Microsphere Measurement of Intrarenal Circulation of the Dog

Distribution of cortical blood flow was measured in the dog by a technique based on radionuclide-labeled microspheres. Initially it was necessary to test possible pitfalls of this technique. Completeness of trapping in the kidney, the effect on renal function, and the notion that microsphere distribution reflects blood flow distribution in the kidney cortex were studied. Renal vein blood contained less than 0.2% of the microspheres (16.8μ diameter) found in the renal artery after an aortic injection. No impairment of CPAH (control 167±4; postinjection 179±31 ml/min), CIn (control 39.3%6; postinjection 37.6±2 ml/min), and Tm glucose (control 90.8±13; postinjection 102±24) was found using doses adequate to measure renal blood flow (5 mg/injection × 4 injections). After 4 injections of 50 mg each significant impairment of renal function was observed. Intrarenal blood flow distribution was determined during hemorrhagic hypotension. Yb-labeled microspheres were injected into the root of the aorta before, and Sr-labeled microspheres after, acute hemorrhage. Radioactivity was measured in the outer two thirds and inner one third of kidney slices. Tissue blood flow was calculated and expressed as the ratio of outer cortex to inner cortex counts. Renal blood flow was redistributed to the inner cortex after hemorrhage (ratio before, 3.00; after 1.30, P < 0.01). Finally, the results of this technique were compared to a widely used method of measuring intrarenal blood flow distribution, Xe washout. The first component of the washout technique correlated fairly well with total cortical flow but it was not possible to match the second component with any single anatomical area of the kidney. Limitations of the Xe washout are discussed.