Validation of a single-compartment approach to the calculation of secretion rates of ACTH

A single-compartment model used in this laboratory for continuously calculating ACTH secretion rates from measured plasma ACTH concentrations has been tested for its ability to follow changing rates of ACTH entry (rapid departure from steady state). ACTH was infused at known moderately high but physiological rates into anaesthetized dogs (Nembutal). Under such conditions endogenous secretion is initially less than 5% of infused rates. Orthogonal polynomials (ACTHt) were fitted to plasma ACTH vs. time data. Then secretion ratet = (ACTHt X MCR) + (dACTHt/dt X V) where it was previously shown that the metabolic clearance rate of ACTH (MCR) lacked significant inter-animal or concentration-dependent variation, and its distribution volume (V) was also constant. The calculated ACTH entry rate curves (a) followed a 10-fold increase in infusion rate over 4 min and subsequent rapid decline with a lag of only about 1 min and, despite some blurring, gave an integrated response equal to 94 +/- 5.5% of the known signal, and (b) followed a sinusoidal change in infusion rate (amplitude, 1.7 X base rate; period, 40 min) with a few percent error and negligible lag. These signals imitate (a) an abrupt stress response, and (b) other rapid departures from steady state.