Increased hypoxia and reduced renal tubular response to furosemide detected by BOLD magnetic resonance imaging in swine renovascular hypertension

Oxygen consumption beyond the proximal tubule is mainly determined by active solute reabsorption, especially in the thick ascending limb of the Loop of Henle. Furosemide-induced suppression of oxygen consumption (FSOC) involves inhibition of sodium transport in this segment, which is normally accompanied by a marked decrease in the intrarenal deoxyhemoglobin detectable by blood oxygen level-dependent (BOLD)-magnetic resonance imaging (MRI). This study tested the hypothesis that the magnitude of BOLD-MRI signal change after furosemide is related to impaired renal function in renovascular hypertension. In 16 pigs with unilateral renal artery stenosis, renal hemodynamics, function, and tubular function (FSOC and fluid concentration capacity) were evaluated in both kidneys using MR and multidetector computerized tomography (MDCT) imaging. Animals with adequate FSOC (23.6 +/- 2.2%, P > 0.05 vs. baseline) exhibited a mean arterial pressure (MAP) of 113 +/- 7 mmHg, and relatively preserved glomerular filtration rate (GFR) of 60 +/- 4.5 ml/min, comparable to their contralateral kidney (66 +/- 4 ml/min, P > 0.05). In contrast, animals with low FSOC (3.1 +/- 2.1%, P = NS vs. baseline) had MAP of 124 +/- 9 mmHg and GFR (22 +/- 6 ml/min) significantly lower than the contralateral kidneys (66 +/- 4 ml/min, P < 0.05). The group with preserved GFR and FSOC showed an increase in intratubular fluid concentration as assessed by MDCT that was greater than that observed in the low GFR group, suggesting better preservation of tubular function in the former group. These results suggest that changes in BOLD-MRI after furosemide can differentiate between underperfused kidneys with preserved tubular function and those with tubular dysfunction. This approach may allow more detailed physiologic evaluation of poststenotic kidneys in renovascular hypertension than previously possible.