Impaired Renal Endothelin‐1 Excretion in Diet‐induced Obesity Contributes to Salt Sensitivity in Male Sprague‐Dawley Rats

Abstract

The renal endothelin system is an important regulator of sodium excretion in response to high salt intake. Urinary Na+ excretion (UNaV) as well as endothelin‐1 (ET‐1) excretion follow a diurnal rhythm, which is controlled by central and peripheral clock genes. Decreased renal medullary ET‐1 secretion has been attributed to salt sensitive (SS) hypertension. Diet‐induced obesity leads to clock gene dysfunction and Na+ imbalance. However, it is not clear how a high fat (HF) diet could influence the diurnal regulation of these systems. Our overarching hypothesis is that HF diet contributes to the development of salt‐sensitive hypertension through renal ET‐1 dysfunction. Male Sprague‐Dawley rats were put on either high fat (45 Kcal% fat, HF) or normal fat (10 Kcal% fat, NF) diet for 8 weeks duration starting at 6 weeks of age. Half‐way through the feeding protocol, rats were implanted with telemetry transmitters for continuous blood pressure measurement. For the last 2 weeks of feeding, HF and NF groups were divided into 2 subgroups to receive an additional high salt (4% NaCl, HS) diet or continue on the normal salt (0.3% NaCl, NS) diet with continuous blood pressure monitoring. For the last 4 days of the feeding protocol, rats were put in metabolic cages for 2 days of acclimation, and then urine was collected and food and water intakes were monitored in 12‐hr increments (inactive‐lights on/active‐lights off) for the subsequent 2 days. Systolic blood pressure (BP) was significantly increased in HF compared to NF rats after 6 weeks of feeding (145 ± 1 vs 138 ± 2 mmHg, n =14 and 11 respectively) during their active period. Chronic HS diet led to a further increase in active‐time systolic BP in HF rats compared to their NS control (153 ± 2 vs 146 ± 2 mmHg, n =6/group), while addition of HS to the NF group showed no significant increase in systolic BP (145 ± 2 vs 140 ± 3 mmHg, n =6/group) during the active period. The NF group on HS diet had a significant increase in active period ET‐1 excretion compared to NF/NS (4.34 ± 0.35 vs 1.65 ± 0.38 pg/12hrs, n= 6/group). However, HS diet failed to increase active period ET‐1 excretion in the HF (2.56 ±0.29 vs 1.32 ± 0.12 pg/12 hrs, n=6/group) compared to their NS controls. Rats on HF/NS and NF/NS diets were given an acute salt load of 900 μEq NaCl in 1 ml water, i.p. to assess their acute natriuretic response. The natriuretic response to an acute salt load given at the beginning of the active period was significantly reduced in rats on HF vs. NF with ⊗UNaV of 629 ± 91 vs 985 ± 75 μEq/12 hrs in the first 12 hours post‐load, respectively, n= 6/group. This was associated with attenuated ET‐1 excretion in HF versus NF diet (0.93 ± 0.15 vs 1.69 ± 0.16 pg/12hrs, respectively, n=11/group). There were no significant differences in the natriuretic response when the salt load was given at the beginning of the inactive period. These data show that HF diet reduces renal ET‐1 and salt handling in a time of day dependent manner. These findings are also consistent with our hypothesis that timing of Na+ handling may contribute to salt‐sensitive hypertension in diet‐induced obesity.Support or Funding InformationFunded by P01 HL136267 to DMP