Abstract P173: Sex Differences in the Diurnal Natriuretic Response to Benzamil in Sprague Dawley Rats

Abstract

Epithelial sodium channel (ENaC) expression follows a circadian pattern, but how this impacts activity is not known. In addition to the well-established sex differences in renal Na handling, recent data show that ENaC expression is higher (Western blot) in female rats. Further, we do not know if there are time of day differences or sex differences in ENaC activity. Therefore, the aim of this study was to determine if the diuretic response to ENaC inhibition (benzamil) is different between sexes at different times of day. SD rats (12-16 wk old) were placed in metabolic cages, where 12-hour urine collections were obtained to measure baseline urine volume and Na excretion as well as water and food intake. On day 3, benzamil was given at a dose of 1mg/kg (i.p.) either at the beginning of their inactive period/lights on (Zeitgeber Time 0, ZT0) or their active period/lights off (ZT12). The natriuretic response to benzamil was significantly greater in male compared to female rats (909 ± 302 vs 523 ± 83 μEq/kg/hr n=8) at ZT0 and (934 ± 94 vs 714 ± 151 μEq/kg/hr n=8) at ZT12. The diuretic response followed natriuresis being more prominent in male than female rats regardless of time of day (4.2 ± 0.7 vs 3.3 ± 0.5 ml/kg/hr n=8) at ZT0 and (3.6 ± 0.6 vs 2.5 ± 0.3 ml/kg/hr n=8) at ZT12. However, the larger response to benzamil given at the beginning of the inactive period (ZT0) compared to active period (ZT12) was not statistically significant (311 ± 98 vs 354 ± 30 μEq/hr and 120 ± 17.75 vs 174 ± 37 μEq/hr n=8) in male and female rats respectively. Given that endothelin-1 (ET-1) is an upstream inhibitor of ENaC, we measured urinary ET-1 levels to assess intrarenal production. ET-1 excretion significantly increased following benzamil administration in both sexes but was significantly greater in females. ET-1 excretion increased from 0.06 ± 0.01 to 0.29 ± 0.05 pg/hr in males (n=8) and from 0.10 ± 0.01 to 0.57 ± 0.28 pg/hr in females (n=8) at ZT0. At ZT12, ET-1 increased from 0.09 ± 0.02 to 0.23 ± 0.06 pg/hr in males (n=8) and from 0.14 ± 0.05 to 0.34 ± 0.06 pg/hr in females (n=8). These results demonstrate that the response to ENaC inhibition is less prominent in females independent of renal ET-1. This suggests less ENaC activity independent of expression in females or differences in non-ENaC related effects of benzamil.