Circadian rhythm of hepatic cytosolic and nuclear estrogen receptors

Abstract

The distribution of estrogen receptor between the cytosolic and nuclear compartments were evaluated in liver of male rats to determine whether a circadian rhythm exists. Cytosolic receptor reached a maximum level at 400 hours and a minimum at 2000 and 2400 hr. Nuclear receptor reached a maximum level at 800 hr and was lowest at 1600 and 2000 hr. Serum estradiol levels were also highest at 800 hr and lowest at 1600 hr. The variations in cytosolic and nuclear receptors are not reciprocal; in fact, the overall content of receptor in the liver is not constant and also displays a circadian rhythm.

Figure 1. Variations in cystosolic and nuclear content of estrogen receptor as a function of circadian phase

Panel A, Protamine sulfate precipitates of rat liver cytosol were prepared and incubated with varying concentrations of [³H]E₂ in the absence (total binding) and presence (nonspecific binding) of a 100-fold excess of unlabeled DES for 16 hr at 4°C. Specific binqing is calculated as the difference of these two values. Each point is the mean ± S.E.M. of cER in five animals, using duplicate determinations for each animal. Scatchard plots of the specific binding values were used to determine the number of binding sites and affinity of binding. Panel B, Nuclear suspensions were incubated with 10 nM [³H]-E₂ in the absebce and presence of a 250-fold excess of unlabeled DES for 1 hr at 30°C, as detailed in Methods. Also shown (open bars) are the serum E₂ concentrations as determined by radioimmunoassay.

Figure 2. Variations in total hepatic content of estrogen receptor at different times of day

The height of each bar represents the total receptor content (cytosolic and nuclear) corrected for and expressed as binding per gram of liver. The shaded area of each bar represents the contribution of the nuclear receptor to the total amount.