activity, will oversupply the tissue with oxygen resulting in an increase in tissue pO2. BOLD MRI has previously been used to monitor changes in intrarenal blood flow and medullary oxygenation in both human (13) and animal (14) studies. In humans, attenuated responses to waterload have been observed using BOLD MRI in elderly subjects (15), diabetics (16), and in young healthy subjects pretreated with a prostaglandin inhibitor (15). The technique has also been used in rats to monitor changes in renal oxygenation following the administration of various pharmacologic agents that either increase (17) or decrease (14) intrarenal blood flow. The goal of our current work is to determine whether BOLD MRI can be used to detect differences in medullary blood flow between hypertensive subjects and normotensive controls. In this preliminary study, we made use of a nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME). Nitric oxide (NO) is a soluble gas that is continuously synthesized by the endothelium (18) and has a wide range of biological properties including relaxation of vascular tone. In healthy subjects, administration of L-NAME blocks the synthesis of nitric oxide and results in a constriction of the vessels. There is also a corresponding reduction in blood flow and an increase in blood pressure (6,8,19–22). In hypertensive subjects, however, it is known that both NO production and basal NO bioavailablity is reduced (23), due to either impaired synthesis or excessive oxidative degradation (24). Administration of nitric oxide synthase inhibitors such as L-NAME, therefore, should have little effect in such individuals. The hypothesis of this study was that a response to L-NAME administration should be detectable in normotensive subjects using BOLD-MRI, but that the response should be diminished in hypertensive individuals. Because L-NAME has not yet been approved for human use, the study was performed in spontaneously hypertensive rats (SHR), a commonly used animal model of essential hypertension. SHR were originally derived from Wistar-Kyoto (WKY) rats. They exhibit an upregulation of nitric oxide synthase (NOS) (25), an enzyme present in endothelial cells that metabolizes L-arginine to produce NO. However, due to excessive oxidative degradation, SHR actually have reduced bioavailability and activity of NO (26,27). Inhibition of NO synthase, therefore, should have minimal effect on their renal blood flow, a result that has been confirmed using invasive measurements (5,6,8,19–22,28). It was our hypothesis that a reduced response to NOS inhibition should also be detectable with BOLD MRI. In our experiments, we performed BOLD imaging in SHR before and after administration of L-NAME. We then compared the results with data obtained in WKY rats used as normotensive controls.