Male Sex is Associated with a Reduced Alveolar Epithelial Sodium Transport

Respiratory distress syndrome (RDS) is the most frequent pulmonary complication in preterm infants. RDS incidence differs between genders, which has been called the male disadvantage. Besides maturation of the surfactant system, Na(+) transport driven alveolar fluid clearance is crucial for the prevention of RDS. Na(+) transport is mediated by the epithelial Na(+) channel (ENaC) and the Na,K-ATPase, therefore potential differences in their expression or activity possibly contribute to the gender imbalance observed in RDS. Fetal distal lung epithelial (FDLE) cells of rat fetuses were separated by sex and analyzed regarding expression and activity of the Na(+) transporters. Ussing chamber experiments showed a higher baseline short-circuit current (I(SC)) and amiloride-sensitive ΔI(SC) in FDLE cells of female origin. In addition, maximal amiloride-sensitive ΔI(SC) and maximal ouabain-sensitive ΔI(SC) of female cells were higher when measured in the presence of a permeabilized basolateral or apical membrane, respectively. The number of FDLE cells per fetus recoverable during cell isolation was also significantly higher in females. In addition, lung wet-to-dry weight ratio was lower in fetal and newborn female pups. Female derived FDLE cells had higher mRNA levels of the ENaC- and Na,K-ATPase subunits. Furthermore, estrogen (ER) and progesterone receptor (PR) mRNA levels were higher in female cells, which might render female cells more responsive, while concentrations of placenta-derived sex steroids do not differ between both genders during fetal life. Inhibition of ER-β abolished the sex differences in Na(+) transport and female cells were more responsive to estradiol stimulation. In conclusion, a higher alveolar Na(+) transport, possibly attributable to a higher expression of hormone receptors in female FDLE cells, provides an explanation for the well known sex-related difference in RDS occurrence and outcome.