AJP - Renal Physiology, Vol 260, Issue 2 283-F289, Copyright © 1991 by American Physiological Society
Suitability of tritiated inulin for determination of glomerular filtration rate
M. Shalmi, H. E. Lunau, J. S. Petersen, M. Bak and S. Christensen
Department of Pharmacology, University of Copenhagen, Denmark.
Purity of different batches of [3H]inulin delivered from leading
manufacturers was elevated with a chromatographic method (Sephadex G-25
column) that allowed simultaneous analysis of cold inulin, [3H]inulin, and
[14C]inulin in the same run. Among four batches of [3H]inulin received
within 5 mo, two were found relatively pure, whereas two were partly
decomposed to lower-molecular-weight fragments. The chromatographic profile
of pure isotopes was not significantly affected by redistribution and
freeze drying, nor by subsequent storage in the freeze-dried state at -20
degrees C for up to 5 mo, nor by incubation in aqueous solution at 37
degrees C for 24 h. Three batches of [3H]inulin with different grades of
decomposition (noninulin percentages 13%, 38%, and 61%, respectively) were
selected for clearance experiments and infused simultaneously with cold and
undecomposed [14C]inulin to conscious rats. [14C]inulin had a significantly
higher clearance than cold inulin (+7.6 +/- 0.6%) and relatively pure
[3H]inulin (+12.4 +/- 0.4%). Decomposed [3H]inulin isotopes progressively
underestimated clearance of cold inulin to an extent related to the degree
of decomposition. Thus at the end of the 5-h clearance experiment, ratios
between clearance of tracer and of cold inulin were 0.92, 0.71, and 0.60
for the three 3H isotopes, respectively. This study indicates that
[3H]inulin delivered from leading manufacturers may be decomposed to an
extent that invalidates its use as a marker for glomerular filtration rate
(GFR). It is thus necessary to check the purity routinely before use.
Within the same rat, clearance of undecomposed [3H]inulin and [14C]inulin
may differ by 12%, and for this reason they should not be used
interchangeably as GFR markers.
