Glucocorticoids and the vascular molecular clock: Implications for renal artery function and blood pressure rhythm

Molecular clocks mediate a daily transcriptional schedule that maintains normal tissue physiology. Glucocorticoids synchronise peripheral clocks with the master clock in the suprachiasmatic nucleus in response to ambient light. Evidence from humans and mice suggests that arrhythmic glucocorticoids induce non-dipping blood pressure and vascular dysfunction. The mechanisms of this are poorly understood. I hypothesized that arrhythmic activation of the glucocorticoid receptor attenuates the molecular clock mechanism in the renal artery, a major component of blood pressure control, dysregulating its normal function. I tested this hypothesis in a mouse model with clamped corticosterone rhythm, first by defining the circadian transcriptome of the renal artery and then by measuring the day-night circadian differences in renal artery physiology RNA sequencing revealed the circadian transcriptome of renal arteries under control conditions and with arrhythmic glucocorticoids. In the control group 465 (3%) transcripts out of 14425 protein-coding genes were rhythmic. 195 of those genes lost their rhythm under arrhythmic corticosterone and were related to pathways including circadian transcription, TGFβ signalling and extracellular matrix. Paradoxically, several transcripts in the arrhythmic corticosterone group gained rhythmicity and were indicative of mitochondrial respiratory chain and oxidative phosphorylation pathways. In parallel, messenger RNA (mRNA) levels of all elements of the core clock machinery were significantly dysregulated. Additionally, transcripts of mediators of the arterial vasomotor machinery were significantly changed in the arrhythmic corticosterone mice compared to controls, while levels of the glucocorticoid receptor (GR) were significantly downregulated. To investigate underlying mechanisms of glucocorticoid control of renal artery function I followed two approaches. First, I compared the day/night reactivity to vasoactive stimuli in isolated renal arteries from control and arrhythmic corticosterone mice. Arteries from control mice displayed greater endothelial-dependent (via acetylcholine) and -independent vasodilation (via nitric oxide donor sodium nitroprusside) was higher during the dark phase (active period) compared with the light phase (inactive phase). In the arrhythmic corticosterone group time of day variability in response to both vasodilators was attenuated. Second, to interrogate the role of glucocorticoid signalling within the smooth muscle in circadian vascular reactivity, renal arteries from mice lacking the glucocorticoid receptor in the smooth muscle (SMGRKO) were used. Time of day dependent vascular reactivity to endothelial-independent vasodilation seen in the controls, was attenuated in the SMGRKO animals. I speculate that temporal misalignment of the vascular molecular circadian machinery, due to arrhythmic glucocorticoid signalling in the arrhythmic corticosterone and SMGRKO mice, may cause subsequent temporal changes in gene expression and have a direct effect on dilatory capacity. In summary I found a robust effect of arrhythmic glucocorticoid signalling both at the transcriptional and functional level of the renal artery circadian physiology. Arrhythmic glucocorticoids may achieve this via several potential pathways including the ability of the renal artery to respond to nitric oxide. Together, this novel study extends our understanding of circadian vascular physiology and underlines the impact of glucocorticoid rhythm on temporal changes in gene expression. This may be clinically relevant in the pathogenesis of vascular dysfunction associated with elevated glucocorticoids in metabolic syndrome and chronic stress

High salt intake activates the hypothalamic-pituitary-adrenal axis, amplifies the stress response, and alters tissue glucocorticoid exposure in mice

Aims: High salt intake is common and contributes to poor cardiovascular health. Urinary sodium excretion correlates directly with glucocorticoid excretion in humans and experimental animals. We hypothesized that high salt intake activates the hypothalamic-pituitary-adrenal axis activation and leads to sustained glucocorticoid excess. Methods and results: In male C57BL/6 mice, high salt intake for 2-8 weeks caused an increase in diurnal peak levels of plasma corticosterone. After 2 weeks, high salt increased Crh and Pomc mRNA abundance in the hypothalamus and anterior pituitary, consistent with basal hypothalamic-pituitary-adrenal axis activation. Additionally, high salt intake amplified glucocorticoid response to restraint stress, indicative of enhanced axis sensitivity. The binding capacity of Corticosteroid-Binding Globulin was reduced and its encoding mRNA downregulated in the liver. In the hippocampus and anterior pituitary, Fkbp5 mRNA levels were increased, indicating increased glucocorticoid exposure. The mRNA expression of the glucocorticoid-regenerating enzyme, 11β-hydroxysteroid dehydrogenase Type 1, was increased in these brain areas and in the liver. Sustained high salt intake activated a water conservation response by the kidney, increasing plasma levels of the vasopressin surrogate, copeptin. Increased mRNA abundance of Tonebp and Avpr1b in the anterior pituitary suggested that vasopressin signalling contributes to hypothalamic-pituitary-adrenal axis activation by high salt diet. Conclusion: Chronic high salt intake amplifies basal and stress-induced glucocorticoid levels and resets glucocorticoid biology centrally, peripherally and within cells.</p