High resolution spectral analysis of plasma adrenocorticotropin reveals a multifactorial frequency structure

Time series of plasma ACTH concentrations were analyzed with a high resolution spectral analysis program based on digital Fourier transforms. Both coherent signal and stochastic aspects of the time series were analyzed. Samples were collected at 2- and 15-min intervals in control rats and rats immunoneutralized against CRH. The individual and composite spectral distributions revealed significant structure at both the higher and lower ranges of frequencies studied, with multiple periodicities between 4-220 min in both groups. CRH immunoneutralization consistently reduced the amplitude by 82% and compressed the frequency distribution for waveforms with periods longer than 15 min by 23%. A systematic break in the slopes of the background continua occurred between 10 and 15 min in the 2-min time series. This break was unaffected by CRH immunoneutralization. Digital Fourier transform analysis of our ACTH time series suggests a system with a more complex high frequency structure than has previously been appreciated. Our analyses suggest a biological system with the following characteristics: 1) both a fast and a slow response to a fairly constant unspecified fast forcing; 2) the slow response is initiated by the fast response and represents an imperfect integration due to feedback processes; 3) CRH alters the ability of the fast forcing to elicit a slow response without altering the fast response or ACTH clearance; and 4) this alteration consists of both amplitude and frequency modulation in the signal output. This view of ACTH secretion suggests an adaptive and energy-efficient system.