Differences in renal BMAL1 contribution to Na+ homeostasis and blood pressure control in male and female mice.

The renal circadian clock has a major influence on the function of the kidney. Aryl hydrocarbon receptor nuclear translocator-like protein 1 [ARNTL; also known as brain and muscle ARNT-like 1 (BMAL1)] is a core clock protein and transcription factor that regulates the expression of nearly half of all genes. Using male and female kidney-specific cadherin BMAL1 knockout (KS-BMAL1 KO) mice, we examined the role of renal distal segment BMAL1 in blood pressure control and solute handling. We confirmed that this mouse model does not express BMAL1 in thick ascending limb, distal convoluted tubule, and collecting duct cells, which are the final locations for solute and fluid regulation. Male KS-BMAL1 KO mice displayed a substantially lower basal systolic blood pressure compared with littermate control mice, yet their circadian rhythm in pressure remained unchanged [male control mice: 127 ± 0.7 mmHg (n = 4) vs. male KS-BMAL KO mice: 119 ± 2.3 mmHg (n = 5), P < 0.05]. Female mice, however, did not display a genotype difference in basal systolic blood pressure [female control mice: 120 ± 1.6 mmHg (n = 5) vs. female KS-BMAL1 KO mice: 119 ± 1.5 mmHg (n = 7), P = 0.4]. In addition, male KS-BMAL1 KO mice had less Na+ retention compared with control mice in response to a K+-restricted diet (15% less following 5 days of treatment). However, there was no genotype difference in Na+ handling after a K+-restricted diet in female mice. Furthermore, there was evidence indicating a sex-specific response to K+ restriction where female mice reabsorbed less Na+ in response to this dietary challenge compared with male mice. We propose that BMAL1 in the distal nephron and collecting duct contributes to blood pressure regulation and Na+ handling in a sex-specific manner.

Female C57BL/6J mice lacking the circadian clock protein PER1 are protected from nondipping hypertension.

The circadian clock is integral to the maintenance of daily rhythms of many physiological outputs, including blood pressure. Our laboratory has previously demonstrated the importance of the clock protein period 1 (PER1) in blood pressure regulation in male mice. Briefly, a high-salt diet (HS; 4% NaCl) plus injection with the long-acting mineralocorticoid deoxycorticosterone pivalate (DOCP) resulted in nondipping hypertension [<10% difference between night and day blood pressure (BP) in Per1-knockout (KO) mice but not in wild-type (WT) mice]. To date, there have been no studies that have examined the effect of a core circadian gene KO on BP rhythms in female mice. The goal of the present study was to determine whether female Per1-KO mice develop nondipping hypertension in response to HS/DOCP treatment. For the first time, we demonstrate that loss of the circadian clock protein PER1 in female mice does not significantly change mean arterial pressure (MAP) or the BP rhythm relative to female C57BL/6 WT control mice. Both WT and Per1-KO female mice experienced a significant increase in MAP in response to HS/DOCP. Importantly, however, both genotypes maintained a >10% dip in BP on HS/DOCP. This effect is distinct from the nondipping hypertension seen in male Per1-KO mice, demonstrating that the female sex appears to be protective against PER1-mediated nondipping hypertension in response to HS/DOCP. Together, these data suggest that PER1 acts in a sex-dependent manner in the regulation of cardiovascular rhythms.

Renal Na-handling defect associated with PER1-dependent nondipping hypertension in male mice.

Many physiological functions have a circadian rhythm, including blood pressure (BP). BP is highest during the active phase, whereas during the rest period, BP dips 10-20%. Patients that do not experience this dip at night are termed "nondippers." Nondipping hypertension is associated with increased risk of cardiovascular disease. The mechanisms underlying nondipping hypertension are not understood. Without the circadian clock gene Per1, C57BL/6J mice develop nondipping hypertension on a high-salt diet plus mineralocorticoid treatment (HS/DOCP). Our laboratory has shown that PER1 regulates expression of several genes related to sodium (Na) transport in the kidney, including epithelial Na channel (ENaC) and Na chloride cotransporter (NCC). Urinary Na excretion also demonstrates a circadian pattern with a peak during active periods. We hypothesized that PER1 contributes to circadian regulation of BP via a renal Na-handling-dependent mechanism. Na-handling genes from the distal nephron were inappropriately regulated in KO mice on HS/DOCP. Additionally, the night/day ratio of Na urinary excretion by Per1 KO mice is decreased compared with WT (4 × vs. 7×, P < 0.001, n = 6 per group). Distal nephron-specific Per1 KO mice also show an inappropriate increase in expression of Na transporter genes αENaC and NCC. These results support the hypothesis that PER1 mediates control of circadian BP rhythms via the regulation of distal nephron Na transport genes. These findings have implications for the understanding of the etiology of nondipping hypertension and the subsequent development of novel therapies for this dangerous pathophysiological condition.