NADPH Oxidase p67phox Subunit Contributes to the Maintenance of Diurnal Blood Pressure Rhythms in Dahl Salt‐Sensitive Rats

Abstract

Loss of diurnal blood pressure (BP) rhythm, such as absence of dipping during the inactive phase is a known risk factor for cardiovascular disease. Salt‐sensitive individuals have higher susceptibility for dysfunctional diurnal BP rhythms as well as for developing hypertension. In this study, we hypothesized that NADPH oxidase, specifically the p67phox subunit, impairs diurnal BP rhythm and circadian clock gene mRNA expression in the Dahl salt‐sensitive (SS) rats. We utilized SS and p67phox knockout SS (SSp67phox−/−) rats (n=8 per group) to determine 24‐hr BP, heart rate and locomotor activity by telemetry while on normal (0.4% NaCl), low (0.04% NaCl) and high (4% NaCl) salt diets (Dyets, AIN‐76A). Our results show that SSp67phox−/− rats and SS rats have similar diurnal BP rhythms (average amplitude 3.43 vs. 3.13 mmHg, respectively) when on a normal salt diet. Cosinor analysis was used to calculate the amplitude, which is defined as the difference between peak and mean value of a 24‐hr oscillation. 24‐hr mean arterial pressure (MAP) was lower in SSp67phox−/− than SS (116 ± 1 vs. 129 ± 1 mmHg, respectively, p<0.0001). Heart rate groups and locomotor activity did not differ across genotypes or diets. On one week of high salt diet, the increase in 24‐hr MAP from normal salt diet was larger in SS rats compared to SSp67phox−/− (+11.6 ± 0.8 vs. +7.0 ± 1.1 mmHg, respectively, p=0.017). The diurnal BP rhythm (day vs. night) in SS and SSp67phox−/− on high salt diet became more pronounced than on normal salt diet (average amplitude 7.99 and 6.11 mmHg, respectively). Low salt diet did not change 24‐hr MAP in either genotype, however diurnal BP rhythm was abolished in SS rats and maintained in the SSp67phox−/− rats (average amplitude 2.35 and 3.30 mmHg, respectively). We also measured 12‐hr urinary sodium (Na+) excretion via metabolic cages under normal, low and high salt diets. During the dark/active phase on high salt diet, SSp67phox−/− had less water intake and urine output compared to SS rats, while light/inactive phase measurements were similar. Urinary Na+ excretion was also lower in SSp67phox−/− compared to SS controls on high salt diet during the light/active phase, while Na+ excretion was similar during the light/inactive phase. Food intake was similar between genotypes, regardless of time of day. On normal and low salt diets, SSp67phox−/− and SS rats had similar urine output, food and water intake, and urinary Na+ excretion. To assess if core clock gene, Bmal1, was altered with salt diets, we examined Bmal1 mRNA expression in renal vessels at 12‐hr intervals under either low or high salt feeding. Bmal1 mRNA expression was higher at zeitgeber time 0 (ZT0) compared to ZT12 in renal vessel and buccal cells. In contrast to our prior findings using renal inner medullary tissue, neither salt diet or genotype changed the magnitude of Bmal1 mRNA expression. Taken together, these data suggest that NADPH oxidase p67phox expression contributes to the maintenance of diurnal BP rhythms in SS rats on low salt diet.Support or Funding InformationThis work was supported by the American Heart Association Strategically Focused Research Network grant (15SFRN2390002)