Interpretation of tracer data: some factors which reduce the number of terms in the specific activity functions in n-pool systems.

Abstract

The number of exponential terms in the function which describes the change in the specific activity of a pool following the injection of an isotopically labeled tracer is usually considered to equal the number of pools in which the labeled compound is distributed. However, the number of exponential terms may be smaller than the actual number of pools in the system, even if all pools exchange material with each other. This study is concerned with the derivation of relationships among the fractional rates of transfer between pools which are necessary and sufficient for a reduction in the number of exponential terms for all exchanging pools. The concept of linear dependence among the specific activity functions is basic to this analysis. By considering these relationships, it is possible to interpret the data on the basis of models consisting of a large number of pools which satisfy the condition of fast internal mixing. Such models may be necessary for a meaningful interpretation of tracer data obtained from biological systems.