Modeling circadian rhythms of glucocorticoid receptor and glutamine synthetase expression in rat skeletal muscle

Purpose. The circadian rhythm of endogenous corticosterone (CS) may produce fluctuations of downstream gene expression in normal rats. This study examined changes in glucocorticoid receptor (GR) and glutamine synthetase (GS) expression in rat skeletal muscle in relation to plasma CS over a 24-h period. Methods. Fifty-four normal male Wistar rats were sacrificed at 18 time points (n = 3) over 24 h. Plasma CS concentrations and gastrocnemius muscle GR and GS mRNA and GS activity were measured. Results. The circadian rhythm of plasma CS was captured by a two-harmonic function. The expression of GR and GS mRNA and GS activity follow a circadian rhythm in normal rat skeletal muscle. GR mRNA reaches a trough at 4 h after the peak of plasma CS and it fluctuates between 0.55 and 0.9 fmol g tissue(-1). GS mRNA and activity reach peaks at 6 and 12 It after the endogenous CS peak. GS mRNA oscillates between 3 and 6 fmol g tissue(-1), whereas GS activity fluctuates between 17 and 23 mu mol min(-1) g protein(-1). Mechanistic receptor/gene-mediated plarmacodynamic models were applied to describe the temporal patterns of GR mRNA, GS mRNA, and GS activity within the circadian cycle. Conclusions. The integrated models were able to capture the circadian expression patterns of plasma CS, and GR and GS in normal rat skeletal muscle showing a dependence of tissue gene expression on plasma CS.