Abstract Body: Hypothesis The diurnal rhythmicity of blood pressure (BP) and sodium excretion is important to blood pressure regulation and is strongly influenced by the circadian clock genes. We hypothesized that the diurnal control of BP and natriuresis is impaired in aged male Sprague Dawley (SD) rats and may be associated with altered kidney expression of circadian clock gene proteins.

Methods Radiotelemetry (to record mean arterial BP (MAP)) and metabolic balance (to assess sodium excretion) was conducted in 3-month-old (3M) and 16-month-old (16M) male SD rats. On a separate day, 3M and 16M rats received an acute 900 μEq NaCl load by oral gavage at ZT0 and sodium excretion was assessed. Total renal cortex expression of Per1 and Bmal1 was assessed by immunoblotting (N=4-6/group ± SD).

Results 16M rats exhibit significantly increased MAP and 24-h sodium retention vs. 3M rats [MAP (mmHg) 3M 102±4 vs. 16M 127±5, P<0.05; 24h Na retention (mEq/24h) 3M 0.3±0.1 vs. 16M 0.8±0.2, P<0.05). 16M rats exhibit an attenuation of the circadian MAP pattern compared to 3M rats. Aged rats have a significant decrease in the blood pressure rise during the dark (active) vs. 3M rats (Dark phase increase in MAP (mmHg) 3M +13.6±2.1 vs. 16M +4.8±1.9, P<0.05). 3M rats exhibit a classical diurnal pattern of 24-h sodium excretion with increased natriuresis during the dark (active) phase – a response that is attenuated in 16M rats that exhibit a significant reduction in the increase in natriuresis in the dark phase (Light phase UNaV (μEq/12h) 3M 380±48 vs. 16M 410±36, Dark phase natriuresis 3M 620±31 vs 16M 502±27, P<0.05). 16M rats had an impaired natriuretic response to an acute oral NaCl load in the first 12h post-gavage (UNaV 12h (μEq/12h) 3M 681±42 vs. 16M 243±39, P<0.05). In 16M old rats there was no change in renal Per1 and decreased renal Bmal1 protein (Bmal1 fold change 16M vs. 3M, 0.65±0.7, P<0.05).

Conclusions We observed impaired circadian control of blood pressure and renal sodium excretion in aged male SD rats. These changes in renal sodium handling may reflect endogenous decreases in renal Bmal1 levels with age and contribute to the observed hypertensive phenotype in aged rats.