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Articles in PresS, published online ahead of print September 26, 2002
Am J Physiol Regu Physiol, 10.1152/ajpregu.00061.2002
Submitted on January 31, 2002
Accepted on September 24, 2002
1 Physiology, Monash University, Melbourne, VIC, Australia
2 Physiology, Institute of Physiology and Pharmacology, University of Goteborg, Goteborg, Sweden
* To whom correspondence should be addressed. E-mail: gabriela.eppel{at}med.monash.edu.au.
We examined the extent of renal medullary blood flow (MBF) autoregulation in pentobarbital anesthetized rabbits. Two methods for altering renal arterial pressure (RAP) were compared; the conventional method of graded supra-renal aortic occlusion, and an extracorporeal circuit that allows RAP to be increased above systemic arterial pressure. Changes in MBF were estimated by laser Doppler flowmetry, which appears to predominantly reflect red cell velocity, rather than flow per se, in the kidney. We compared responses using a dual fiber needle probe held in place by a micromanipulator, with those from a single fiber probe anchored to the renal capsule, to test whether RAP-induced changes in kidney volume confound medullary laser Doppler flux (MLDF) measurements. MLDF responses were similar for both probe types, and both methods for altering RAP. MLDF changed little as RAP was altered from 50 to at least 170 mmHg (24 ± 22% change). Within the same RAP range, RBF increased by 296 ± 48%. Urine flow and sodium excretion also increased with increasing RAP. Thus, pressure diuresis/natriuresis proceeds in the absence of measurable increases in medullary erythrocyte velocity estimated by laser Doppler flowmetry. These data do not, however, exclude the possibility that MBF is increased with increasing RAP in this model, because vasa recta recruitment may occur.
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