The amiloride-sensitive epithelial Na+ channel (ENaC) plays a critical role in Na+ uptake and fluid homeostasis in epithelial tissues. In the kidney, Na+ uptake via ENaC is essential in maintaining Na+ balance and the regulation of arterial blood pressure. Glucocorticoid hormones are known to mediate changes in ENaC mRNA levels in the kidney and the lung. More recently, it has been demonstrated that female gender hormones can regulate ENaC expression in the lung (Sweezy et al. 1998). As renal Na+ handling is reported to differ between males and females particularly during the menstrual cycle (Stachenfeld et al. 2001), we have investigated changes in the abundance of mRNAs encoding the three subunits of ENaC (α, β and λENaC) between male and female rats (fed with normal Na+-containing diets) during postnatal development. Hooded/Lister rats were humanely killed by cervical dislocation and kidneys removed. RNA was extracted from the tissue using Trizol (Sigma) and analysed by northern blotting as previously described (Baines et al. 2000).
Immediately after birth, there was no significant difference in mRNA abundance of any of the subunits in male or female rat kidneys. By 10 weeks of age, however, there were significantly higher levels of αENaC, βENaC and λENaC mRNA in female compared with male kidneys (P = 0.001, n = 10, P = 0.004, n = 10 and P = 0.02, n = 10, respectively, Student’s unpaired t test). At 16 weeks, all three subunits remained significantly higher in females than males (P < 0.01, n = 10, P < 0.01, n = 10 and P = 0.002, n = 10, respectively).
In order to establish if female gonadal hormones were important in maintaining the elevated levels of ENaC subunits in female rats, we performed ovariectomy on rats of 12 weeks in age. Female Wistar rats were anaethetised with halothane (3 % in O2) and ovariectomy performed as previously described (Murray et al. 2000). After a 2 week recovery period, kidneys were removed and analysed as described above. Following ovariectomy levels of α, β and λENaC were not significantly different between females and males (P = 0.9, n = 6, P = 0.2, n = 6 and P = 0.8, n = 6, respectively).
Taken together these data suggest that female development and the associated elevation of female gender hormones (such as oestrogen and progesterone) at sexual maturity regulate ENaC mRNA levels in the kidney.