Eppel GA, Jacono DL, Shirai M, Umetani K, Evans RG, Pearson JT. Contrast angiography of the rat renal microcirculation in vivo using synchrotron radiation. Am J Physiol Renal Physiol 296: F1023-F1031, 2009. First published March 4, 2009 doi:10.1152/ajprenal.90499.2008.-We have developed a new method for contrast microangiography of the rat renal circulation using synchrotron radiation. The method was applied to determine responses of the renal arterial vasculature to angiotensin II and electrical stimulation of the renal nerves (RNS). Iodinated contrast agent was administered directly into the renal artery of pentobarbital-anesthetized rats before and during 1) intravenous infusion of angiotensin II (1.6 g ⅐ kg Ϫ1 ⅐ min Ϫ1 ) or 2) its vehicle, or 3) RNS at 2 Hz. Images were obtained at 30 Hz, before and during these treatments, and vascular caliber was determined by use of a newly developed algorithm described herein. Up to four levels of branching could be observed simultaneously along the arterial tree, comprising vessels with resting diameter of 28 -400 m. Vessel diameter was not significantly altered by vehicle infusion (ϩ3.1 Ϯ 3.5% change) but was significantly reduced by angiotensin II (Ϫ24.3 Ϯ 3.4%) and RNS (Ϫ17.1 Ϯ 3.8%). Angiotensin II-induced vasoconstriction was independent of vessel size, but RNS-induced vasoconstriction was greatest in vessels with a resting caliber of 100 -200 m and least in vessels with a resting caliber 40 -100 m. In conclusion, the method we describe herein provides a new approach for assessing responses of the renal arterial circulation to vasoactive factors along several orders of branching.angiography; angiotensin II; vascular caliber; kidney circulation; renal nerves RENAL VASCULAR TONE IS A CRITICAL controller of renal perfusion, glomerular filtration rate, and tubular function (21) and thus long-term regulation of body fluid homeostasis (9). Many available methodologies allow quantification of changes in the caliber of resistance vessels in vivo, but all have important limitations. For example, vascular casting techniques allow quantification of vascular caliber at multiple points along the renal vascular tree, potentially allowing analysis of regulation of blood flow within networks of blood vessels in the kidney (4, 5). But an important disadvantage of this approach is that measurements can only be made at a single time point so that the effects of vasoactive factors can only be inferred from cross-sectional comparisons between blood vessels in different animals. Real-time analysis of vascular caliber (17) and capillary blood velocity (33) within the kidney has been achieved with intravital microscopy, but currently the measurements derived from such techniques are limited to single points along an identified blood vessel. Magnetic resonance imaging and X-ray microtomographic techniques currently lack the spatial and/or temporal resolution required for real-time analysis of resistance vessel caliber in the kidney in vivo (1).Herein we present a new approach that overcom...