Sex Differences in Diurnal Sodium Handling During Diet-Induced Obesity in Rats

Abstract

Background: Emerging evidence over the past several years suggests that diurnal control of sodium excretion is sex dependent and involves the renal endothelin system. Given recent awareness of disruptions of circadian function in obesity, we determined whether diet-induced obesity impairs renal handling of an acute salt load at different times of day and whether this varies by sex and is associated with renal endothelin dysfunction.

Methods: Male and female Sprague-Dawley rats were placed on a high-fat diet for 8 weeks before assessing renal sodium handling and blood pressure.

Results: Male, but not female, rats on high fat had a significantly reduced natriuretic response to acute NaCl injection at the beginning of their active period that was associated with lower endothelin 1 (ET-1) excretion, lower ET-1 mRNA expression in the cortex and outer medulla as well as lower ET_B receptor expression in the outer medulla of the high-fat rats. Obese males also had significantly higher blood pressure (telemetry) that was exacerbated by adding high salt to the diet during the last 2 weeks. While female rats developed hypertension with a high-fat diet, they were not salt sensitive and ET-1 excretion was unchanged.

Conclusions: These data identify diet-induced obesity as a sex-specific disruptive factor for maintaining proper sodium handling. Although high-fat diets induce hypertension in both sexes, these data reveal that males are at greater risk of salt-dependent hypertension and further suggest that females have more redundant systems that can be productive against salt-sensitive hypertension in at least some circumstances.

Keywords: blood pressure; endothelin-1; kidney; obesity; sodium.

Figure 1:. High fat (HF) diet increases mean arterial pressure (MAP; A) but not heart rate (B), body temperature (C) or activity (D) in both male and female rats.

Blood pressure and heart rate were measured in Sprague-Dawley rats (telemetry) following 8 weeks on either NF or HF diet protocol. Data for individual animals along with mean ± SE were obtained from continuous recordings during the final 2 days of treatment and are presented as 12-hour averages for inactive (lights on) and active (lights off) periods. Each symbol represents an individual animal. Data were analyzed by repeated measures 2-way ANOVA, *Indicates p < 0.05 from Sidak’s post-hoc test. DxT: Diet x Time interaction.

Figure 2:. High fat (HF) diet attenuates urine ET-1 excretion (A) only in male rats during the active period, increases plasma aldosterone in female rats during the inactive period (B) and increases aldosterone excretion (C) only in females during the active period.

Urine was collected in 12-hr increments by placing rats in metabolic cages. Data were analyzed by repeated measures 2-way ANOVA, *Indicates p < 0.05 from Sidak’s post-hoc test. DxT: Diet x Time interaction.

Figure 3:. Acute natriuretic response to an i.p. injection of NaCl (900μEq; 1ml 0.9% NaCl) at ZT0 or ZT12 in male and female Sprague-Dawley rats on either NF or HF diets (A) along with corresponding excretion rates for ET-1 (B) and aldosterone (C).

The acute salt load was given to rats either at the beginning of their inactive period when lights come on (ZT0) or the active period when lights go off (ZT12). Urine was collected in 12-hr increments by placing rats in metabolic cages. The change in sodium excretion (ΔUNaV) were calculated as the difference from baseline for the corresponding time of day. Data were analyzed by repeated measures 2-way ANOVA, *Indicates p < 0.05 from Sidak’s post-hoc test. DxT: Diet x Time interaction.

Figure 4:. Combining high salt (HS) and high fat diet increases MAP (A) in male but not female Sprague-Dawley rats without any changes in heart rate (B), body temperature (C) or locomotor activity (D).

Rats were maintained on HF diet for 8 weeks with HS being added during the final two weeks. Telemetry data are presented as the average 12-hour means for the final two days of the protocol. Each symbol represents an individual animal. Data were analyzed by repeated measures 2-way ANOVA. *Indicates p < 0.05 from Sidak’s post-hoc test. SxT: Salt x Time interaction.

Figure 5:. Chronic HS diet increased ET-1 excretion in both sexes that was more evident in the active period although the increase was significantly attenuated in rats on HF only during the active period (A, B). Aldosterone excretion was suppressed by HS similarly in both sexes (C, D).

Urine was collected in 12-hr increments by placing rats in metabolic cages. Each symbol represents an individual animal. Data were analyzed by repeated measures 2-way ANOVA, *Indicates p < 0.05 from Sidak’s post-hoc test. SxD: Salt x Diet interaction.