Human epidermal cell proliferation with regard to circadian variation of plasma cortisol

This study was performed in 6 healthy young men under analogous experimental conditions to investigate whether or not the epidermal proliferative activity is influenced by the physiological circadian rhythmicity of cortisol concentration in blood. Every 6 h of total duration of the experiments, blood samples were taken for radioimmunoassay of serum cortisol and skin punch biopsies (upper arm inside) were labelled in vitro by 3H-thymidine. In 4 out of 6 individuals, an inverse and time-shifted correlation between the combined serum cortisol levels and the epidermal 3H-LI could be found, i.e., a rise or decline of serum cortisol were followed by a lowered or elevated rate of epidermal LI 6 h later. In particular, a steep increase of serum cortisol from midnight to 6 a.m. correlated to a decrease of 3H-LI from 6 a.m. to noon. This indicates that in man the rate of the epidermal proliferative activity inversely depends, apart from other mechanisms of regulation, on the concentration and circadian periodicity of endogenous cortisol in blood. However, we failed to demonstrate a clear circadian rhythm of the epidermal proliferation, possibly due to the short duration of the experiments extended in each individual to 48 h only.