Relationship of Extraluminal Tubular Deposition of Ferrocyanide to Peritubular Perfusion Rate in Cortical and Medullary Nephron Segments of the Rat Kidney

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Abstract
Ferrocyanide can be precipitated in the kidney and retained in microdissected tubules. By means of 14C-ferrocyanide it is possible to measure nephron glomerular filtration rate, but radioactivity is also detected in tubular segments beyond the reach of filtered ferrocyanide. The present study evaluated this radioactivity with the assumptions that it derives from peritubular capillary and vasa recta blood and that it may be proportional to the perfusion rate to single tubular segments. The findings rule out axial streaming of filtrate and transtubular diffusion of filtered ferrocyanide as factors and therefore point to an extraluminal deposition of ferrocyanide diffusing out of peritubular capillary and vasa recta blood. It was also found that the extraluminal tubular radioactivity is not affected by processing of the tissue, that it decreases from cortex to outer medulla to inner medulla, and that distal convoluted tubules have a greater radioactivity per unit length than the thick ascending limbs at the same level. A comparison with data obtained by others using a method to estimate intrarenal blood flow distribution lends support to the assumption that extraluminal tubular radioactivity is proportional to the capillary perfusion rate to single tubular segments.