ABSTRACTS PRESENTADOS AL 11th INTERNATIONAL SYMPOSIUM "RADIONUCLIDES IN NEPHRO-UROLOGY", REALIZADO EN MONTEREY, CALIFORNIA, 13-17 MAYO 2001.
11. IS THE STANDARD MODEL OF Tc-99m MAG3 HANDLING CORRECT? A HYPOTHESIS.
Andrew J.W. Hilson, Royal Free Hospital, London NW3 2QG.
The standard teaching is that Tc-99m MAG3 is filtered in the glomerulus and further secreted, and that no other renal handling occurs. With the realization that other radiopharmaceuticals have more complex handling - HIDA shows clear-cut bidirectional transport into and back from the hepatocyte - the question arises as to how Tc-99m MAG3 is handled. There are the following observations to explain:
In ATN, there may be significant output shown on the cumulative output curve with no renal excretion, suggesting that tracer is returning to the circulation.
In
normal renography, the "pure" plateau-type retention function
is unusual (only 1 seen in a consecutive series of 50 studies in our
department). More usually, there is a curve, which falls more or less
steadily from the time of injection, with a variable degree of plateau
being seen. This again implies that tracer is leaving the kidney,
but not appearing in the urine.
In the only data directly measuring the renal extraction of Tc-99m
MAG3, this changed with time, being lower at 30 minutes than immediately
after injection. This is explicable if there is retrograde flow of
Tc-99m MAG3 from the tubular cell to the blood with a delay.
This hypothesis is compatible with the available data on the cellular transport of molecules such as Tc-99m MAG3.
I propose that the "standard" model of renal handling of Tc99m MAG3 be modified to include a component for back-flow.
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