Greater natriuretic response to ENaC inhibition in male versus female Sprague Dawley rats.

Genes for the epithelial sodium channel (ENaC) subunits are expressed in a circadian manner, but whether this results in time of day differences in activity is not known. Recent data show that protein expression of ENaC subunits is higher in kidneys from female rats, yet females are more efficient in excreting an acute salt load. Thus, our in vivo study determined whether there is a time of day difference as well as a sex difference in the response to ENaC inhibition by benzamil. Our results showed that the natriuretic and diuretic responses to a single dose of benzamil was significantly greater in male compared to female rats whether given at the beginning of the inactive period (Zeitgeber time 0, ZT0, 7am) or active period (ZT12, 7pm). However, the response to benzamil was not significantly different between ZT0 and ZT12 dosing in either male or female rats. Given previous reports of flow-induced stimulation of endothelin-1 (ET-1) production and sex differences in the renal endothelin system, we measured urinary ET-1 excretion to assess the effects of increased urine flow on intrarenal ET-1. ET-1 excretion was significantly increased following benzamil administration in both sexes but this increase was significantly greater in females. These results support the hypothesis that ENaC activity is less prominent in maintaining Na+ balance in females independent of renal ET-1. Since ENaC subunits genes and protein expression vary by time of day and is greater in female rat kidneys, this suggests a clear disconnect between ENaC expression and channel activity.