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

Abstract

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 h after the endogenous CS peak. GS mRNA oscillates between 3 and 6 fmol g tissue(-1), whereas GS activity fluctuates between 17 and 23 micromol min(-1) g protein(-1). Mechanistic receptor/gene-mediated pharmacodynamic 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.

Fig. 1

Pharmacodynamic models of glucocorticoid effects in rat skeletal muscle. Pharmacodynamic model of (A) GS induction by glucocorticoids and (B) GR mRNA suppression by glucocorticoids. Symbols are defined in the text.

Fig. 2

Plasma corticosterone concentration vs. time profile in normal male Wistar rats. Symbols are mean data (±SD) and the lines are model fitted profiles. The onset of light period was t = 0 h, whereas the onset of dark period was 12 h. The shaded area represents the dark period.

Fig. 3

Plasma dexamethasone (●) or hydrocortisone (○) concentration vs. time profiles following i.v. injection of 0.1 mg kg⁻¹ dexamethasone or 50 mg kg⁻¹ hydrocortisone to ADX rats. Symbols are mean data (±SD) and the lines are model-fitted profiles. Data were taken from Mager et al. (11).

Fig. 4

Time course of muscle GS mRNA and activity profiles in normal rats (circadian rhythm study) or following i.v. injection of 0.1 mg kg⁻¹ dexamethasone or 50 mg kg⁻¹ hydrocortisone in ADX rats (single bolus dose study). Symbols are mean data (± SD) and the lines are model-fitted profiles. The dashed line represents simulated plasma CS concentrations over the 24-h period. The shaded areas represent the dark period. Note differences in scales.

Fig. 5

Time course of muscle GR mRNA profiles in normal rats (circadian rhythm study) or following i.v. injection of 0.1 mg kg⁻¹ dexamethasone or 50 mg kg⁻¹ hydrocortisone in ADX rats (single bolus dose study). Symbols and lines are defined as in Fig. 4. Note differences in scales.