Noninvasive evaluation of intrarenal oxygenation with BOLD MRI.
BACKGROUND
The countercurrent arrangement of capillary blood flow in the medulla of mammalian kidneys generates a gradient of oxygen tension between the renal cortex and the papillary tip that results in a state of relative hypoxia within the renal medulla. Exploration of the pathophysiological implications of medullary hypoxia has been hampered by the absence of a noninvasive technique to estimate intrarenal oxygenation in different zones of the kidney. In the present study, we demonstrate the feasibility of such a method on the basis of blood oxygenation level-dependent (BOLD) MRI, which allows sequential measurements in humans in response to a variety of physiological/ pharmacological stimuli in health and disease.


RESULTS
BOLD MRI measurements were obtained in healthy young human subjects (n = 7), and the effects of three different pharmacological/physiological maneuvers that induce diuresis were studied. Spin-spin relaxation rate, R2*, was measured, which is directly related to the amount of deoxyhemoglobin in blood and in turn to tissue PO2. Furosemide but not acetazolamide (n = 6 each) increased medullary oxygenation (delta R2* = 7.62 Hz; P < .01), consistent with the separate sites of action of these diuretics in the nephron and with previous direct measurements of their effects in anesthetized rats with oxygen microelectrodes. A new finding is that water diuresis improves medullary oxygenation (delta R2* = 6.43 Hz; P < .01) in young human subjects (n = 5).


CONCLUSIONS
BOLD MRI can be used to monitor changes in intrarenal oxygenation in humans in a noninvasive fashion.
Prasad
P V
PV
Department of Radiology and Medicine, Beth Israel Hospital, Boston, MA 02215, USA. pprasad@bidmc.harvard.edu
Edelman
R R
RR
Epstein
F H
FH
D000086:Acetazolamide / Q000494:pharmacology; D000328:Adult; D000818:Animals; D004231:Diuresis; D004232:Diuretics / Q000494:pharmacology*; D005260:Female; D005665:Furosemide / Q000494:pharmacology; D006801:Humans; D007668:Kidney / Q000033:anatomy & histology / Q000187:drug effects / Q000502:physiology*; D007672:Kidney Cortex / Q000502:physiology; D007679:Kidney Medulla / Q000502:physiology; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008322:Mammals; D008839:Microelectrodes; D009399:Nephrons / Q000502:physiology; D010100:Oxygen / Q000097:blood*; D010108:Oxyhemoglobins / Q000378:metabolism; D010313:Partial Pressure; D051381:Rats; D012016:Reference Values
D004232:Diuretics; D010108:Oxyhemoglobins; D005665:Furosemide; D000086:Acetazolamide; D010100:Oxygen