VASCULAR EXCHANGE IN THE KIDNEY - REGIONAL CHARACTERIZATION BY MULTIPLE INDICATOR TOMOGRAPHY

In previous work with the method of multiple indicator dilution (MID), we have established that a spatially distributed model of transcapill ary exchange proposed by Goresky, Ziegler, and Bach (GZB) accurately d escribes, at the in vivo whole-organ level, the handling of extracellu lar indicators in the canine renal cortex. To date, however, it has no t been possible to assess the key hypothesis that GZB corresponds to t he actual local mechanism of exchange in vivo and is not just a compac t summary of the kidney's average whole-organ behavior. By adapting th e MID method to high speed computed tomography (CT), we are now able t o report that the GZB mechanism is an accurate description of renal co rtical transcapillary exchange down to volumes of cortical tissue comp rising no more than a few per cent of the total cortical mass, i.e., c ontaining no more than a few thousand nephrons. A small bolus of iohex ol (radiopaque extracellular indicator) or iodipamide ethyl ester micr oparticles (radiopaque plasma indicator) injected into the renal arter y was followed by CT as it passed through the kidney and into the rena l vein. Time-attenuation value curves of the two contrast media obtain ed from the renal vein and from regions of interest in the cortex were then modeled with the GZB mechanism and with a more complex formulati on that includes GZB as a limiting case. When applied to the data, the models converged to GZB as the best fit for each region examined. The GZB mechanism is found to provide excellent agreement with the region al data.