THE DISPOSAL OF PLASMA 17-HYDROXYCORTICOSTEROIDS. I. EXPONENTIAL DISPOSAL FROM A SINGLE COMPARTMENT*

Abstract

Many investigators have stated or implied that the distribution and disposal of plasma 17-hydroxycorticosteroids (17-OH-CS) can be approximated by assuming that these substances are distributed in the body as though they were in a single compartment and are disposed of at a rate proportional to concentration. In the present investigation, cortisol was given by intravenous infusion and the changes in plasma 17-OH-CS concentrations were examined. The data obtained were then compared with the predictions of the single compartment-exponential disposal theory. The concentration of plasma 17-OH-CS was studied in 4 experimental situations: 1) after cessation of constant intravenous infusions of cortisol at slow rates; 2) after administration of large amounts of cortisol; 3) after equilibration attained during constant infusions of cortisol; and 4) during the early period of infusions of cortisol. In all 4 experimental situations, there was a poor correspondence between the predicted and the actual data. It is concluded that 17-OH-CS do not disappear from the plasma as though they were distributed in a single compartment and disposed of at a rate proportional to concentration. Since plasma 17-OH-CS disposal does not follow first-order kinetics, the half-time of plasma 17-OH-CS is not independent of concentration. Methods for quantitation of adrenocortical function that utilize the assumptions of single-compartment distribution and exponential disposal of plasma 17-OH-CS are based on a theory which does not describe the data accurately.