Journal of Applied Physiology, Vol 68, Issue 6 2434-2442, Copyright © 1990 by American Physiological Society
Modeling flux of free and protein-bound radioisotopes into the pulmonary interstitium
W. R. Riddle, R. J. Roselli and N. A. Pou
Department of Radiology and Radiological Sciences, School of Medicine, Vanderbilt University, Nashville, Tennessee 37232.
Several groups of investigators are using external detection of
radiolabeled protein to study the flux of protein from plasma into the
pulmonary interstitium. A basic assumption for these studies has been that
the unbound (free) tracer concentration is small and insignificant. The
purpose of this study is to evaluate how free tracer influences the
determination of normalized slope index. A five-compartment model for the
lung was used with transport equations for both unbound and bound nuclide
flux. Parameters of the unbound and bound transport equations were varied
to evaluate the sensitivity of normalized slope index to each parameter.
The model was also compared with published protein flux data to investigate
the validity of the transport model. Application of the model to external
scan data provides a sensitive method for evaluating the flux of bound and
unbound tracers into the pulmonary interstitium. We conclude that because
the distribution volume for unbound tracer is large with respect to protein
distribution volume, even a small amount of unbound tracer (2-5%) can
create large errors in the determination of normalized slope index.
