Vascular exchange in the kidney. Regional characterization by multiple indicator tomography

Area of Science:

• Renal physiology
• Pharmacokinetics
• Medical imaging

Background:

• The Goresky, Ziegler, and Bach (GZB) model previously described whole-organ transcapillary exchange in the canine renal cortex.
• The local accuracy of the GZB model had not been previously assessed in vivo.

Purpose of the Study:

• To validate the GZB model as a local mechanism of renal cortical transcapillary exchange.
• To assess the GZB model's accuracy at a finer tissue resolution than previously possible.

Main Methods:

• Adaptation of the multiple indicator dilution (MID) method to high-speed computed tomography (CT).
• Injection of radiopaque extracellular (iohexol) and plasma (iodipamide ethyl ester microparticles) indicators into the renal artery.
• Analysis of time-attenuation curves from CT data using the GZB model and a more complex formulation.

Main Results:

• The GZB mechanism accurately describes transcapillary exchange in small volumes of renal cortical tissue (a few percent of total mass).
• CT-based MID successfully resolved exchange processes at the level of a few thousand nephrons.
• Both the GZB model and a more complex model converged to the GZB mechanism as the best fit for regional data.

Conclusions:

• The GZB model represents an accurate, local mechanism for renal cortical transcapillary exchange.
• High-speed CT combined with MID provides unprecedented resolution for studying organ-level exchange dynamics.
• The findings support the GZB model's applicability beyond whole-organ averaging.