Plasma free cortisol during secretory episodes.

Plasma free cortisol fractions were determined in normal men during spontaneous cortisol secretory episodes and ACTH- or exercise-provoked cortisol peaks in the physiological range. The percentage of free cortisol was measured using ultrafiltration by centrifugation at 37 C, and total cortisol concentrations were measured by RIA. The percent free cortisol varied with the time of day, as did total cortisol. It increased parallel to total cortisol increases, whether they were spontaneous, as occurring in the early morning or postprandially, or provoked by external stimulation. A highly significant positive correlation between relative increases in free and total cortisol was found throughout the experiment. As reported previously, total cortisol increases, according to their origin, did or did not reduce the effects of subsequent stimuli. The present results indicate that the variations in the percent free cortisol are not responsible for the differences in the cortisol responses to these stimuli.